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Short-term effects of graminaceous cover crops on autumn soil mineral nitrogen cycling in western lower… Nafuma, Leonard Simiyu 1998

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SHORT-TERM EFFECTS OF GRAMINACEOUS COVER CROPS ON AUTUMN SOIL MINERAL NITROGEN CYCLING IN WESTERN LOWER FRASER VALLEY SOILS by Leonard Simiyu Nafuma B.Sc. The University of Nairobi, 1981 M.Sc. The University of Manitoba, 1987 A Thesis Submitted in Partial Fulfilment of the Requirements for the Degree of Doctor of Philosophy in THE FACULTY OF GRADUATE STUDIES (DEPARTMENT OF SOIL SCIENCE) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA May 1998 © Leonard Simiyu Nafuma, 1998 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia Vancouver, Canada Date DE-6 (2/88) 11 A B S T R A C T Proper cover crop management practices in autumn can minimize N03"-N leaching. Three experiments to study the effect of cover crop management on autumn soil mineral N conservation were conducted in the 1991-92, 1992-93 and 1993-94 winter seasons on a silty clay loam Rego Humic Gleysol in the western Lower Fraser Valley, British Columbia, Canada. The study tested short-term effects of planting date, autumn soil mineral N content and type of cover crop on biomass production and N uptake, residual soil mineral N (0-60 cm layer), plant composition of various N fractions of autumn-planted spring species at winter-kill, retention of accumulated N by autumn-planted spring species after winter-kill, and the C/N ratio of cover crops. Treatments involved two planting dates (late August and September), two simulated autumn residual mineral N levels (0 and 100 kg N ha"1) and types of cover crops. In the first two seasons, the cover crop treatments were spring barley (Hordeum vulgare L.) and winter rye (Secale cereale L.) plus fallow for comparison purposes. In the third season, planting date was omitted and six cover crop treatments tested were spring barley, spring wheat (Triticum aestivum L.), spring oat (Avena sativa L.), winter rye and annual ryegrass (Lolium multiflorum Lam.) including fallow. Planting crops in August as compared to a month later increased biomass production by 56 to 135% and N uptake by 38 to 93% before winter leaching period. Large N uptake by cover crops that were planted in August was generally accompanied by significant reduction in soil mineral N (0-60 cm) from August to November. August-planted spring species N at winter-kill was largely composed of the protein fraction (insoluble and soluble) which increased with N supply in autumn when the initial mineral N contents in 0-60 cm layer of soil were suboptimal (< 100 kg N ha1) but was not affected when soil mineral N content was 200 kg N ha"1 and more or when the cover crops were planted in I l l September. There were indications that August-planted spring species can retain some of the soluble protein N fraction in the winter-killed residues during winter. Maximum plant N0 3 ' -N content represented about 15% (~ 20 kg N ha"1) of the total N in the plant when cover crops were planted in August and autumn soil mineral N content (0-60 cm) was about 200 kg N ha-' or more. The proportion of NffV-N averaged only 3%. Spring species can be included in winter cropping systems in western Lower Fraser Valley. Spring species that were planted in August and winter-killed in late autumn showed greater potential to retain the N accumulated before winter-kill compared to the cover crops that were planted a month later. August-planted spring species increased soil mineral N (by 40 to 76%) in the 0-60 cm layer in spring relative to fallow plots while September-planted crops had little effect. It appears, spring species can play a significant role in autumn mineral N conservation by accumulating large amounts of autumn soil mineral N before winter leaching period, retaining it in winter-killed residues until spring and releasing the N in plant available form through decomposition and mineralization. iv TABLE OF CONTENTS Abstract ii Table of Contents iv List of Tables viii List of Figures xi List of Appendices xv Acknowledgements xvi Chapter 1 : GENERAL INTRODUCTION 1 1-1. Background 1 1- 2. Objectives and Hypotheses 6 1-2.1. Statement of Objectives 6 1-2.2. Hypotheses 7 1-2.2.1. Comparison of Spring Species and Winter Species 7 1-2.2.2. Retention of Accumulated Nitrogen in Spring Species Residues During Winter. 8 Chapter 2: LITERATURE REVIEW 10 2- 1. Factors Affecting Mineral Nitrogen Accumulation in Soil 10 2-2. Factors Affecting Nitrate Leaching 12 2-3. Factors Affecting Denitrification Loss 13 2-4. Effect of Grasses on Nitrate Leaching 14 2-5. Grass Cover Crop Nitrogen Accumulation 16 2-6. Effect of Planting Date on Soil Nitrogen 17 V 2-7. Research on Nitrogen in the Lower Fraser Valley 18 2-8. Factors Affecting Nitrate Assimilation in Plants 18 2- 9. Nitrogen Fractions and Chemical Composition of Plants 20 Chapter 3: MATERIALS AND METHODS 22 3- 1. Experimental Layout and Soil Type Description 22 3-2. The Rationale for Treatment Choices 28 3-3. Autumn Soil Mineral Nitrogen Dynamics 29 3-3.1. Soil Sampling and Initial Conditions 29 3-3.2. Soil Mineral Nitrogen Extraction and Analysis 33 3-3.3. Plant Sampling and Biomass Measurement 34 3-3.4. Cover Crop Total Nitrogen and Carbon Analysis 37 3-3.5. Statistical Analysis 38 3- 4. Nitrogen Dynamics of Spring Graminaceous Cover Crops 40 3-4.1. Nitrogen Retention Study 40 3-4.2. Nitrogen Fractionation Study 43 3-4.2.1. Sample Preparation and Extraction Techniques 43 3-4.2.2. Preparation of Homogenizing Medium 43 3-4.2.3. Phosphate Buffer (Cold-Water) Extraction 44 3-4.2.4. Sample Nitrogen Determination 47 3-4.2.5. Hot Water Extraction 48 3-4.3. Statistical Analysis 49 Chapter 4: EXPERIMENTAL CONDITIONS 50 4- 1. Weather During Study Period 50 4-2. Daily Soil and Air Temperatures 50 4-3. Monthly Minimum and Maximum Temperatures 59 vi Chapter 5: RESULTS 64 5-1. Dynamics of Autumn Soil Mineral Nitrogen 64 5-1.1. Soil Mineral Nitrogen Status in Fallow Plots 64 5-1.2. Cover Crop Biomass and Nitrogen Before Winter Leaching Period 66 5-1.3. Soil Mineral Nitrogen Content Before Winter Leaching Period 71 5-1.4. Impact of Cover Cropping on Soil N0 3" Before Winter Leaching Period 78 5-1.5. Cover Crop Biomass and Nitrogen After Winter Leaching Period 80 5-1.6. Soil Mineral Nitrogen Content After Winter Leaching Period 86 5-1.7. Changes in Cover Crop Biomass and Nitrogen During Winter 92 5-1.8. Changes in Soil Mineral Nitrogen During Winter 97 5-1.9. Fertilizer Nitrogen Balance 103 5-1.10. Cover Crop Carbon to Nitrogen Ratios 108 5- 2. Nitrogen Dynamics of Spring Graminaceous Cover Crops 115 5-2.1. Nitrogen Retention by Spring Species During Winter 115 5-2.2. Spring Species Nitrogen Fractions At Winter-Kill 119 5-2.3. Protein Nitrogen Fractions and Nitrogen Retained in Meshbag Residues in the Field 123 Chapter 6: DISCUSSION 125 6- 1. Cover Crop and Soil Nitrogen Before Winter Leaching Period 125 6-2. Impact of Cover Cropping on Soil N0 3" Before Winter Leaching Period 127 6-3. Cover Crop and Soil Nitrogen After Winter Leaching Period 129 6-4. Fertilizer Nitrogen Balance 132 6- 5. Recommendations 135 Chapter 7: CONCLUSIONS AND FUTURE RESEARCH 136 7- 1. Conclusions 136 7-1.1. Cover Crop and Soil Mineral Nitrogen Before Winter Leaching 136 7-1.2. Cover Crop and Soil Mineral Nitrogen After Winter Leaching 136 7-1.3. Spring Species Nitrogen Fractions At Winter-Kill 138 vii 7-2. Future Research 139 REFERENCES 141 TABLES OF APPENDICES 152 vm LIST OF T A B L E S Table 3-1. Summary of field operations and precipitation measurements for 1991-92, 1992-93 and 1993-94 winter cropping seasons 27 Table 3-2. Some soil chemical properties at first planting for 1991-92 experimental location.... 30 Table 3-3. Some soil physical and chemical properties at first planting for 1992-93 experimental location 31 Table 3-4. Some soil physical and chemical properties at planting for 1993-94 experimental location 32 Table 3-5. Summary of sampling dates and growth stages of cover crops for the three winter cropping seasons (1991-94) 36 Table 5-1. Main growing season residual soil mineral N for samples taken immediately before planting in the three cover cropping seasons 64 Table 5-2. Effect of planting date, autumn soil nitrogen content and crop species on biomass and cover crop N before winter leaching period (November 21, 1991) 67 Table 5-3. Effect of planting date, autumn soil mineral nitrogen content and crop species on biomass and cover crop N before winter leaching period (November 24, 1992) 68 Table 5-4. Effect of autumn soil mineral nitrogen content and cover crop species on biomass production and cover crop N before winter leaching period (November 24, 1993).. 70 Table 5-5. Effect of planting date, autumn soil nitrogen content and cover crop species on residual soil mineral N before winter leaching period (November 21, 1991) 72 Table 5-6. Effect of planting date, autumn soil mineral nitrogen content and crop species on residual soil mineral N before winter leaching period (November 24, 1992) 73 Table 5-7. Effect of autumn soil mineral nitrogen content and crop species on residual mineral N before winter leaching period (November 24, 1993) 76 Table 5-8. Analysis of variance (P > F values) for planting date (D), autumn soil mineral nitrogen content (N) and crop species effects on percent N0 3"-N reduction (PR) in the 0-60 cm layer before winter leaching period for the three winter cropping seasons 78 Table 5-9. Influence of crop species on NOV reduction (% - PR) in the 0-60 cm layer before winter leaching period of 1993 79 Table 5-10. Effect of planting date, autumn soil nitrogen content and crop species on biomass and cover crop N after winter leaching period (April 08, 1992) 80 ix Table 5-11. Effect of planting date, autumn soil mineral nitrogen content and cover crop species on biomass and cover crop N after winter leaching period (April 30, 1993) 83 Table 5-12. Effect of autumn soil mineral nitrogen content and cover crop species on biomass and cover crop N after winter leaching period (April 14, 1994) 85 Table 5-13. Effect of planting date, autumn soil nitrogen content and crop species on residual soil mineral N after winter leaching period (April 08, 1992) 87 Table 5-14. Effect of planting date, autumn soil mineral nitrogen content and crop species on residual soil mineral N after winter leaching period (April 30, 1993) 89 Table 5-15. Effect of autumn soil mineral nitrogen content and crop species on residual mineral N after winter leaching period (April 14, 1994) 91 Table 5-16. Repeated measures analysis of variance (P > F values) for planting date (D), autumn soil nitrogen content (N), crop species (C) and time of sampling (T) effects on variations in biomass and cover crop N during the three cropping winters 92 Table 5-17. Repeated measures analysis of variance (P > F values) for planting date (D), autumn soil nitrogen content (N), crop species (C) and time of sampling (T) effects on variations in mineral N in the 0-60 cm soil layer 98 Table 5-18. Balance sheet for applied fertilizer N0 3"-N (100 kg N ha') for 1991-92 season as estimated by the difference method 104 Table 5-19. Balance sheet for applied fertilizer N0 3 ' -N (100 kg N ha"') for 1992-93 season as estimated by the difference method 106 Table 5-20. Balance sheet for applied fertilizer N0 3 ' -N (100 kg N ha"') for 1993-94 season as estimated by the difference method 107 Table 5-21. Analysis of variance for the effect of planting date, autumn soil mineral nitrogen content and crop species on C/N ratio during the 1991-92 and 1992-93 cover cropping seasons 108 Table 5-22. Analysis of variance for the effect of autumn soil mineral nitrogen content and crop species on C/N ratio in the 1993-94 cover cropping season 113 Table 5-23. Analysis of variance (P > F values) for effects of planting date (D) and autumn soil mineral nitrogen content (N) on residue biomass remaining and nitrogen retention by spring barley over the 1991-92 and 1992-93 winter seasons 115 Table 5-24. Analysis of variance (P > F values) for effects of autumn soil mineral nitrogen content (N) and crop species (C) on winter-killed crop residue biomass remaining and nitrogen retention over the 1993-94 winter 117 X Table 5-25. Analysis of variance (P > F-values) for effects of planting date (D) and autumn soil mineral nitrogen content (N) on the various nitrogen fractions of spring barley at winter-kill during the 1992-93 winter cropping season 119 Table 5-26. Effect of planting date and autumn soil mineral nitrogen content on organic nonprotein N (NPN) fraction (kg ha"1) at winter-kill in late November 1992 121 Table 5-27. Analysis of variance (P > F-values) for effects of autumn soil mineral N content (N) and cover crop species (C) on the various spring species nitrogen fractions at winter-kill in late November 1993 122 Table 5-28. Effect of cover crop species (C) on the various spring species nitrogen fractions at winter-kill in late November 1993 122 Table 5-29. Comparison of cold-water insoluble N (WIN) and total protein N fractions with N retained in meshbag residues in the field in 1992-93 and 1993-94 seasons 123 xi LIST OF FIGURES Figure 3-1. Map of delta showing locations of experiments ( • ) for the three winter cropping seasons. L91 = Nottingham farm (1991-92); L92 = Kamlah farm (1992-93); L93 = Swenson farm (1993-94) 23 Figure 3-2. Layout of experiments established in August 1991 and August 1992 at Nottingham (L91) and Kamlah (L92) locations. (N-0, no N applied; N-100, 100 kg N ha"' applied as N03"; F = fallow; SB = spring barley; WR = winter rye;) 24 Figure 3-3. Layout of the experiment established in August 1993 at swenson (L93) location (N-0, no N applied; N-100, 100 kg N ha"1 applied as N03"; F = fallow; SB = spring barley; SW = spring wheat; SO = spring oat; WR = winter rye; A R G = annual ryegrass) 26 Figure 3-4. Nitrogen fractionation scheme to determine total N in residue (a), total N in homogenate (b), total N in supernatant (C), N H 4 + - N (d) and N0 3"-N (e) 46 Figure 4-1. Mean monthly air temperatures and total monthly precipitation for 1937-90 normal as recorded at Vancouver International Airport (Source: Environment Canada, Climate Services Vancouver, and Climate Normals for British Columbia) 51 Figure 4-2. Total daily precipitation for 1991-92 (above) and 1992-93 (below) winter cropping seasons as recorded at Vancouver International Airport (Source: Environment Canada, Climate Services Vancouver, and Climate Normals for British Columbia). 52 Figure 4-3. Total daily precipitation during 1993-94 winter cropping season as recorded at Vancouver International Airport (Source: Environment Canada, Climate Services Vancouver, and Climate Normals for British Columbia) 53 Figure 4-4. Mean monthly air temperatures during 1991-92, 1992-93 and 1993-94 winter cropping seasons as recorded at Vancouver International Airport (Source: Environment Canada, Climate Services Vancouver, and Climate Normals for British Columbia) 54 Figure 4-5. Daily air and soil temperature means during the 1991-92 winter cropping season under fallow (above) and autumn-planted spring barley (below) 55 Figure 4-6. Daily air and soil temperature means during the 1991-92 winter cover cropping season under autumn-planted winter rye 56 Figure 4-7. Mean daily air and soil temperatures for the 1992-93 winter cropping season under fallow (above) and autumn-planted spring barley (below) 57 Figure 4-8. Mean daily air and soil temperature variations during the 1992-93 winter cropping season under autumn-planted winter rye 58 X l l Figure 4-9. Monthly minimum (above) and maximum (below) temperatures at 3 cm depth during the 1991-92 winter cropping season, (values above the bars for spring barley and winter rye represent averages over the N treatment of the amount of cover in t ha"1). 60 Figure 4-10. Monthly minimum (above) and maximum (below) temperatures at 40 cm depth during the 1991-92 winter cropping season 61 Figure 4-11. Monthly minimum (above) and maximum (below) soil temperatures at 3 cm depth during the 1992-93 winter cropping season (values above the bars for spring barley and winter rye represent averages over the N treatment of the amount of cover in t ha' ') 62 Figure 4-12. Monthly minimum (above) and maximum (below) soil temperatures at 40 cm depth during the 1992-93 winter cropping season 63 Figure 5-1. Temporal pattern of changes in soil mineral N in the 0-60 cm layer in fallow plots at the primary level (N-0) of autumn soil mineral N for the three winter cropping seasons 65 Figure 5-2. Biomass production (above) and N uptake (below) of autumn-planted spring barley and winter rye before winter leaching period (November 24, 1992) as influenced by planting date and autumn soil mineral N content. Error bars represent standard error of the mean (n = 8) 69 Figure 5-3. Variations in residual mineral N in the 0-60 cm layer in late November 1992 under fallow, spring barley and winter rye as influenced by planting date (above) and, autumn soil N content (below). Error bars represent standard error of the mean (n = 8). D x C C = planting date x crop species interaction; N x C C = nitrogen x crop species interaction; 74 Figure 5-4. Variations in residual mineral N in late November 1993 in the 0-20 (above) and 0-40 cm layers under spring wheat and spring oat as influenced by autumn soil nitrogen content. Error bars represent standard error of the mean (n = 4) 77 Figure 5-5. Influence of planting date on cover crop impact on soil N0 3" in the 0-60 cm layer before winter leaching period. Error bars represent standard error of the mean (n = 8).79 Figure 5-6. Photo showing winter-killed August-planted spring barley (a) and September-planted spring barley (b) in early tillering stage (left) and a close-up of spring barley mulch on February/15/1993. [L.S. Nafuma] 81 Figure 5-7. Photo showing freezing damage on August-planted winter rye that received 100 kg N ha"1 (left) and no N (right) at planting. [February/15/1993 - L.S. Nafuma] 81 Figure 5-8. Spring barley and winter rye biomass (above) and nitrogen (below) in the spring of 1992. Error bars represent standard error of the mean (n = 8) 82 Xl l l Figure 5-9. Spring barley and winter rye biomass (above) and nitrogen (below) in the spring of 1993 as influenced by planting date and autumn soil mineral N content. Error bars represent standard error of the mean (n = 8) 84 Figure 5-10. Soil mineral N in spring of 1992 in the 0-20 cm layer under fallow, spring barley and winter rye as influenced by planting date. Error bars represent standard error of the mean (n =8) 88 Figure 5-11. Soil mineral N in spring of 1993 in the 0-60 cm layer under fallow, spring barley and winter rye as influenced by planting date. Error bars represent standard error of the mean (n =8) 90 Figure 5-12. Changes in biomass (above) and cover crop N (below) during 1991-92 winter as influenced by planting date and crop species. Error bars represent standard error of the mean (n = 8) 94 Figure 5-13. Changes in biomass (above) and cover crop N (below) during 1992-93 winter season as influenced by planting date and crop species. Error bars represent standard error of the mean (n = 8) 95 Figure 5-14. Changes in biomass (above) and cover crop N (below) during 1993-94 winter season as influenced by crop species. Error bars represent standard error of the mean (n = 8) 96 Figure 5-15. Changes in soil mineral N in the 0-60 cm layer during 1991-92 winter as influenced by crop species (a) and by planting date and autumn soil mineral N content (b). Error bars represent standard error of the mean [n = 16 for (a) and n = 12 for (b)] 99 Figure 5-16. Changes in soil mineral N in the 0-60 cm layer during 1992-93 winter season as affected by planting date and crop species. Error bars represent standard error of the mean (n = 8) 100 Figure 5-17. Changes in soil mineral N in the 0-60 cm layer over the 1993-94 winter as affected by autumn soil mineral N content (above) and cover crop species (below). Error bars represent standard error of the mean (n = 24 for above and n = 8 for below) 102 Figure 5-18. Cover crop C/N ratios in late November 1991 as influenced by planting date and autumn soil N content (above) and, planting date and crop species (below). Error bars represent standard error of the mean (n = 8). D x N = planting date x nitrogen interaction; D x CC = planting date x crop species interaction 109 Figure 5-19. The C/N ratios for spring barley residues and winter rye in the spring of 1992. Error bars represent standard error of the mean (n = 16) 110 Figure 5-20. Autumn-planted spring barley and winter rye C/N ratios in late November 1992 as influenced by planting date and autumn soil mineral N content. Error bars represent standard error of the mean (n = 4) I l l xiv Figure 5-21. Cover crop C/N ratios in the spring of 1993 as influenced by planting date and autumn soil N content (above) and, planting date and crop species (below). Error bars represent standard error of the mean (n = 8). D x N = planting date x nitrogen interaction; D x CC = planting date x crop species interaction 112 Figure 5-22. Cover crop C/N ratios in late November 1993 (above) and in the spring of 1994 (below). Error bars represent standard error of the mean (n = 8) 114 Figure 5-23. Influence of planting date on biomass remaining (above) and N retention (below) by spring barley over the 1991-92 and 1992-93 cover cropping seasons. Error bars represent standard error of the mean (n = 8) 116 Figure 5-24. Residue biomass remaining (above) and nitrogen retention (below) in the spring of 1994 for surface and meshbag residues not in contact with soil. Error bars represent standard error of the mean (n = 8). MSDoos is the minimum significant difference according to Tukey's (HSD) test at P < 0.05 118 Figure 5-25. Effect of planting date and autumn soil mineral nitrogen content on cold-water insoluble-, protein-, N03"- and N H 4 + - N fractions of spring barley at winter-kill in late November 1992. Error bars represent standard error of the mean (n = 4). CWIN, cold-water insoluble N; PN, TCA-precipitable protein N; NPN, organic nonprotein N; NN, nitrate N; A N , ammonium N 120 XV LIST OF APPENDICES Table A - l . List of common terms and abbreviations 152 Table A-2. Cold-water, hot-water and K C L extractable ammonium and nitrate N for spring barley in late November 1992 153 Table A-3. Cold-water, hot-water and K C L extractable ammonium and nitrate N for spring species in late November 1993 153 Table A-4. Soil ammonium nitrogen content in late autumn 1991 and in spring 1992 154 Table A-5. Soil ammonium nitrogen content in late autumn 1992 and in spring 1993 154 Table A-6. Soil ammonium nitrogen content in late autumn 1993 and in spring 1994 155 Table A-7. Soil N0 3"-N exprressed as a proportion (%) of total mineral N ( N H 4 + + N03"-N) before winter leaching period in late November 1991 155 Table A-8. Soil N0 3"-N exprressed as a proportion (%) of total mineral N ( N H 4 + + N03"-N) before winter leaching period in late November 1992 156 Table A-9. Soil N0 3"-N expressed as a proportion (%) of total mineral N (NH 4 + + N03"-N) before winter leaching period in late November 1993 156 Table A-10. Various N fractions and total N determined on 100 mg of plant material to check recovery of N for samples taken at winter-kill (November 24, 1992) 157 Table A - l l . Various N fractions and total N determined on 100 mg of plant material to check recovery of N for samples taken at winter-kill (November 24, 1993) 158 Table A-12. Comparison of cold-water (CW) and hot-water (HW) for extraction of protein- and organic nonprotein-N fractions from plant materials 159 Table A- l3 . Comparison of cold-water (CW), hot-water (HW) and 2M K C L (KCL) for extraction of N0 3"-N and N H 4 + - N fractions from plant materials 159 Table A-14. Carbon content (%DM) of cover crops for the 1991-92 and 1992-93 winter cropping seasons 160 Table A-15. Carbon content (%DM) of cover crops for the 1993-94 winter cropping seasons.. 160 ACKNOWLEDGEMENTS xvi Sincere thanks to my research supervisor, Dr. A .A. Bomke, for his interest and advice throughout the course of this work. I am also most grateful to members of my graduate committee, Dr. M.C. Fortin, Dr. C G . Kowalenko and Dr. T . M . Ballard. I also wish to thank Dr. S.M. Berch for her initial contribution to the research at the time she served on my committee, and Dr. L . E . Lowe for his constructive comments on protein extraction and for purchasing the Freeze Drier which made plant nitrogen fractionation possible. My appreciation also goes to Dr. W.D. Temple, Project Coordinator of Delta Farmers' Soil and Water Conservation Group, for some guidance and for establishment of experimental plots. My thanks also go to Rick Ketler for his direct involvement in the work and for installing thermocouples in the plots for temperature measurements. Special thanks to my beloved wife, Eunice Nafuma, for her patience, unswerving support and faith in me throughout the study period, and to my daughter, Donna and sons, Darryl, Lyall and Russell. This thesis is dedicated to them. Finally, I wish to thank the Kenya Government through Kenya Agricultural Research Institute (KARI) for granting me study leave. Financial support was provided by the Kenya Government through KARI and the Canadian Government through Canadian International Development Agency (CIDA) until January 1996. This research was funded by CIDA and Canada-British Columbia Soil Conservation Agreement with costs shared by the governments of Canada and British Columbia through Agriculture Canada and the British Columbia Ministry of Agriculture, Fisheries and Food. I also greatly appreciate financial assistance from the University of British Columbia through Awards and Financial Aid Office and International House, that enabled me complete my thesis. General Introduction 1 Chapter 1: GENERAL INTRODUCTION 1-1. BACKGROUND The western Fraser Valley (Fraser River delta) has one of the most productive agricultural lands in Canada. The region has the longest period of frost-free days in Canada (Luttmerding, 1981) and its temperate climate is generally characterized by warm rainy winters and relatively cool dry summers (Hare and Thomas, 1979). Long growing seasons, highly productive soils, and flat topography which allows for mechanization, installation of subsurface drainage and use of irrigation makes it ideally suitable for a wide range of agricultural practices such as arable crop production (beans, peas, potatoes, corn and small cereals), dairy, sheep, beef and fruit production (Luttmerding, 1981). The agricultural capability of western Fraser Valley ranks within the top 20% of agricultural land in British Columbia (Klohn Leonoff Ltd., 1992). Despite climatic conditions that favour agriculture, current farming in western Fraser Valley is below its crop production potential due to a number of political and soil factors (Klohn Leonoff Ltd., 1992). The majority of the land (about 65%) is not owned by farmers but leased from the government and private landowners on short-term tenures. Lack of direct or long-term tenure on the land has forced farmers to manage the land on an annual basis with minimum conservation practices. Soil degradation, especially with respect to organic matter and structural stability, is a consequence of the lack of incentive for the farmers to invest in long-term management inputs such as subsurface drainage. In the western Fraser Valley intensive crop production is practised and one of the major concerns associated with this type of farming is efficient utilization of nitrogen (N). Generally, General Introduction 2 fertilizer N use in British Columbia in intensively farmed areas like the Lower Fraser Valley exceeds maximum recommended rates (Kowalenko, 1987a). Large N fertilizer application rates are also a consequence of decline in soil organic matter levels and compensate for reduced N mineralization from organic matter. This results in high amounts of residual soil mineral N after harvest of main season crops, thus increasing the amount of N0 3" available for leaching. Annual rainfall distribution in the Lower Fraser Valley is biased towards winter season with over 70% occurring in the off-season1 (November to March). Annual maximum temperature and minimum rainfall are synchronized around July. Conversely, minimum temperature and maximum rainfall occur in January. Thus main growing seasons are relatively dry, minimizing the probability of N0 3" leaching (Kowalenko, 1987b). Lack of optimum soil moisture conditions during the main season limits crop growth and N uptake. This may lead not only to residual fertilizer N in soil but also accumulation of mineralized N during the season and in the period succeeding the main growing season. Loss of N from agricultural production systems has become a major concern since the cost of N fertilizers has gone up over the years. Loss of N can occur through NOV leaching, denitrification, volatilization and erosion. The chances of loss of N from Lower Fraser Valley soils through NOy leaching during winter are high (Kowalenko, 1987b and 1989). This is due to favourable climatic conditions and rapid rates of nitrification (Kowalenko, 1987b and 1989) in Lower Fraser Valley soils. The appearance of NOy in groundwater as a result of agricultural practices has become a major concern with respect to groundwater pollution. The loss of N03" through leaching beyond the effective root zone is also an economic loss to farmers and is not compatible with sustainable agriculture (Lai et al., 1991). General Introduction 3 Extensive research has been done on the efficiency of winter rye to accumulate residual fertilizer N in a corn (Zea mays L.) production system (Staver and Brinsfield, 1990; Shipley et al., 1992; Ditsch et al., 1993). However, wide differences exist in climate, soil types and management systems. Most of the evaluation of biomass production and N uptake by barley (Holderbaum et al., 1990) in Maryland, oat (Jones, 1942; Mitchell and Teel, 1977) in Alabama and Delaware and, wheat (Neely et al., 1987; Tyler et al., 1987) in Georgia and Tennessee, respectively, have involved spring sampling and in many cases, no information on the dynamics of accumulated N is available when these cover crops were killed during winter. Most research on graminaceous cover crops and their effectiveness in capturing autumn residual N has concentrated on winter rye (Mitchell and Teel, 1977; Muller et al., 1989; Shipley et al., 1992; Ditsch et al., 1993; Staver and Brinsfield, 1990) and annual ryegrass (Nielsen and Jensen, 1985; Shipley et al., 1992). In most studies graminaceous cover crops were included as controls to measure the N-supplying capability of legume cover crops (Mitchell and Teel, 1977; Ebelhar et al., 1984; Hargrove, 1986; McVay et al., 1989; Holderbaum et al., 1990). Under these circumstances, graminaceous cover crop effects were restricted by cover crop management practices that were best suited for the legumes. Legume cover crops have a critical soil mineral N requirement for nodulation and N 2 fixation. High soil mineral N content depresses nodulation and reduces N 2 fixation (Richards, 1987). Soil mineral N content, which in part is related to fertilizer practice, becomes a limiting factor for grass cover crops. Furthermore, information on soil mineral N content in these studies is lacking. More information is needed about the capacity of graminaceous cover crops to accumulate main season residual mineral N with respect to planting dates, residual soil mineral N levels in the autumn and a wide range of cover crop possibilities including autumn-planted spring species. Because there is wide variability in management ' Based on 1937-90 Normals at Vancouver International Airport. General Introduction 4 systems and weather patterns in winter in different regions, recommendations based on these findings will be mostly region specific. One of the goals of this study was to introduce spring species in winter cropping systems in western Lower Fraser Valley because of their rapid growth and N uptake in early autumn. However, due to their susceptibility to freezing damage in winter, it is important to know the composition of the various N fractions at winter-kill and their potential to retain accumulated N in residues following winter-kill. Several reports indicate that proteins, peptides and amino acids can be stabilized through interaction with secondary products, mainly tannins and oxidation products of phenolics (Swain, 1965; Loomis 1974; Lyttleton, 1973; Gegenheimer, 1990; Verma, 1975). When spring species are killed by freezing, their cells lose compartmentalization through membrane damage. This results in mixing of cytoplasmic proteins with the contents of the vacuole. The pH of the vacuole contents may be sufficiently low to allow protein precipitation, and/or vacuole contents may contain phenolics which form complexes with the proteins on mixing (Forsyth, 1964). This may restrict the amount of N leached out of autumn-planted spring species residues during rainfall events following winter-kill and therefore influence retention of the N accumulated prior to winter. Thus determination of the amounts of water-insoluble, protein, nonprotein and mineral N fractions as influenced by various management practices may provide fundamental knowledge about retention of absorbed N during winter. Most of the N in plants is in the form of proteins which can be classified into two components based on their physical properties. The two classes of plant proteins are (i) cold-water insoluble and (ii) cold-water soluble. The N associated with cold-water insoluble proteins can be retained in the residues of spring species following winter-kill. Soluble proteins, on the other hand, interact with plant secondary products (phenolics) following membrane damage General Introduction 5 (Swain, 1965; Lyttleton, 1973; Loomis, 1974; Stevenson, 1982) to form insoluble complexes. Some N associated with soluble (cytoplasmic) proteins can also be retained in the residues of autumn-planted spring species following winter-kill. Short daylengths and low light intensity conditions are prevalent in the autumn and both factors negatively influence the activity of nitrate reductase (Aslam and Huffaker, 1984; Heath et al., 1973; Pearson et al., 1981); the primary enzyme that is involved in the reduction of the NOy-N taken up by plants to N H 4 + during assimilation process. Quantification of NOy-N in autumn-planted spring species crops at winter-kill is important because NGy-N is a readily mobile ion and can easily leach from winter-killed residues and ultimately out of the root zone during winter rainfall events. It is therefore felt that the potential for autumn-planted spring species to retain accumulated N during winter may be explained by plant composition of the three N fractions (insoluble protein-N, soluble protein-N and NOy-N) at winter-kill. There is tremendous documentation on the role of winter species in soil mineral N conservation (Ditsch et al., 1993; Staver and Brinsfield, 1990; Nielsen and Jensen, 1985; Brandi-Dohrn et al.; 1997; Martinez and Guiraud, 1990; Shipley et al., 1992), but currently there is no definitive research on autumn-planted spring species use to conserve autumn mineral N (especially NGy-N). Until the establishment of the University of British Columbia Soil and Water Conservation Group and Delta Farmers' Soil Conservation Group in the spring of 1991 with joint funding from the governments of Canada and British Columbia (Temple, 1992), cover crop research in the western Lower Fraser Valley was lacking and most farms were left bare during the off-season. The goal of the program was to find feasible solutions to local soil degradation problems; especially soil compaction and declining soil organic matter levels (Bomke, 1996). One of the objectives was to develop suitable cover cropping techniques for maintaining soil organic matter General Introduction 6 contents, providing overwinter soil protection, improving soil physical properties, and conserving autumn soil N . This thesis investigated the influence of cover crops on autumn soil mineral N conservation. Apart from soil protection, improvement of soil physical properties (Hermawan, 1995) and reduction of soil erosion, when graminaceous cover crops are included in a crop rotation, management practices that will maximize the capture of main growing season residual mineral N and optimize N-use efficiency of succeeding main season crops should be considered. Identification of management strategies that influence the economic and environmental use of graminaceous cover crops on western Lower Fraser Valley soils during winter to conserve residual mineral N and protect NOV from leaching is necessary. 1-2. O B J E C T I V E S AND H Y P O T H E S E S 1-2.1. Statement of Objectives The overall goal of this research was to contribute to the development and evaluation of appropriate winter cover cropping systems to conserve residual mineral N during winter for the western Lower Fraser Valley using species from the Gramineae family. This thesis tested the effects of various management practices (planting dates, fertilizer practice and crop species) on autumn residual soil mineral N conservation. The general objective was to evaluate the ability of winter tolerant (winter rye and annual ryegrass) and sensitive (barley, wheat and oat traditionally planted in spring) species planted in autumn as cover crops to accumulate and protect residual mineral N (N03"-N in particular) against loss through leaching in silty clay loam soils of western Lower Fraser Valley. The specific objectives were: General Introduction 1 1. to determine the effect of autumn planting date, soil mineral N content and crop species on N uptake by a cover crop before and after winter leaching period. 2. to determine the effect of autumn planting date, soil mineral N content and crop species on residual soil mineral N content before and after winter leaching period. 3. to determine the effect of autumn planting date and soil mineral N content on the retention of the N in the biomass of winter-sensitive crop species following winter-kill. 4. to determine the effect of autumn planting date and soil mineral N content on the partitioning of N into various plant N fractions by winter-sensitive crop species at winter-kill. 5. to determine the effect of autumn planting date, soil mineral N content and crop species on C/N ratio before and after winter leaching. 1-2.2. Hypotheses 1-2.2.1. Comparison of spring species and winter species On the assumption that spring species (winter-sensitive species) can utilize early-autumn warm temperatures and long daylengths to grow rapidly and produce high biomass and that the vigorous growth is accompanied by larger N accumulation before winter leaching period compared with winter species (winter-tolerant species), I studied the effect of crop species on productivity and N uptake. The following specific hypotheses were tested: (i) Spring species produce larger quantities of biomass before winter leaching period than winter species. (ii) Spring species are more effective in capturing main-growing-season residual mineral N before winter leaching period than winter species. General Introduction 8 1-2.2.2. Retention of accumulated nitrogen in spring species residues during winter One of the major advantages of winter cover cropping in the Lower Fraser Valley is protection of N0 3" from leaching during winter. Cover crops are able to meet this goal through assimilation of N 0 3 ' into organic N compounds. The role of autumn-planted spring species in soil mineral N (N03" in particular) cycling during winter is therefore, in large part determined not only by the amount of N taken up before winterkill but by the quantities of various plant N fractions (mainly cold-water insoluble protein-N, soluble protein-N and N03"-N) at winter-kill and thus the amount retained in the cover crop residue after winter season. On the assumption that plant protein content (total amount per ha"') and biochemical composition (hemicellulose, cellulose, lignin and phenolics) increase with plant age and that these constituents (mainly lignin and phenolics) interact with protein to form insoluble complexes following winter-kill, I studied the effect of autumn-planting date on N retention by spring species during winter. The following specific hypotheses were tested: (i) Spring species cover crops planted in August are more effective in capturing main-growing-season residual mineral N than those planted a month later. (ii) August-planted spring species will retain both cold-water insoluble N (structural N) and water-soluble protein N (cytoplasmic protein N) but the cover crops planted one month later will retain only structural N. (iii) Spring species cover crops that are planted in August have greater potential to retain accumulated N following winter-kill in winter than those planted one month later. General Introduction 9 Various spring species N fractions were measured in order to determine the relationship between protein N (water-insoluble and soluble protein N) and N retention by these species during winter. Quantification of plant N0 3"-N was important because N0 3" can readily leach out of the crop residues following membrane damage through freezing and thus can directly influence N retention by spring species during winter. While cover crops can assimilate autumn mineral N and thus minimize N0 3" leaching during winter, it is important to know how readily the organic N can be mineralized to plant available forms ( N H 4 + and N03") in spring. An inverse relationship exists between plant residue decomposition rates and the C/N ratio. Thus, residues with a high C/N ratio are often associated with a slow rate of decomposition. In contrast, residues having a low C/N ratio usually decompose at a more rapid rate. The C/N ratios of cover crops were studied because they are sometimes a useful parameter for predicting mineralization of residue N. However, despite the fact that C/N ratio can be useful in predicting decomposition rates, they should be interpreted cautiously since the C/N ratio indicates nothing about the availability of the C and N to microorganisms. This study will broaden our understanding of the role of various cover crops of the Gramineae family in cycling of main-season residual mineral N so that management strategies can be developed to maintain or improve soil N availability while minimizing input costs and chances of polluting the environment. Literature Review 10 Chapter 2 : LITERATURE REVIEW Cover crop systems can be based upon either legume or nonlegume species. Legumes contribute symbiotically fixed N 2 to soil in the form of N H 4 + and N0 3" through mineralization of plant residue N and the recycling of manure from animals to which legumes are fed (Peterson and Russelle, 1991). Nonlegume cover crops utilize soil N from the mineral N (NH 4 + and NOV) pool. The two cover cropping systems are similar in that they both eventually release N to succeeding summer crops through mineralization. In this thesis, the literature review will cover nonleguminous (Gramineae family) cover crops and their role in soil N conservation during winter. 2-1. Factors affecting mineral nitrogen accumulation in soil When land is left fallow during summer, there is no crop uptake of N and mineral N (NH 4 + + NOV) accumulates in the soil. This mainly occurs as long as: there is decomposable organic matter in soil; the field is weed-free; there is not too much rain to either leach N0 3" out of the root zone or cause loss to the atmosphere through denitrification; and the soil must be moist or subject to alternate wetting and drying. Soil mineral N can also accumulate when crops are growing (Staver and Brinsfield, 1990). Contents of soil mineral N decline in early stages of crop growth due to vigorous uptake and increase as the crop matures especially after flowering when uptake decreases. Transport of N to the root surface is mainly by convective mass flow (for N0 3" only) which is driven by transpiration. Thus, low amounts of rainfall in summer may limit crop yields and Literature Review 11 reduce utilization of N H 4 + and N03". Limited precipitation combined with evapotranspiration during the growing season also minimizes the probability of N03" leaching (Kowalenko, 1987b and 1989). In field experiments with l 5 N , Bartholomew (1971) found that recovery of the labelled fertilizer N by maize was closely related to the total amount of rainfall during the experimental period. Lower recoveries were observed under conditions where water stress was high. This finding is consistent with observations that in dry periods NOV accumulates in the upper soil layers (Page and Talibudeen, 1977) and N03" availability is reduced (Mengel and Casper, 1980). Cropping sequence influences the amount of mineral N in the soil after the growing season. For example, Meek et al. (1994) showed that silage corn followed by winter wheat left less soil mineral N in the surface 45 cm in the autumn than bean following bean. Growing legumes during summer may increase the amount of residual mineral in autumn because legumes will utilize soil mineral N and will fix N2 to an extent necessary to satisfy their requirement. Since summer temperatures favour mineralization of soil organic N, net accumulation of large amounts of mineral N at the end of the growing season is most likely under legume crops. Vaughan and Evanylo (1994), in their study using rye, hairy vetch and rye-hairy vetch in Virginia, to determine the combined effects of cover crops and fertilizer N on residual soil mineral N accumulation after corn, have indicated that combinations of legume cover crop and supplemental fertilizer N cause large amounts of mineral N to accumulate in the soil. Similar results were observed in experiments by Hargrove et al. (1984) who reported greater N0 3" leaching potential when a legume cover was grown in a sorghum production system since a higher mineral N concentration was maintained in soil over a 2-year period compared to fertilizer-N-based systems with either rye or no cover crop. Nitrate contents at the end of the summer growing season in a sorghum production system were greater with legume cover crops compared to a nonlegume cover crop or L iterature Review 12 no cover crop (Hargrove, 1986). In both studies, N03" leaching was not actually monitored therefore autumn soil NO3" is assumed to be the potential for leaching. These studies indicate that for the most part, soil N mineralization increases the amounts of post-harvest mineral N in soil which can be attributed to lack of synchronization of N uptake with conversion of organic N (unavailable to plants) to mineral N (available to plants). Mineralization of N from legume residues occurs late in the season after peak demand for N by summer crops. 2-2. Factors affecting nitrate leaching Among the various combined forms of N only N03~ is leached out of soil in appreciable amounts by percolating water. Ammonium is held by cation exchange on clay and humus and can only be displaced by excess water and/or application of a salt solution. In contrast, NOV is the most mobile ion and remains in solution because of its high solubility, since it is not retained by the negatively charged soil colloidal system (Legg and Meisinger, 1982; Russelle and Hargrove, 1989). The solution is displaced downwards by rainfall or irrigation water. Thus, if sufficient water is added to the soil, dissolved NOV will be leached below the root zone. However, acid soils can have a local positive charge (pH-dependent) on the surface of particles and thus have an anion exchange capacity. In this case, N0 3" adsorption by these soils can limit leaching losses. The preconditions for major losses through N03" leaching is therefore largely determined by soil N03" content, pH and the amount of water passing through the soil. Loss of N0 3" from the rooting zone is also determined by the soil type, since soil texture and structure affect the amount of water retained at field capacity (Wild, 1988). Proportions of sand, silt, clay and organic matter influence water and nutrient-holding capacity. Thus, the rate of N03" leaching from a sandy soil is much greater than from fine textured soils (Widdowson et al., 1987) or soils with high organic matter content. Literature Review 13 Rates of N leaching have been positively correlated with rates of N fertilization in various agricultural systems (Schuman et al., 1975; MacLean, 1977; Baker and Johnson, 1981; Barraclough et al., 1983). The potential for NOY leaching also exists where excessive quantities of N-rich livestock or poultry wastes are applied in crop production systems (Ritter et al., 1986). Nitrate displacement in soil is complicated. Many soils in the field have cracks and biopores through which rain or irrigation water flows quickly, carrying N03" and other ions with it (Bouma and Anderson, 1977; Omoti and Wild, 1979; Smetten et al., 1983). Tillage breaks the continuity of these channels, resulting in slower infiltration rate and less downward movement of N0 3" compared to untilled situations (McMahon and Thomas, 1976; Goss et al., 1978). For the same reason, intense rain in the autumn leaches some N03" rapidly from the soil but bypasses the N0 3" held in the soil matrix (Garwood and Tyson, 1977). The timing and intensity of rainfall events after fertilizer application influence N03" leaching. Light rainfall following fertilizer application promotes movement of N03" into soil micropores; macropores facilitate bypass of water-filled N03"-rich micropores, thus reducing N03" leaching when subsequent rainfall events occur (Kanwar et al., 1985). On the other hand, abundant rainfall following fertilizer application may not allow equilibration of fertilizer N with resident soil solution in micropores and hence promote leaching through macropores. Thus improvement of soil structure may reduce or promote N0 3" leaching after fertilizer application depending on timing and intensity of rainfall events. 2-3. Factors affecting denitrification loss Denitrification is defined as the microbial reduction of N03" or N02" to gaseous nitrogen, either as N 2 or N 2 0 (Soil Sci. Soc. Am., 1979). Occurrence of denitrification requires presence of (1) denitrifying bacteria (2) N03" [electron acceptor] (3) readily decomposable organic Literature Review 14 compounds [energy source] and (4) anaerobic conditions [e.g saturated soils]. Losses of NGy through denitrification during the growing season have been estimated to be small, based on measurements of 1 5 N loss in a field microplot experiment on a medium-textured Lower Fraser Valley soil (Kowalenko, 1989). The amount of mineral N in this study were monitored over the entire year in fallow plots starting in the spring of 1982. Myrold (1988) reported a similar observation during winter in western Oregon in a denitrification study under ryegrass (Lolium multiflorum Lam.) and winter wheat {Triticum aestivwn L.). Denitrification losses are greater in soils fertilized with manure than in non-manured soils (Burford et al., 1976; Rolston et al., 1984; Christensen, 1985; Paul and Zebarth, 1993) during the growing season due to the presence of readily decomposable organic carbon like volatile fatty acids (Paul and Beauchamp, 1989). The effect of temperature on denitrification is great and N03" loss can double with a temperature increase of 10 °C in the range of 10 to 35 °C. In the range of 0 to 5 °C, denitrification rates are small but still measurable (Harris, 1988). In the Fraser Valley (at Agassiz and Sumas), Paul and Zebarth (1993) estimated denitrification loss in the fall averaging 13 kgN ha"1 on sandy loam and silty loam soils. In Oregon, Myrold (1988) measured denitrification losses of about 1.7 and 0.7 kg N ha"1 yr 1 in winter wheat and annual ryegrass systems, respectively, on medium to fine-textured soils. 2-4. Effect of grasses on nitrate leaching Grass cover crops affect N03" leaching through their influence on downward flow of soil water. They reduce water runoff and increase the amount of water infiltration compared to a bare Literature Review 15 soil (Langdale et al., 1979; McVay et al., 1989; Hermawan, 1995). This increases the amount of water moving through the soil profde and thereby increases the potential for NOV leaching. Cover crops also influence downward movement of soil water when they are killed and left on the surface as a mulch. This results in increased water infiltration rate and decreased evaporation from the soil surface (Jones et al., 1969; Blevins et al., 1971; Phillips, 1984; Hargrove, 1985; Radcliffe et al., 1988). Conversely, cover crops transpire soil water and hence dry the soil, reducing the amount of water percolating through the soil profile and decreasing the potential for significant NOV leaching (Wagger and Mengel, 1988). Increased infiltration is important during the winter when cover crop water use is small, but water use by cover crops is a major factor during spring when they are actively growing. Cover crops influence N0 3 ' leaching potential by utilizing residual mineral N in soils in the post-harvest period before the onset of winter. This reduces the amount of N03" available for leaching before winter. Winter rye has been shown to reduce accumulation of residual soil mineral N (NH 4 + + N03") in monoculture crop production systems and to minimize the risk of N0 3" leaching in the Ridge and Valley region of Virginia (Ditsch et al., 1993). Various grass cover crops have been demonstrated to effectively reduce NCY leaching. Morgan et al. (1942), working with tobacco as a summer crop and winter rye (Secale cereale L.), oat (Avena sativa L.) and timothy (Phleum pratense L.) as winter cover crops, showed in a 10-year lysimeter study in Connecticut that the grass cover crops effectively reduced N0 3"-N leaching and soil N03"-N content. In their study rye was the most effective, reducing the amount of N03"-N leached by 66%. The oat crop was less effective than rye because it winter-killed. However, they did not take into account N contribution from the dead mulch of oat that may have mineralized and leached. Similar observations have been reported with winter rye (Karracker et Literature Review 16 al., 1950; Staver and Brinsfield, 1990; Meisinger et al., 1990), annual ryegrass (Nielsen and Jensen, 1985) and perennial ryegrass (Martinez and Guiraud, 1990). The net effect of cover crops on NOV leaching will depend on prevailing weather conditions. In the Fraser Valley the most important cover crop effect on N0 3" leaching is utilization of residual soil mineral N, since winter precipitation is extremely abundant and it is unlikely that there can be significant reduction of water percolating through the soil. 2-5. Grass cover crop nitrogen accumulation One of the major criteria in selection of cover crops to reduce autumn soil NFL + and N0 3" is their effectiveness in taking up the mineral N. Cover crops minimize the problem of N03~ leaching by assimilation of mineral N (NFL+ + N03") to relatively unleachable organic forms. This protects N0 3" from leaching during winter and hold it in the soil until the next spring to become available to a summer crop or stabilized in soil organic matter for future release. This is a simplified version of the concept underlying the role of cover crops in autumn residual-mineral-N cycling. This way, planting of winter cover crops can decrease N03" leaching by reducing the amount available for leaching during the wet winter. A cover crop with slow autumn but rapid spring growth will most likely not be effective in reducing N03"-N leaching from Lower Fraser Valley soils because in south coastal B.C., where winter precipitation is abundant and spread over several months, N03" leaching can be a problem. Patterns of N assimilation during plant growth vary among and within species. Although most rapid N accumulation generally occurs during rapid vegetative growth, some genotypes begin accumulating N earlier or continue to assimilate N later than others (Kurtz, 1982). Literature Review 17 Most research on cover crops and their effectiveness to accumulate autumn residual N has concentrated on winter rye (Mitchell and Teel, 1977; Muller et al., 1989; Shipley et al., 1992; Ditsch et al., 1993; Staver and Brinsfield, 1990) and annual ryegrass (Nielsen and Jensen, 1985; Shipley et al., 1992). Extensive research on winter rye has shown its great potential to utilize residual mineral N present after corn harvest (Ditsch and Alley, 1991; Staver and Brinsfield, 1990; Wagger and Mengel, 1988). Some of the major characteristics of cover crops that would achieve that goal are rapid establishment and high biomass production; deep and high-density root system and ability of species to accumulate N in early stages of vegetative growth. 2-6. Effect of planting date on soil nitrogen Early planting, as compared to late planting, of winter crops results in greater accumulation of N mineralized from soil organic matter during autumn. In Great Britain, Widdowson et al. (1987) compared amounts of N taken up over winter by several winter wheat crops planted early (September) or late (October) in autumn with N03"-N remaining in the soil. They reported greater N uptake and soil N03"-N content reduction (0-90 cm layer) when winter wheat was planted early compared to late. They also found an inverse relationship between N uptake in aboveground portions of wheat and residual N03"-N in soil for early-planted winter wheat but no consistent relationship for the late-planted crop on fine-textured soils. In fact, residual N0 3"-N in the 60-90 cm layer in late-planted plots increased due to a combination of limited uptake and leaching during winter. In Denmark, Sorensen (1992) studied the effects of annual ryegrass planted in mid-July, beginning of August and mid-August on soil mineral N content before and after winter leaching for 4 years. He reported a significant decline in biomass production and N uptake with delayed planting. Soil mineral N content in the 0-100 cm layer before winter leaching (late November) Literature Review 18 was significantly reduced when the cover crop was planted early as compared to late. Early planting exposes cover crops to long daylengths and high temperatures in late summer or early autumn, thus allowing rapid and deep root development, which can exploit a large volume of soil in the root zone. 2-7. Research on nitrogen in the Lower Fraser Valley The climate of the Lower Fraser Valley is characterized by mild winter temperatures and high levels of precipitation. Under these conditions, NOV is subject to leaching. Two studies by Kowalenko (1987b and 1989) in which soil mineral N was monitored on fallow plots over the entire year demonstrated that the (1) potential for NOV leaching during the growing season is low (2) potential for NOV leaching over winter is high (3) nitrification rate is rapid during the main season and (4) soil N mineralization over the summer is significant but denitrification is negligible. These studies clearly indicate that N03" can accumulate in Lower Fraser Valley soils and unless proper management practices for summer crops are used, large losses will occur during winter. 2-8. Factors affecting nitrate assimilation in plants Nitrate and NFL + are the major sources of inorganic N taken up by roots of higher plants. Most of the N H 4 + has to be incorporated into organic compounds in the roots, whereas NOV is readily mobile in the xylem and can be stored in the vacuoles of roots, shoots and storage organs. In arable soils, N0 3" is often the major source of available N to plants. In order to be incorporated into organic structures and to accomplish its essential functions as a plant nutrient, N0 3" has to be reduced to N H 3 . The reduction of NOV to N H 3 is accomplished by two enzymes; nitrate reductase Literature Review 19 which reduces N03~ to N0 2" and nitrite reductase which reduces NOV to N H 3 (Hewitt, 1975; Beevers, 1976). Therefore, factors which influence the activity of nitrate reductase affect incorporation of N0 3" into organic N compounds and consequently cause accumulation of N0 3" in plant tissues. Nitrate assimilation by plants is influenced by environmental factors such as light intensity, (Aslam and Huffaker, 1984), water availability, rate of fertilizer N (Wright and Davison, 1964; Murphy and Smith, 1967) and type of N fertilizer. Nitrate accumulation in plants is greater from N0 3" fertilizers than from (NH) 2S0 4 or urea (Holmes, 1968). Low light intensity and drought conditions increase the concentration of N03" in the plant (Heath et al., 1973). In green plants a correlation exists between light intensity and N03" reduction. For example, a distinct diurnal pattern of reduction has been observed in shoots but not in roots (Pearson et al., 1981). The daytime proportion of nitrate reduction in shoots and roots differs from the proportion at night (Aslam and Huffaker, 1982). Aslam and Huffaker (1984) exposed excised barley leaves to a source of permanent light and found that nearly all of the NOV absorbed was reduced but at low light intensity, only 25% of the NOY absorbed was reduced. This indicates that N0 3" reduction is much more sensitive to low light intensity than is NOY uptake. The location of N03" reduction in plants can have an impact on how plants respond to light in terms of N0 3" assimilation and net accumulation. The proportion of N0 3" reduction carried out in roots and shoots depends on a number of factors, including the rate of N03" supply, plant species and plant age. Generally, when soil solution N0 3 ' concentration is low, a greater proportion of the N03" is reduced in the roots. As soil solution N0 3 ' concentration increases, the capacity of N0 3" reduction in the roots becomes a limiting factor and an increasing proportion of Literature Review 20 the total N is translocated to the shoots in the form of NOV (Wallace and Pate, 1965). Although a high proportion of N03~ reduction in barley occurs in the roots (Bloom et al. 1992), NOY reduction in the roots of oat is greater than that of barley (Pate, 1971). There is a relationship between plant age and nitrate reductase activity. Nitrate reductase activity is generally lower in older plant leaves and this likely increases accumulation of N0 3 ' in more mature plants (Van Egmond and Breteler, 1972; Santoro and Magalhaes, 1983; Kenis et al., 1992). 2-9. Nitrogen fractions and chemical composition of plants Most of the N in plants is in organic forms. The N is incoporated into carbon-containing compounds which include nucleic acids, some vitamins, hormones, membrane components, coenzymes and pigments (chlorophyll). The largest proportion (about 90%) of the N in plants is in the form of proteins (Streeter and Barta, 1984). Proteins, organic nonprotein N compounds, N H 4 + and N0 3" constitute all N fractions in plants. Plant protein N is composed of cold-water insoluble and soluble fractions. Cold-water insoluble N is mainly structural N which comprises of a small fraction of protein N associated with the cellulose of the cell wall and a large amount of insoluble protein N of the chloroplasts in which the protein is associated with lipid material of the membranes and with pigments such as chlorophyll. Plant tissues have phenolic compounds, which constitute by far the largest and most widespread group of secondary plant products. Apart from genetic factors, light, mineral nutrition and other stress conditions (temperature and drought), the content of phenolic compounds in plants is dependent on age and growth stage (Wong, 1973). Lignin, cellulose and Literature Review 21 hemicellulose increase as plants age (Albersheim, 1965; Parr and Papendick, 1978; Mengel and Kirkby, 1987; Walton, 1983). Lignin is a high-molecular-weight aromatic complex which is deposited on cellulose structures in the secondary cell walls of plants. Lignin varies from 2% of dry matter in young plants to 17% or more in fully mature plants (Walton, 1983). Lignin content in grasses may increase rather than decrease with high N supply (Kaltofen, 1988) as amino acids phenylalanine and tyrosine are precursors of lignin synthesis. Similarly, plant polyphenol (tannin) content increases with plant age (Swain, 1965). After homogenization or winter-kill, plant N fractions especially those associated with proteins, can undergo two major transformations. Firstly, proteins can undergo enzymatic degradation to form amino acids. Secondly, proteins can interact with plant secondary products (phenolic compounds) to form insoluble complexes. Two major mechanisms are involved in the reaction of these phenolic compounds with proteins, forming insoluble complexes. Proteins combine with phenolics reversibly by hydrogen bonding and irreversibly following oxidation of the phenols by polyphenol oxidases to quinones (Lyttleton, 1973; Loomis, 1974; Stevenson, 1982). This kind of reactions can occur when plant cells lose their compartmentalization through membrane damage caused by freezing or homogenization, when cytoplasmic proteins mix with phenolic compounds in the vacuole (Forsyth, 1964). Thus the amount of N retained by autumn-planted spring species following winter-kill may be influenced not only by residue N (water-insoluble protein fraction), but also by the amount of N associated with soluble proteins. Chapter 3. Materials and Methods 22 Chapter 3: MATERIALS AND METHODS 3-1. Experimental layout and soil type description Three experiments were conducted in the western Lower Fraser Valley of British Columbia starting from August and continuing to April of the next year in 1991-92, 1992-93 and 1993-94. The studies were conducted at different locations (L91, L92 and L93) each season (Figure 3-1). The soil at all three locations was Westham series. The landscape at the locations was nearly level (1% slope) and the soil was formed from fine-textured Fraser River floodplain deposits overlying coarse-textured deposits and, is generally classified as a Rego Humic Gleysol (Luttmerding, 1981). The L91 and L92 locations were poorly drained and previous summer crops were field peas (Pisum sativa L.) for L91 and beans (Phaseolus vulgaris L.) for L92. The L93 location was well drained due to installation of a subsurface drainage system and the preceding summer crop was potato (Solanum tuberosum L). In the 1991-92 and 1992-93 winter cropping seasons, the experiments (I and II) had similar treatments, randomization and design. The factors tested were planting dates (August, September), with and without autumn-applied fertilizer N (0 and 100 kg N ha'1) and crop species (fallow, spring barley and winter rye). The two planting dates, two simulated mineral N levels and three cover crop treatments were combined into twelve treatments in a factorial experiment that was arranged in a randomized complete block split split plot design with four blocks (Figure 3-2). Planting dates were the main plots (12 m x 36 m), soil mineral N treatments were subplots (12 m x 18 m) and cover crops were sub-subplots (12 m x 6 m). Chapter 3. Materials and Methods 24 September Planting August Planting N-100 N-0 N-0 N-100 SB F W R WR SB F SB WR F W R F SB August Planting September Planting N-0 N-100 N-100 N-0 SB WR F WR F SB F SB WR F WR SB September Planting August Planting N-0 N-100 N-0 N-100 F SB W R F WR SB SB F WR WR SB F August Planting September Planting N-100 N-0 N-100 N-0 SB F WR WR SB F SB W R F W R F SB B l B2 B3 B4 Figure 3-2. Layout of experiments established in August 1991 and August 1992 at Nottingham (L91) and Kamlah (L92) locations. (N-0, no N applied; N-100, 100 kgNha'1 applied as NOi; F =fallow; SB = spring barley; WR = winter rye;) Chapter 3. Materials and Methods 25 In 1993-94 winter cropping season (Expt III), planting date treatment was not tested as a factor because the data from experiments I and II had indicated only a minor effect of late-planting on main growing season residual mineral N. The same N treatment (0 and 100 kg N ha"1) as in the previous two seasons was used and the cover crop treatments tested were increased to five, plus a winter fallow treatment. In addition to winter rye (Secale cereale L. cv. Danko) and spring barley (Hordeum vulgare L. cv. Virden), spring wheat (Triticum aestivum L. cv. Max), spring oat (Avena sativa L. cv. Jasper) and annual ryegrass (Lolium multiflorum Lam. cv. Westerwolds) were included. Spring wheat and spring oat were included because preliminary results from Expts I and II had indicated that spring barley that was planted in August and winter-killed in late November, increased soil mineral N (0-60 cm layer) in the spring. The two mineral N levels and six cover crop treatments were factorially combined into twelve treatments and arranged in a randomized complete block split plot design with four blocks (Figure 3-3). Mineral N levels were main plots (12 m x 36 m) and cover crop treatments were subplots (12 m x 6 m). In all the three experiments, the buffer strip between blocks was 9 m. Cover crops were planted on August 19 and September 16, 1991, August 24 and September 22, 1992 and August 24, 1993, in 10-cm row spacings using a 3-m wide Vicon air seeder (LZ 301) mounted on a tractor. In 1991 and 1992 spring barley and winter rye were seeded at the rate of 100 kg ha"1. The same seeding rate was used in the third year for spring barley, winter rye, spring wheat and spring oat. Annual ryegrass was seeded at 25 kg ha"1. Nitrogen was surface broadcast immediately after planting as pelleted Ca(N0 3) 2 (15.5-0-0 farm grade). A summary of field operations during the three cover cropping seasons is presented in Table 3-1. Chapter 3. Materials and Methods 26 August Planting N-0 N-100 S W SB A R G WR F SO WR SW F SB A R G SO August Planting N-100 N-0 S W SB so A R G WR F W R SW so F A R G SB B2 August Planting N-0 N-100 A R G SW SB F so WR A R G F WR SO SW SB August Planting N-100 N-0 so S W W R F SB A R G SW SB A R G SO F WR B3 B4 Figure 3-3. Layout of the experiment established in August 1993 at Swenson (L93) location (N-0, no N applied; N-100, 100 kg N ha'1 applied as NOi; F = fallow; SB = spring barley; SW = spring wheat; SO = spring oat; WR = winter rye; ARG = annual ryegrass). Chapter 3. Materials and Methods 27 Table 3-1. Summary offield operations and precipitation measurements for 1991-92, 1992-93 and 1993-94 winter cropping seasons Field Operation Date Days after Cumulative planting precipitation (mm)^ l Soil sampling, First planting and N application Soil sampling, Second planting Soil and plant sampling Soil and plant sampling Aug/19/1991 Sep/16/1991 Nov/21/1991 Apr/08/1992 0 27 94 232 0 142 334(398) 862 Soil sampling, First planting and N application Soil sampling, Second planting Soil and plant sampling Soil and plant sampling Soil sampling, Planting and N application Soil and plant sampling Soil and plant sampling Aug/24/1992 Sep/22/1992 Nov/24/1992 Apr/30/1993 Aug/24/1993 Nov/23/1993 Apr/14/1994 0 28 91 248 0 90 232 0 30 290(349) 803 0 106(157) 670 1^ Long-term (1937-90) cumulative precipitation between August and November is 388 mm; Values in brackets include total precipitation for the entire months of August and November; Cumulative precipitation was obtained by summation of daily measurements (Source: Vancouver International Airport); Chapter 3. Materials and Methods 28 3-2. The rationale for treatment choices The study was set up to investigate the role of cover crops in autumn soil mineral N cycling under western Fraser Valley conditions. Winter-sensitive species (autumn-planted spring species) were included because of their rapid growth and high biomass production in early autumn. Winter-tolerant species were chosen because of their overwintering characteristic, to compare with winter-sensitive species which usually winter-kill at the start of winter (late November). Nitrogen was applied in the form of Ca(N0 3) 2 to simulate the larger quantity of residual NCV-N in the soil that can result from fertilizer applications on a main season crop. Thus, the N treatment represented low (N-0) and high (N-100) amount of residual mineral N in the 0-60 cm soil layer, where no N and 100 kg N ha"1 was applied, respectively. This was done to enable me to test the capacity and efficiency of the selected cover crops in capturing autumn residual mineral N and to evaluate the dynamics of fertilizer N under cover cropping during winter on silty clay loam soils of the western Fraser Valley. Therefore, N was applied to test the potential of selected cover crops to capture mineral N remaining after summer crop harvest and thus protecting N0 3" from leaching out of the root zone. The timing of cover crop planting is largely determined by the time of summer crop maturity and harvest. Time of summer crop maturity and harvest is in most part dependent on crop species and environmental factors. The scope of the experiment was further increased by including two planting dates (August and September) to represent early- and late-harvested summer crops. Chapter 3. Materials and Methods 29 3 - 3 . AUTUMN SOIL MINERAL NITROGEN DYNAMICS 3-3.1. Soil sampling and initial conditions Soil sampling was done immediately before first and second plantings to determine initial amounts of main-growing-season residual mineral N (NH„+ + NOV) in the 0-60 cm soil layer after summer crop harvest in 1991 and 1992 (August 19, 1991; September 16, 1991; August 24, 1992; September 22, 1992). Since there was only one planting date in 1993, soil was sampled once on August 24, 1993. Ten soil samples were taken randomly from each of the four blocks at 0-20, 20-40 and 40-60 cm depth intervals and composite samples of corresponding depths for each block were analyzed for various chemical and physical properties (Tables 3-2, 3-3 and 3-3). Soil sampling was also done in late November and April of each winter cropping season to determine the fate of N in the soil over time. Late November sampling was assumed to represent the beginning of winter in the Lower Fraser Valley when spring species would usually winter-kill while April was selected to represent the time of land preparation for summer crops. In all cases, soil was sampled with a 2.5-cm diameter Oakfield probe. In November and April, six cores were taken from each plot and samples from corresponding depth intervals were bulked in plastic bags and immediately placed in ice coolers. Soil samples were stored in a refrigerator at 1 °C overnight before they were thoroughly mixed and subsamples weighed for NOV and exchangeable NFL + extraction. Analysis of soil samples was done within 5 days after sampling in all cases. Chapter 3. Materials and Methods 30 Table 3-2. Some soil chemical properties at first planting for 1991-92 experimental location. Soil sampling depth intervals (cm)f Parameter measured 0-20 20-40 40-60 pH 4~9 4~6 4/7 EC(dsm"') 0.2 0.1 0.1 C E C (Cmol c kg"1) 17.0 15.7 21.6 Total C (g kg"1) 26.4 14.0 7.3 Total N ( g kg"1) 1.6 1.2 0.6 Available elements (mg kg"1) P 120 30 10 K 370 190 80 Ca 1950 1580 1320 Mg 120 170 290 Na 10 10 20 S0 4-S 10 10 10 % Values are means of four blocks; pH, determined on 1:1 soil to water samples; EC, electrical conductivity determined on 1:1 soil to water filtrate x 2; CEC, total cation exchange capacity determined using 1M NH 4Ac and 1M K G ; C, LECO analyzer carbon; P, phosphorus (Bray 1); K, Ca, Mg and Na, base cations determined on NH 4Ac extracts; S04-S, 0.01M C a G 2 extracts determined colorimetrically; Chapter 3. Materials and Methods 31 Table 3-3. Some soil physical and chemical properties at first planting for 1992-93 experimental location Soil sampling depth intervals (cm)^ ] Parameter measured 0-20 20-40 40-60 Sand (%) L7 L7 3~3 Silt(%) 58.7 57.5 64.2 Clay(%) 39.6 40.9 32.6 Textural class SiCL SiC SiCL pH 5.4 5.2 4.5 EC(dsm"') 0.6 0.5 0.5 C E C (Cmol c kg"1) 19.6 19.8 18.9 Total C ( g kg"1) 17.1 15.1 12.8 Total N (g kg"1) 1.7 1.5 1.1 Available elements (mg kg"') P 190 100 30 K 290 180 110 Ca 1530 1230 780 Mg 250 240 210 Na 260 260 280 S0 4-S 20 20 30 % Values are means of four blocks; Particle size determined by Hydrometer method; pH, determined on 1:1 soil to water samples; EC, electrical conductivity determined on 1:1 soil to water filtrate x 2; C E C , total cation exchange capacity determined using 1M N H 4 A C and 1M K G ; C, L E C O analyzer carbon; P, phosphorus (Bray 1); K , Ca, M g and Na, base cations determined on N H 4 A C extracts; SO4-S, 0.01M CaCb extracts determined colorimetrically; Chapter 3. Materials and Methods 32 Table 3-4. Some soilphysical and chemical properties at planting for 1993-94 experimental location Soil sampling depth intervals (cm)TJ Parameter measured 0-20 20-40 40-60 Sand (%) 3^ 6 IJ 3^ 8 Silt(%) 66.5 71.4 71.5 Clay(%) 29.6 26.9 24.7 Textural class SiCL SiL SiL pH 5.1 5.3 4.7 EC(dsm"') 1.3 0.7 0.6 C E C (Cmol c kg"') 18.8 18.4 18.6 Total C (gkg') 16.7 17.0 12.9 Total N ( g kg"') 1.5 1.4 1.0 Available elements (mg kg"') P 130 120 30 K 290 240 120 Ca 1590 1600 860 Mg 230 220 160 Na 30 20 30 S C v S 60 20 30 % Values are means of four blocks; Particle size determined by Hydrometer method; pH, determined on 1:1 soil to water samples; EC, electrical conductivity determined on 1:1 soil to water filtrate x 2; C E C , total cation exchange capacity determined using 1M N H 4 A c and 1M K G ; C, L E C O analyzer carbon; P, phosphorus (Bray 1); K, Ca, M g and Na, base cations determined on N H 4 A c extracts; S04-S, 0.01M C a C h extracts determined colorimetrically; Chapter 3. Materials and Methods 3-3.2. Soil mineral nitrogen extraction and analysis 33 Ten-gram wet samples were weighed into 125-mL Nalgene plastic bottles and shaken with 100 mL of 2M KC1 solution (Keeney and Nelson, 1982) on a reciprocating shaker for 1 h. Samples (including blanks) were then filtered through Whatman No. 42 filter paper into 60-mL Nalgene bottles and stored in a refrigerator at 1 °C. Samples were analyzed on automated flow injection ion analyzer (QuickChem AE) within 24 h after extraction. Nitrate was determined colorimetrically as NOV by modified Griess-Ilosvay Cd reduction procedure by passage of the samples through a copperized cadmium column. The N0 2" concentration was determined by diazotizing with sulfanilamide followed by coupling with N-l-naphthyl-ethylenediamine dihydrochloride to form a water soluble reddish purple azo dye which was read at 520 nm. Sample readings were calibrated against standard solutions of K N 0 3 within appropriate concentration ranges. Extractable N H 4 + was also analyzed on the automated flow injection ion analyzer (QuickChem AE). Ammonium cation was converted to N H 3 by raising the pH to 13.5 with a concentrated buffer. Ammonia produced was heated with salicylate and hypochlorite to produce blue colour which was intensified by sodium nitroprusside. The blue colour is proportional to N H 3 concentration and was read at 660 nm. Sample readings were calibrated against standard solutions of NFLCl within appropriate concentration ranges. Subsamples of the soil of about 20 g were used to determine the water content by oven-drying at 105 °C for 48 h. This water content was used to correct soil NH 4 + -N and N0 3"-N concentration measurements to an oven-dry weight basis. Core samples (10 cm diam.) were taken from 0-20, 20-40 and 40-60 cm soil layers at each planting in November and April for bulk density determination. Bulk density values were used to Chapter 3. Materials and Methods 34 convert N H 4 + - N and N03"-N concentrations (mg kg"1) to kg ha"' values. Apparent fertilizer N recovery in the soil (ANR S O IL) was calculated according to equaton 3-1: [(Mineral N)N.ioo] - [(Mineral N)N.o] ANRson.(%) = x 100 (3-1) applied N rate where, N-0 = no N applied and N-100 = 100 kgN ha"' applied as NOV. Residual N0 3" + N H 4 + or N 0 3 ~ N under fallow was compared with that under cover crop by calculating percent reduction in soil N0 3" + N H 4 + or NOY-N that was attributed to the cover crop. A similar procedure was used by Meisinger et al. (1991). The impact of cover crops on soil N0 3" + N H 4 + or N0 3"-N was calculated as percent reduction in N (PR) according to equation 3-2: [(Soil N), IFALLOW. ,] - [(Soil N)CROP] PR(%) = x 100 (3-2) (Soil N), FALLOW where, soil N (N03" + N H 4 + or N03"-N) was estimated from the 0-60 cm soil layer. 3-3.3. Plant sampling and biomass measurement Estimates of biomass production of cover crops and N uptake were based on aboveground plant material. In November and April of each season, plant samples of living cover crops Chapter 3. Materials and Methods 35 (winter rye and annual ryegrass) were harvested at ground level from centre rows using clippers from a 0.5 m x 0.5 m quadrate. In November of each season, two sets of winter-sensitive species (spring barley, spring wheat or spring oat) were harvested from the 0.25 m 2 area. One set of samples was weighed fresh, placed in meshbags and left in the field until spring. The other set was used for moisture, biomass and total N determinations. Moisture content was used to calculate the dry-weight of meshbag samples left in the field. In the spring of 1992, biomass and residue N for spring barley were measured on meshbag samples that were anchored on the soil surface. In the spring of 1993, measurements of biomass and residue N for spring barley were obtained from meshbag samples placed on wire mesh tables. Measurements of biomass and residue N for winter-sensitive species (spring barley, spring wheat and spring oat) were obtained from samples collected from a 0.5 m x 0.5 m quadrat in the spring of 1994. A summary of sampling dates and growth stages, according to Zadoks decimal code (Zadoks et al., 1974), for cover crops during the three study seasons is presented in Table 3-5. Plant samples were oven-dried at 70 °C, weighed and ground in a Wiley mill (1.0-mm sieve) in preparation for dry weight biomass measurement, and total N and carbon (C) analysis. Chapter 3. Materials and Methods 36 Table 3-5. Summary of sampling dates and growth stages of cover crops for the three winter cropping seasons (1991-94). Sampling Date Cover Crop Morphological Zadoks Growth Species Planting Date Stage Stage Nov/21/1991 Spring Barley August/19 Ear emergence 51 Spring Barley September/16 Stem elongation 33 Winter Rye August/19 Tillering 28-29 Winter Rye September/16 Tillering 23-25 Apr/08/1992 Spring Barley August/19 WK WK Spring Barley September/16 WK W K Winter Rye August/19 Booting 45 Winter Rye September/16 Booting 45 Nov/24/1992 Spring Barley August/24 Booting 41 Spring Barley September/22 Tillering 23-24 Winter Rye August/24 Tillering 29 Winter Rye September/22 Tillering 25-26 Apr/30/1993 Spring Barley August/24 WK WK Spring Barley September/22 Ear emergence 51 Winter Rye August/24 Flowering 61 Winter Rye September/22 Flowering 61 Nov/23/1993 Spring Barley August/24 Booting 45 Spring Wheat Booting 45 Spring Oat Stem elongation 45 Winter Rye Tillering 29 Annual Ryegrass Stem elongation -Apr/14/1994 Spring Barley August/24 WK WK Spring Wheat WK W K Spring Oat WK WK Winter Rye Ear emergence 51 Annual Ryegrass Stem elongation -W K , Winter-killed Chapter 3. Materials and Methods 3 7 3-3.4. Cover crop total nitrogen and carbon analysis Duplicate 1-g samples were weighed into 100-mL digestion tubes and 5 mL of digestion mix (Parkinson and Allen, 1975) were added. Samples were digested for 2.5 h at 360 °C. Nitrogen (as N H / - N ) concentrations were determined as described in section 3-3.2 on automated flow injection ion analyzer (QuickChem AE). Using dry weight biomass (kg ha"1) and N concentrations (mg kg"1) the data were extrapolated to a kg ha"1 (kg per unit surface area) basis. Plant samples (50 mg) were analyzed by LECO™ C R - 1 2 C analyzer (LECO Corp., St. Joseph MI) for total carbon content. The effectiveness of the cover crops to accumulate main-growing-season residual soil N was assessed by determining productivity (dry weight biomass) and N accumulation (dry weight biomass x N concentration) of the autumn-planted cover crops. Apparent fertilizer N recovered (ANRCROP) in cover crop was calculated from equation 3-3: [(N uptake)N.ioo] - [(N uptake)N.0] ANRCROP (%) = xlOO (3-3) applied N rate where, N-0 = no N applied and N-100 = 100 kg N ha"' applied as N0 3 "-N. Chapter 3. Materials and Methods 3 8 3-3.5. Statistical analysis The data was analyzed using the general linear model procedure (SAS/STAT Software, 1989). The mathematical model of the procedure applied to the split-split plot design used in the experiment is represented by the equation: Yijkl = u + Bj + Pj + BP// + N £ + PN/jfc + BPNy* + C/ + PC// + NC*/ + PNC/*/ + BPNCy*/ where, Yy'yfc/ = observation associated with the y'Mh experimental unit (eu); u = overall mean (common effect in all observations); B/ = effect due to the /th block (Block effect, / = 1, 2, 3, 4); Py = effect due to they'th level of planting date (Planting date effect, 7=1, 2); Bpy = random error associated with i/'th main plot (main plot error (a)); N £ = effect due to the Ath level of nitrogen treatment; (Nitrogen effect, & = 1, 2); PNy£ = interaction associated with the jkth planting date-nitrogen combination (planting date x nitrogen interaction effect); B P N y £ = random error associated with the ijkth subplot (subplot error (b)); Q = effect due to the /th level of cover crop treatment (/ = 1,2, 3); PCy/ = interaction associated with the y'/th planting date-cover crop combination (Planting date x cover crop interaction effect); NCyt/ = interaction associated with the kith nitrogen-cover crop combination (nitrogen x cover crop interaction effect); Chapter 3. Materials and Methods 39 PNCyyt/ = interaction associated with the jkhh planting date-nitrogen-cover crop combination (planting date x nitrogen x cover crop interaction effect); BPNCyjfc/ = random error associated with ijklth subsub-plot (subsubplot error (c)). Bartlett's test was carried out to test homogeneity of variances. Test of independence of the means and the standard deviations were also used to determine the relationship between the means and standard deviations. Most of November soil mineral N data failed Bartlett's test of homogeneity of variances and all soil mineral N data (November and April) had means that were highly correlated with the standard deviations (0.80 < r < 0.95). Logarithm transformation was applied to soil mineral N contents to minimize correlation between the mean and the standard deviation. In this thesis, means of original data are presented but interpretation is based on A N O V A from data transformed logarithmically (Antal. Kozak, Faculty of Forestry, University of British Columbia, personal communication). Since soil data were collected from the same experimental units in November and April of each season, a repeated-measures analysis of variance was used to evaluate the changes in cover crop productivity, N uptake and soil mineral N (0-60 cm layer) over the winter as randomization of sampling dates was not possible (Littell, 1989; George. W. Eaton, Department of Plant Science, University of British Columbia, personal communication). A similar approach has been used by Schomberg et al. (1994) to evaluate the influence of water on decomposition and N dynamics for surface and buried residues. General linear model options in SAS (SAS/STAT, 1989) were set to test planting date, autumn soil mineral N content and cover crop species main effects using main plot M S E [error (a)], subplot M S E [error (b)] and subsubplot MSE, [error (c)], respectively, according to split split plot design for 1991-92 and 1992-93 data. Similarly, G L M options were set to test soil Chapter 3. Materials and Methods 40 mineral N content and cover crop main effects using main plot MSE, [error(a)] and subplot M S E [error (b)], respectively, according to split plot design, for 1993-94 data. Repeated-measures analysis of variance was done using the general linear models procedure in SAS (SAS/STAT, 1989). Univariate tests of hypotheses for time (T) and interactions with planting date (D), autumn soil mineral N content (N) and crop species (C) (i.e Within Subjects Effects) were based on repeated measures A N O V A , but tests of hypotheses for planting date (D), autumn soil mineral N content (N) and crop species (C) (i.e Between Subjects Effects) were based on A N O V A for data at each sampling date. 3-4. NITROGEN DYNAMICS OF SPRING GRAMINACEOUS COVER CROPS 3-4.1. Nitrogen retention study The following sections (sec. 3-4.1 and 3-4.2) describe the procedures for determining the potential of autumn-planted spring species to conserve mineral N (N03"-N in particular) during winter. Spring barley (Hordeum vulgare L. cv. Virden), spring wheat (Triticum aestivum L . cv. Max) and spring oat (Avena sativa L. cv. Jasper) were harvested in the experiments described in the materials and methods (see. sec. 3-1). In 1991-92 and 1992-93 seasons, the study involved only spring barley and, therefore, the experimental design was reduced to split plot with planting date as the main plot and autumn soil mineral N content as the subplot. But in the 1993-94 winter cropping season, planting date was omitted as a treatment and the study included spring barley, spring wheat and spring oat as subplots and autumn soil mineral N content as the main plot. The N treatments (N-0, N-100) were administered to create a secondary level of residual mineral N in autumn to test the capacity of N uptake by cover crops. Chapter 3. Materials and Methods 41 Two sets of aboveground plant material were harvested from two randomly selected areas (0.5 m x 0.5 m quadrat each) in each of the spring species plots on November 21, 1991, November 24, 1992 and November 24, 1993 for the three winter cropping seasons. Fresh weights of both sets of samples were determined immediately in the field. One set of samples was placed in fiberglass meshbags of 0.5 m x 0.5 m dimension having mesh size of 1.5 mm x 1.5 mm. These bags of plant material were left in the field to measure changes in biomass and total N of the winter-killed mulch over the winter. The other set of samples was brought into the laboratory and oven-dried at 70 °C for 96 h. The oven-dry weight was used both for biomass and moisture content determinations. The moisture content was then used to calculate the dry weight of original biomass in the meshbags left in the field. The oven-dried samples were ground in a Wiley mill to pass through a 1-mm sieve and duplicate digestions using the method described by Parkinson and Allen (1975) were run to determine N concentration as described in section 3-3.2. This allowed me to calculate total N of meshbag materials at the beginning of the experiment (November). In November 1991, the meshbags were anchored on the soil surface in harvested areas within the plots. However, because the original objective was to estimate the amount of N lost from the plant materials through leaching after damage caused by freezing, it was decided in 1992-93 and 1993-94 seasons to place the meshbags on a wire mesh table with an area of 3 m x 2 m raised on wooden legs 0.5 m above the ground. The purpose of the wire mesh table was to avoid contact of the residues with the soil and thus an environment conducive to decomposition. The wire mesh size was 12 mm x 12 mm. The mesh table was laid out in the buffer strips between blocks above winter rye which was at late tillering stage in November so that contamination from soil particles during rainfall events was avoided. Chapter 3. Materials and Methods 42 In spring of each cropping season, the meshbag samples were removed from the field at the time the winter cover crops were harvested. The removal dates of meshbag samples from the field were April 8, 1992, April 30, 1993 and April 14, 1994 for the three seasons. In the spring of 1994, undisturbed spring-species-cover-crop residues in field plots were collected using 0.5 m x 0.5 m quadrates. The materials were oven-dried at 70 °C for 96 h, and biomass and total N were determined as described above. The difference in residue N between late November (time of winter-kill) and April (spring) was assumed to have been that which was leached from the materials in meshbags on mesh tables. Biomass and N retention by cover crop residues were calculated according to equations 3-4 and 3-5 as follows: Biomass remaining in April (kg ha") Biomass remaining (%) = x 100 (3-4) Biomass measured in November (kg ha" ) Residue N in April (kg ha" ) Nitrogen retention (%) = x 100 (3-5) Cover crop N in November (kg ha") Chapter 3. Materials and Methods 43 3-4.2. NITROGEN FRACTIONATION STUDY 3-4.2.1. Sample preparation and extraction techniques. Separate plant materials were randomly harvested from each of the spring-species plots in experiments II (1992-93) and III (1993-94). Materials were immediately frozen in liquid N 2 to minimize protease activity. Samples were transported to the laboratory in ice coolers and stored at -50 °C until they were freeze-dried (Edwards Freeze Dryer, M O D U L Y O ) for 72 h and chopped into small pieces with scissors. Freeze-dried samples were then frozen with liquid N 2 , ground while still frozen in a chilled commercial waring blender and stored at -50 °C until the time of extraction and analysis. The major aim was to extract plant tissue as completely as possible to evaluate the relative contribution of the various fractions to the total N of the cover crops at winter-kill. A cold extraction with 0.05M phosphate buffer at pH of 7.4 was used to determine the various N fractions. The fractions were cold-water insoluble N (WIN) remaining in residue, T C A -precipitable protein N (PN), organic nonprotein N (NPN), NOy-N (NN) and NH 4 + -N (AN). Cold-and hot-water extraction procedures were compared for total protein (WIN + PN) and organic nonprotein N (NPN). Hot-water extraction (Goering and Van Soest, 1970) and 2M KC1 extraction (Keeney and Nelson, 1982) were compared with cold-water extraction for N03"-N and NH 4 + -N. 3-4.2.2. Preparation of homogenizing medium Two stock solutions of 0.05M monosodium phosphate (NaH 2P0 4.2H 20 - solution A) and 0.05M disodium phosphate (Na 2HP0 4 - solution B) were prepared using deionized water. To make 1 L of 0.05M phosphate buffer solution, 190 mL of solution A was mixed with 810 mL of Chapter 3. Materials and Methods 44 solution B (Bollag and Edelstein, 1991). Homogenizing medium was then made with the 0.05M phosphate buffer solution by adding protease inhibitors and antioxidants. To a 1 L volumetric flask, protease inhibitors, l.OmM phenylmethylsulfonyl fluoride - PMSF (Fahrney and Gold, 1963; Turini et al., 1969), l.OmM p-chloromercuribenzoate (Wallace and Cotta, 1988) and lOmM ethylenediaminetetraacetic acid (EDTA) were added (Bollag and Edelstein, 1991). Further, 0.4% sodium isoascorbate (w/v) and 4mM sodium metabisulfite (Anderson and Rowan, 1967) were added as antioxidants and the mixture was made to volume with phosphate buffer solution. These antioxidants were included to prevent oxidation of low molecular weight phenolic compounds to quinones, which in turn could combine with proteins, thus modifying their biochemical and physical properties. Higher molecular weight phenolics, frequently called tannins, can form insoluble complexes with proteins (Swain, 1965; Loomis and Battaile, 1966; Loomis, 1974; Gegenheimer, 1990). This removes the proteins from solution. To overcome this problem, insoluble polyvinylpyrrolidone (PVP) was used (Loomis, 1974). 3-4.2.3. Phosphate buffer (cold-water) extraction The nitrogen fractionation scheme is presented in Figure 3-4. Three-gram samples were weighed into a waring blender and 4.5 g of insoluble polyvinylpyrrolidone (PVP) was added. The mixture was homogenized for 1 min. in 70 mL of cold 0.05M phosphate buffer homogenizing medium, cooling intermittently for 1 min in an ice bath after every 15 seconds. A blank, consisting of 4.5 g PVP, was similarly processed. The homogenized slurry was transferred into 250-mL centrifuge bottles using 100 mL of homogenizing medium and centrifuged immediately at 14,000 x g for 45 min at 3 °C. The homogenate was decanted carefully from plant residue into 500-mL volumetric flasks. Plant residue remaining in the centrifuge tubes was rinsed Chapter 3. Materials and Methods 45 twice by suspension in 80-mL aliquots of homogenizing medium each time and centrifuged as stated above. Blanks, to correct for reagents used, were similarly processed. During the period of rinsing and centrifuging of the residue, homogenates were kept in the refrigerator at 1 °C. Immediately after completion of extraction, 80 mL of the homogenate were measured into 100-mL centrifuge tubes and proteins were precipitated using 15% (w/v) trichloroacetic acid - T C A (Bhatty, 1972). The mixture was centrifuged at 10,000 x g for 45 min at 3 °C. The supernatant was decanted into 125-mL Nalgene plastic bottles and stored at -18 °C until analysis for total N, N H 4 + and N 0 3 ' (see Figure 3-4). The methods of analyses are given in the following section. The pellet of protein precipitate was discarded. Chapter 3. Materials and Methods RESIDUE (H 2SO 4 + 30%H!O2)| Digestion (FIA) T O T A L N IN RESIDUE (A) PROTEIN PRECIPITATE (DISCARDED) T O T A L N IN S U P E R N A T A N T (C) A M M O N I U M N (D) PLANT S A M P L E (+ PVP) Homogenization Centrifugation (I4,000xg Max.,45 Min) H O M O G E N A T E E X T R A C T (HjSOj + 30% Hj0 2 ) H O M O G E N A T E E X T R A C T (+ 15% T C A , -20"C) Pretreatment (KMn04 + Fe) Digestion (H 2SOj + 30% H 2 0 2 ) Centrifugation (10,000xg Max.,45 Min.) Digestion (FIA) T O T A L N IN H O M O G E N A T E (B) S U P E R N A T A N T Activated Charcoal (FIA) NITRATE N (E) T C A = Trichloroacetic Acid FIA = Flow Injection Analyzer PVP = Insoluble polyvinylpyrrolidone Figure 3-4. Nitrogen fractionation scheme to determine total N in residue (A), total N in homogenate (B), total N in supernatant (C), NH/-N (D) andN03'-N (E). Chapter 3. Materials and Methods 47 3-4.2.4. Sample nitrogen determination Residue samples and the PVP-only blank were dried on previously tared drying pans at 100 °C for 8 h and weighed. These dried residues were then transferred into 100-mL Kjeldahl flasks and digested for total N according to the method described by Parkinson and Allen (1975). Total digestion time was 3 h. Total N was determined as NH 4 + -N colorimetrically with an automated flow injection ion analyzer (Lachat Instruments, QuickChem AE) as described in section 3-3.2. Homogenate and supernatant extracts (5 mL) were pretreated for 1 h with K M n 0 4 and reduced Fe to include N02"-N and N0 3 ' -N (Bremner and Mulvaney, 1982; McGill and Figueiredo, 1993) before digestion and total N was determined as described above. Cold-water soluble protein N was estimated from the difference between total N for homogenate extract and supernatant. Supernatant (100 mL) was treated with 0.5 g of activated carbon (Darco-60G). Blanks of similar volume of homogenizing medium were also treated with 0.5 g of activated carbon. Charcoal-treated samples and reagent blanks were filtered through Whatman No. 42 paper. Ammonium N and N03"-N were determined colorimetrically on automated flow injection ion analyzer (Lachat Instruments, QuickChem AE). Organic nonprotein N was estimated from the difference between supernatant total N and mineral N (NH 4 +-N + N03"-N). To check recoveries of total N, N02"-N and N03"-N from plant material, 100-mg plant samples and 1 mg N kg"1 of standard K N 0 2 and K N 0 3 were similarly digested to include N0 2" and N0 3" as described for homogenate extract and supernatant. Nitrogen of the various fractions was first calculated on freeze-dry-weight basis (g kg'l). Values of the sum of all fractions were compared with total N of 100-mg plant material. Total N recovery from plant material was 96 and 95% of all fractions for November 1992 and 1993 Chapter 3. Materials and Methods 48 samples, respectively. Total N recovery from K N 0 2 and K N 0 3 was 99%. Individual fractions were therefore, expressed as percentages of the sum of all fractions (%TN). The means (n = 4) of field N uptake (kg ha"1) for November sampling were used to calculate N for all fractions on a kg ha"1 basis. 3-4.2.5. Hot water extraction Extraction of hot-water protein-N (True protein-N), organic nonprotein-N, NH„ +- and N0 3 '-N fractions was based on the method described by Goering and Van Soest (1970) with modification to the original apparatus. Two-gram samples were weighed into 500-mL round bottom flasks and 200 mL of distilled water added. Refluxing condensers were attached vertically at the top and the contents were boiled for 1 h at 100 °C in heating mantles connected to variable autotransformers (Type 3PN 1010, Staco Inc., Dayton, Ohio) in order to regulate the temperature. The contents were then filtered with vacuum through 12.5 cm Whatman No. 54 filter papers set in 60° funnels. Residue on the filter paper was washed four times with 50 mL aliquots of hot water. Residue samples were dried on previously tared drying pans at 100 °C for 8 h and weighed. These dried residues (including filter paper) were then transferred into 100-mL Kjeldahl flasks and digested for total N according to the method described by Parkinson and Allen (1975). Blank digestions of filter paper alone were similarly processed. Total digestion time was 3 h. Two sets of filtrate samples were placed in 125 mL Nalgene bottles. One set was treated with 0.5 g of activated carbon (Darco G-60) to remove colour in order to determine NH 4 + -N and N0 3"-N. Blanks with 0.5 g of activated carbon in a similar volume of hot water were included. The samples and blanks were filtered through Whatman No. 42 paper. The filtrates were stored Chapter 3. Materials and Methods 49 in the freezer at -18 °C until NH„ +-N and NOY-N analysis. The other set was used for total N measurement. Total N was determined as described in section 3-4.2.4 for homogenate and supernatant N to include N03". Hot-water extractable organic nonprotein N (NPN) was estimated from the difference between filtrate total N and mineral N (NH 4 + + N03"). In addition to cold- (see Sec.3-4.2.3) and hot-water (see sec. 3-4.2.5) extracts for N H 4 + and N03", plant samples were extracted with 2M KC1 (Keeney and Nelson, 1982) by shaking 0.25 g of tissue samples in 100 mL of 2M KC1 solution for 1 h. The mixtures were filtered through Whatman No. 42 filter paper. Nitrogen for all the fractions was determined colorimetrically with an automated flow injection ion analyzer (Lachat Instruments, QuickChem AE) as described in section 3-3.2. 3-4.3. Statistical analysis Since plant materials were harvested in a split split plot design (Expts I and II) and the sub-subplot had only one spring species treatment (spring barley), the data for N retention and N fractionation studies were analyzed according to a split plot design for 1991-92 and 1992-93 seasons with planting date as the main plot and autumn soil mineral N content (N-0 and N-100) as the subplots. In the 1993-94 season (Expt III) the data were also analyzed as a split plot design, but with autumn soil mineral N content as main plots and spring cover crop species (barley, wheat and oat) as subplots. In the fractionation study, cold- (CW - method A) and hot-water (HW - method B) extracts were compared for protein-, organic nonprotein- N H 4 + - and N0 3 ' -N measurements as paired observations using a two-tailed T - test (P < 0.05). Similarly, cold-water insoluble N (WIN) and total protein N (WIN + PN) fractions were compared with proportions of N retained in meshbag residues that were placed on tables, as paired observations using a two-tailed T - test (P < 0.05). Chapter 4. Experimental Conditions 50 Chapter 4 : EXPERIMENTAL CONDITIONS 4-1. Weather during study period The Lower Fraser Valley has a temperate climate, generally characterized by warm, rainy winters and relatively cool, dry summers (Hare and Thomas, 1979) and, has the highest annual temperature ( about 10 °C 2) in Canada (Schaefer, 1978). Western Fraser Valley also has the longest period of frost-free days in Canada (Luttmerding, 1981). Mean monthly temperatures and total monthly precipitation for long-term 1937-90 normals are shown in Figure 4-1. Total daily precipitation for 1991-92, 1992-93 and 1993-94 winter cropping seasons are shown in Figures 4-2 and 4-3. August 1991 was very wet compared to long-term records and, August 1992 and 1993. In that month, total rainfall was 170 mm with 134 mm occurring between August 26 to 31. Generally, most of the rainfall events occurred during the winter months (November to March). Mean monthly air temperatures for the three winter cropping seasons are shown in Figure 4-4. The lowest temperatures occurred between November and February. In the 1991-92 and 1993-94 seasons the lowest temperatures were generally above normal while those for 1992-93 season were below normal, with the lowest temperature recorded in January (-0.4 °C). 4-2. Daily soil and air temperatures Daily soil temperatures were monitored during the 1991-92 and 1992-93 winter cropping seasons at 3 and 40 cm below the surface using thermocouple (copper-constantan) sensors. Thermocouples (one in each plot) were installed in fallow, August-planted spring barley and winter rye plots and readings were recorded with a Campbell Scientific 21X Datalogger (Campbell Scientific Inc., Logan, Utah). Daily air temperatures were similarly measured using 2 Based on 1937-90 normals at Vancouver International Airport (Source: Environment Canada, Climate Services Vancouver, and Canadian Climate Normals for B.C) Chapter 4. Experimental Conditions 51 May Jul Sep Nov Jan Figure 4-1. Mean monthly air temperatures and total monthly precipitation for 1937-90 normal as recorded at Vancouver International Airport (Source: Environment Canada, Climate Services Vancouver, and Climate Normals for British Columbia). Chapter 4. Experimental Conditions 52 M a y l S Junl5 Jul l5 A u g l 5 Sepl5 Oct l5 N o v l 5 Dec lS JanlS Febl5 M a r l 5 A p r l 5 Date (1991-92) 60 40 30 2 20 1992-93 season ILILL L T I M a y l 5 Junl5 Jul 15 A u g l 5 Sepl5 Oct l5 N o v l 5 Dec l5 Janl5 Febl5 M a r l 5 A p r l 5 Date (1992-93) Figure 4-2. Total daily precipitation for 1991-92 (above) and 1992-93 (below) winter cropping seasons as recorded at Vancouver International Airport (Source: Environment Canada, Climate Services Vancouver, and Climate Normals for British Columbia). Chapter 4. Experimental Conditions 53 Mayl5 Junl5 Jul 15 Augl5 Sepl5 Octl5 Novl5 Decl5 Janl5 Febl5 Marl5 Aprl5 Date (1993-94) Figure 4-3. Total daily precipitation during 1993-94 winter cropping season as recorded at Vancouver International Airport (Source: Environment Canada, Climate Services Vancouver, and Climate Normals for British Columbia). Chapter 4. Experimental Conditions 54 1991-92 May Jul Aug Sep Oct Nov Month A p r Figure 4-4. Mean monthly air temperatures during 1991-92, 1992-93 and 1993-94 winter cropping seasons as recorded at Vancouver International Airport (Source: Environment Canada, Climate Services Vancouver, and Climate Normals for British Columbia). copper-constantan sensors shielded from radiation by a Stevenson Screen. Mean daily air and soil temperatures for the 1991-92 season are shown in Figures 4-5 and 4-6. Average daily soil temperatures (at 3 cm depth) approached 0 °C in December 1991 and January 1992, and generally increased above 8 ° C from the beginning of March. Mean daily air and soil temperatures at the 3 and 40 cm depth for the 1992-93 season are presented in Figures 4-7 and 4-8. Mean daily soil temperatures (at the 3 cm depth) remained near Chapter 4. Experimental Conditions 55 Figure 4-5. Daily air and soil temperature means during the 1991-92 winter cropping season under fallow (above) and autumn-planted spring barley (below). Chapter 4. Experimental Conditions Figure 4-6. Daily air and soil temperature means during the 1991-92 winter cropping season under autumn-planted winter rye. Chapter 4. Experimental Conditions 57 Figure 4-7. Mean daily air and soil temperatures for the 1992-93 winter cropping season under fallow (above) and autumn-planted spring barley (below). Chapter 4. Experimental Conditions 58 -15 Nov 3Nov 15 Nov 30 Dec 15 Dec 31 Jan 15 Jan31 Febl5Feb28 Marl5 Mar31 Apr 15 A p r 3 0 M a y l 5 Time (1992 - 93) Figure 4-8. Mean daily air and soil temperature variations during the 1992-93 winter cropping season under autumn-planted winter rye. Chapter 4. Experimental Conditions 59 0 C in the first and last half of January and February, respectively, and thereafter increased above 5 °C in March. In both winter cropping seasons, soil temperatures measured at the 3 cm depth generally reflected changes in air temperature, except under fallow in 1992-93 season. It appears the temperature sensor in the fallow plot was exposed to direct radiation. 4-3. Monthly minimum and maximum temperatures Monthly minimum and maximum soil temperatures at the 3 cm depth for the 1991-92 season are shown in Figure 4-9. In October and November 1991, both monthly minimum and maximum soil temperatures at the 3 cm depth were higher under spring barley than under fallow or winter rye. Tremendous growth of spring barley and early snow fall (October 28, 1991) which resulted in lodging of the cover crop provided sufficient insulation that limited heat exchange between surface soil and air during cooling and warming. Thereafter, minimum and maximum temperatures under fallow and cover crops were not considerably different, although fallow plots appeared to cool and warm faster than those under cover crops. Monthly minimum and maximum soil temperatures at the 40 cm depth were not greatly influenced by cover cropping (Figure 4-10). In the 1992-93 winter cropping season, comparison of soil temperature at the 3 cm depth between fallow and cover cropped plots was not valid as it was apparent that the temperature sensor was exposed to direct radiation (Figure 4-11). Monthly minimum temperatures were not influenced by crop species but maximum temperatures under spring barley were generally higher than those under winter rye. Similar to the 1991-92 season, monthly minimum and maximum temperatures (at the 40 cm depth) were not greatly influenced by cover cropping, although maximum temperatures under winter rye were generally cooler than either under fallow or autumn-planted spring barley residues (Figure 4-12). Chapter 4. Experimental Conditions 60 Figure 4-9. Monthly minimum (above) and maximum (below) temperatures at 3 cm depth during the 1991-92 winter cropping season, (values above the bars for spring barley and winter rye represent averages over the N treatment of the amount of cover in t ha'1). Chapter 4. Experimental Conditions 61 Time (1991-92) Figure 4-10. Monthly minimum (above) and maximum (below) temperatures at 40 cm depth during the 1991-92 winter cropping season. Chapter 4. Experimental Conditions 62 Figure 4-11. Monthly minimum (above) and maximum (below) soil temperatures at 3 cm depth during the 1992-93 winter cropping season (values above the bars for spring barley and winter rye represent averages over the N treatment of the amount of cover in t ha'1). Chapter 4. Experimental Conditions 63 Nov Dec Jan Feb Mar Apr Time (1991-92) Figure 4-12. Monthly minimum (above) and maximum (below) soil temperatures at 40 cm depth during the 1992-93 winter cropping season. Chapter 5. Results 64 Chapter 5: RESULTS 5-1. DYNAMICS OF AUTUMN SOIL MINERAL NITROGEN 5-1.1. Soil mineral nitrogen status in fallow plots Soil mineral N (NH 4 + + NOf) content in the 0-60 cm layer for the three seasons, determined on samples taken before planting of cover crops in August and September, was variable among the three locations (L9l, L92 and L93) and between the two planting dates (Table 5-1). Table 5-1. Main growing season residual soil mineral N for samples taken immediately before planting in the three cover cropping seasons. Soil sampling depth intervals (cm) Sampling date N level 0-20 20-40 40-60 (^ 60 .. (kg ha"1) August/19/1991 N-0 64 30 30 124 N-100J 164 30 30 224 September/16/1991 N-0 58 53 36 148 N-100 113 89 43 245 August/24/1992 N-0 38 18 18 74 N-1001 138 18 18 174 September/22/1992 N-0 46 21 23 90 N-100 153 22 25 200 August/24/1993 N-0 96 61 38 196 N-100| 196 61 38 296 N-0, autumn residual mineral N only; N-100, autumn residual mineral N + 100 kg N ha"1 as N03"; % Theoretical sum of amount determined before N application and 100 kg N ha"' added after planting; Changes in soil mineral N (0-60 cm layer) in fallow plots that received no fertilizer N at planting for the three winter cropping seasons are shown in Figure 5-1. At the beginning of the seasons, a higher proportion of the N was in the surface 20 cm and decreased gradually with depth. Chapter 5. Results 65 Aug. Sep. Oct. Nov. Dec. Jan. Sampling Date Mar. Apr. Figure 5-1. Temporal pattern of changes in soil mineral N in the 0-60 cm layer in fallow plots at the primary level (N-0) of autumn soil mineral Nfor the three winter cropping seasons. Chapter 5. Results 66 Soil N mineralization from organic matter was evident at the late planting date (September) at the 1991 location (L91) and up to November at the 1992 location (L92). At both locations, soil mineral N content in late November increased with depth, indicating the start of leaching. In late November 1993, soil mineral N content gradually decreased down the profile, indicating N03" leaching into the subsurface layers was minor. At all three locations (L91, L92 and L93), the least amount of mineral N was measured in the spring and generally increased with depth. Soil mineral N decrease in fallow plots (N-0) in the 0-60 cm layer between late November and April of 1991-92, 1992-93 and 1993-94 winter seasons was 51, 59 and 70% (of that measured in November), respectively. 5-1.2. Cover crop biomass and nitrogen before winter leaching period The effects of planting date, autumn soil mineral N content and crop species on biomass production and N uptake before winter leaching period for 1991-92 and 1992-93 seasons are summarized in Tables 5-2 and 5-3, respectively. At the start of 1991 and 1992 winter seasons (late November), variability in cover crop biomass production and N uptake were independent of crop species but were largely due to planting date and autumn soil mineral N content. In late November 1991, planting of cover crops in the third week of August as compared to a month later increased cover crop biomass production by 135% and N uptake by 38%. Similarly, increasing autumn soil mineral N content by 100 kg ha"1 increased cover crop productivity by 28%o and N uptake by 41%. In late November 1992, cover crop biomass production and N uptake increased with increase in autumn soil mineral N content when cover crops were planted in August but changed little when planted a month later (Figure 5-2). When averaged over cover crops, biomass and N content of August-planted cover crops increased by 56 and 93%, respectively, with increasing autumn soil N content; the average biomass of September-planted Chapter 5. Results 67 cover crops was not influenced by soil mineral N content and N uptake increased by only 9%. In both years, autumn-planted spring barley was not different from winter rye in terms of productivity and capturing main-growing-season residual mineral N before winter leaching period. Maximum N uptake capacity averaged 95 and 107 kg ha"1 in late November 1991 and j 1992, respectively, and occurred when cover crops were planted in August. Table 5-2. Effect of planting date, autumn soil nitrogen content and crop species on biomass and cover crop N before winter leaching period (November 21, 1991). Treatment Biomass Cover crop N Mean values^ ] Planting date August/19/1991 4.0 95 September/16/1991 1.7 69 Nitrogen level N-0 2.5 68 N-100 3.2 96 Treatment effects (P > F values) Planting date (D) 0.0024 0.0460 Nitrogen (N) 0.0325 0.0144 D x N NS NS Cover crop (C) NS NS C x D NS NS C x N NS NS C x D x N NS NS C V (%) 25 35 \ Biomass is expressed as t ha and cover crop N as kg ha ; NS, not significant (P > 0.05); Chapter 5. Results 68 Table 5-3. Effect of planting date, autumn soil mineral nitrogen content and crop species on biomass and cover crop N before winter leaching period (November 24, 1992). Treatment Biomass Cover crop N Mean values^ } Planting date August/24/1992 3.0 107 September/22/1992 1.0 48 Nitrogen level N-0 1.7 59 N-100 2.3 95 Treatment effects (P > F values) Planting date (D) 0.0007 0.0017 Nitrogen (N) 0.0021 0.0001 D x N 0.0022 0.0002 Cover crop (C) NS NS C x D NS NS C x N NS NS C x D x N NS NS \ Biomass is expressed as t ha"1 and cover crop N as kg ha"'; NS, not significant (P > 0.05); Chapter 5. Results 69 N-0 N-100 Autumn soil nitrogen content Figure 5-2. Biomass production (above) and N uptake (below) of autumn-planted spring barley and winter rye before winter leaching period (November 24, 1992) as influenced by planting date and autumn soil mineral N content. Error bars represent standard error of the mean (n -8). Chapter 5. Results 70 In the 1993-94 winter cropping season all crops were planted on August 24, 1993, the date that represented early planting in the previous two seasons. Fallow plots were heavily infested with chickweed (Stellaria media L.) and thus a bare control was not maintained. Variations in cover crop biomass production and N uptake before winter leaching period were mainly due to crop species (Table 5-4). Table 5-4. Effect of autumn soil mineral nitrogen content and cover crop species on biomass production and cover crop N before winter leaching period (November 24, 1993). Treatment Biomass Cover crop N Crop species Fallow/Chickweed (F) Spring barley (SB) Spring wheat (S W) Spring oat (SO) Winter rye (WR) Annual ryegrass (ARG) Nitrogen (N) Cover crop (C) C x N C V (%) F vs SB+SW+SO+WR+ARG SB vs SW+SO SW vs SO WR vs A R G SB+SW+SO vs WR+ARG Mean values^ ] 2.8 4.2 5.4 5.3 3.7 4.3 115 157 142 144 146 Treatment effects (P > F values) NS NS 0.0001 0.0001 NS NS 10 14 Orthogonal contrasts for cover crop 0.0001 0.0019 0.0001 0.0001 NS NS 0.0040 NS 0.0001 0.0045 H Biomass is expressed as t ha"1 and cover crop N as kg ha"1; NS, not significant (P > 0.05); Chapter 5. Results 71 Generally, cover crop biomass production and N accumulation were greater relative to those of chickweed in fallow plots. Among spring species, wheat and oat produced greater biomass and took up more mineral N when compared with barley. In contrast, annual ryegrass produced greater biomass than winter rye but the two cover crops were similar in terms of N uptake. A general statistical contrast analysis of spring species with winter species indicates that despite significantly less biomass production than that for spring species, winter species absorbed relatively greater amounts of residual mineral N. 5-1.3. Soil mineral nitrogen content before winter leaching period The influence of planting date, autumn soil mineral N content and crop species on residual mineral N in the 0-60 cm layer varied greatly among the three years. In late November 1991, variations in soil mineral N content were not influenced by planting date but were independently affected by early autumn soil mineral N content and crop species (Table 5-5). The main effect of the N treatment indicated that, on average, a significant proportion (47%) of fertilizer N was recovered in the 0-60 cm layer before winter leaching period. When averaged over planting date and autumn soil N content, spring barley and winter rye reduced soil mineral N by 53 and 54%, respectively. In contrast, planting date, autumn soil mineral N content and crop species influenced soil mineral N in late November 1992 (Table 5-6). Residual soil mineral N under autumn-planted spring barley and winter rye increased with delay in planting but a greater increase occurred under winter rye than spring barley (Figure 5-3). Residual mineral N under winter rye and spring barley increased by 246 and 87%>, respectively, when planting was delayed by one month. The data shows that planting of winter rye in August Chapter 5. Results 72 Table 5-5. Effect of planting date, autumn soil nitrogen content and cover crop species on residual soil mineral N before winter leaching period (November 21, 1991) Soil sampling depth intervals (cm) Treatment 0-20 0-40 0-60 Mean values (kg ha"') Nitrogen level N-0 10 30 51 N-100 19 56 98 Crop species Fallow (F) 13 65 116 . Spring Barley (SB) 20 36 55 Winter Rye (WR) 11 28 Treatment effects (P > F values)^ 53 Planting date (D) NS NS NS Nitrogen (N) 0.0048 0.0007 0.0001 D x N NS NS NS Cover crop (C) 0.0047 0.0001 0.0001 C x D NS NS NS C x N NS NS NS C x D x N NS NS NS C V (%) 16 8 Orthogonal contrasts for cover crop 6 F vs SB + WR 0.0018 0.0060 0.0001 SB vs WR NS 0.0001 0.0001 ^Analysis of variance performed on data transformed to natural logarithm scale; NS, not significant (P < 0.05); Chapter 5. Results 73 Table 5-6. Effect of planting date, autumn soil mineral nitrogen content and crop species on residual soil mineral N before winter leaching period (November 24, 1992). Soil sampling depth intervals (cm) Treatment 0-20 0-40 0-60 Nitrogen level N-0 N-100 Crop species Fallow/chickweed Spring barley Spring wheat Spring oat Winter rye Annual ryegrass Planting date (D) Nitrogen (N) D x N Cover crop (C) C x D C x N C x D x N C V (%) F vs SB+WR SB vs WR (F vs SB+WR)x(Dl vs D2) (SB vs WR)x(Dl vs D2) (F vs SB+WR)x(Nl vs N2) (SB vs WR)x(Nl vsN2) 39 79 84 53 51 63 58 44 Mean values (kg ha" ) 72 102 120 152 146 194 90 121 88 116 97 123 80 102 74 106 Treatment effects (P > F values)^ } NS 0.0238 0.0261 0.0027 0.0013 0.0024 NS NS NS 0.0001 0.0001 0.0001 NS 0.0001 0.0001 0.0381 0.0132 0.0223 NS NS NS 1 1 5 4 Orthogonal contrasts for cover crop and interactions NS 0.0075 0.0046 0.0001 0.0001 0.0001 NS NS NS NS 0.0001 0.0001 NS NS NS NS 0.0078 0.0079 1 Analysis of variance performed on data transformed to natural logarithm scale; F = fallow; SB : NS, not significant (P < 0.05); spring barley; WR = winter rye; Chapter 5. Results 74 250 200 150 A is 100 50 A D x C C 1 .--j Fallow Spring barley | | Winter rye Aug/24/92 Sep/22/92 Planting date 250 200 150 •a 100 A 50 N x C C N-0 N-100 Autumn soil mineral nitrogen content Figure 5-3. Variations in residual mineral N in the 0-60 cm layer in late November 1992 under fallow, spring barley and winter rye as influenced by planting date (above) and, autumn soil N content (below). Error bars represent standard error of the mean (n = 8). D x CC = planting date x crop species interaction; Nx CC = nitrogen x crop species interaction; Chapter 5. Results 75 caused greater reduction in soil mineral N than spring barley but the two cover crops were not different when planted a month later. Similarly, residual mineral N increase under winter rye and spring barley was 91 and 136%, respectively, when an additional 100 k g N ha"' was applied. The data indicates that under low N supply, winter rye resulted in greater soil mineral N reduction than spring barley but that the two cover crops were comparable when an additional 100 kg N ha" 1 was applied. On average, significant proportions (47 to 75%) of applied fertilizer N were recovered (ANRSO.L) in the 0-60 cm soil layer in late November in all the three years. Generally, the proportions recovered increased sharply with depth in late November 1991 but gradually decreased with depth in late November 1992. This suggested substantial downward movement of N0 3" in both years. In contrast, the proportions of fertilizer N recovered in 1993 decreased sharply with depth, indicating little movement into the subsurface layers. In late November 1993, variations in residual mineral N were dependent on autumn soil mineral N content and crop species (Table 5-7). When averaged over the N treatment, cover cropping reduced soil mineral N by 42% in the 0-60 cm soil layer, despite considerable chickweed (Stellaria media L.) growth on fallow plots. Corresponding reduction in soil N0 3" was 47% in the 0-60 cm layer. This is not surprising as cover crops generally accumulated greater amounts of soil mineral N compared to chickweed by November 24, 1993 (see Table 5-4). The data also indicates that spring wheat was more effective in reducing soil mineral N in the 0-40 cm layer than spring oat when 100 kg ha"' was applied (Figure 5-4.). Chapter 5. Results 76 Table 5-7. Effect of autumn soil mineral nitrogen content and crop species on residual mineral N before winter leaching period (November 24, 1993). Soil sampling depth intervals (cm) Treatment 0-20 0-40 0-60 Mean values (kg ha"1) Nitrogen level N-0 39 72 102 N-100 79 120 152 Crop species Fallow/Chickweed (F) 84 146 194 Spring Barley (SB) 53 90 121 Spring Wheat (SW) 51 88 116 Spring oat (SO) 63 97 123 Winter Rye (WR) 58 80 102 Annual Ryegrass (ARG) 44 74 106 Treatment effects (P > F values)^ Nitrogen (N) 0.0065 0.0132 0.0039 Cover crop (C) 0.0001 0.0001 0.0001 C x N 0.0007 0.0324 NS C V (%) 6 4 4 Orthogonal contrasts for cover crop and interactions F vs SB+SW+SO+WR+ARG 0.0001 0.0001 0.0001 SB vs SW+SO NS NS NS SW vs SO 0.0034 NS NS WR vs A R G NS NS NS SB+SW+SO vs WR+ARG NS NS NS (F vs SB+SW+SO+WR+ARG)x(NlxN2) 0.0447 NS NS (SBvsSW+SO)x(NlxN2) NS NS NS (SWvs SO)x(NlxN2) 0.0001 0.0039 NS (WR vsARG)x(NlxN2) NS NS NS (SB+SW+SO vs WR+ARG)x(NlxN2) NS NS NS ^Analysis of variance performed on data transformed to natural logarithm scale; NI = N-0; N2 = N-100; NS, not significant (P > 0.05); Chapter 5. Results 11 200 N-0 N-100 Autumn soil mineral nitrogen content Figure 5-4. Variations in residual mineral N in late November 1993 in the 0-20 (above) and 0-40 cm layers under spring wheat and spring oat as influenced by autumn soil nitrogen content. Error bars represent standard error of the mean (n = 4). Chapter 5. Results 78 5-1.4. Impact of cover cropping on soil N03~ before winter leaching period Table 5-8 shows that percent N03" reduction due to cover cropping was significantly influenced by planting date and crop species in late November 1992 and by crop species in 1993. The trend for the 1992 data indicates that when cover crops were planted in August, winter rye was more efficient than spring barley in reducing soil N03" levels in the 0-60 cm layer before winter leaching period but when planted one month later the two cover crops were equally less effective (Figure 5-5). Before winter leaching period of 1993, winter rye, annual ryegrass and spring wheat had comparable impact on soil N03" content in the 0-60 cm layer but winter rye was more effective than either spring barley or spring oat (Table 5-9). Table 5-8. Analysis of variance (P > F values) for planting date (D), autumn soil mineral nitrogen content (N) and crop species effects on percent NO3-N reduction (PR) in the 0-60 cm layer before winter leaching period for the three winter cropping seasons. Treatment November/21/1991 November/24/1992 November/24/1993 D NS 0.0017 N NS NS NS D x N C C x D C x N C x D x N NS NS NS NS NS NS 0.0263 0.0384 NS NS 0.0248 NS Chapter 5. Results 79 Spring Barley Winter Rye Cover Crop Species Figure 5-5. Influence of planting date on cover crop impact on soil NO; in the 0-60 cm layer before winter leaching period. Error bars represent standard error of the mean (n = 8). Table 5-9. Influence of crop species on NOi reduction (% - PR) in the 0-60 cm layer before winter leaching period of 1993. Crop species NCy Reduction (%) Spring barley 43b Spring wheat 45ab Spring oat 40b Winter rye 57a Annual ryegrass 53ab Chapter 5. Results 80 5-1.5. Cover crop biomass and nitrogen after winter leaching period In late November 1991, August- and September-planted spring barley winter-killed and August-planted winter rye was severely damaged by freezing. In contrast, only August-planted spring barley was killed by freezing in late November 1992 (Figure 5-6) and August-planted winter rye was damaged by freezing, especially where additional 100 kg N ha"' was added (Figure 5-7). Cover crop biomass and N in the spring of 1992 were influenced by planting date and crop species (Table 5-10). The biomass of spring barley residue declined drastically (by 83% of that measured in November 1991) with delayed planting but that of winter rye increased (33%). Similarly, total N of spring barley residue decreased considerably (86%) with delayed planting but that of winter rye increased (42%) (Figure 5-8). Table 5-10. Effect of planting date, autumn soil nitrogen content and crop species on biomass and cover crop N after winter leaching period (April 08, 1992). Treatment Biomass Cover crop N Treatment effects (P > F values) Planting date (D) NS NS Nitrogen (N) NS NS D x N NS NS Cover crop (C) 0.0001 0.0001 C x D 0.0111 0.0029 C x N NS NS C x D x N NS NS C V (%) 44 42 NS, not significant (P > 0.05); C O Chapter 5. Results 82 Figure 5-8. Spring barley and winter rye biomass (above) and nitrogen (below) in the spring of 1992. Error bars represent standard error of the mean (n = 8). Chapter 5. Results 83 Effects of planting date, autumn soil mineral N content and crop species on biomass and cover crop N in the spring of 1993 are summarized in Table 5-11. The strong interaction between planting date and crop species indicates that the biomass of spring barley increased with delay in planting but that of winter rye decreased (Figure 5-9). Despite the fact that August-planted spring barley winter-killed in late November and all September-planted cover crops survived the winter, cover crop N increased with increase in autumn N supply when planted in August but was least affected when planted a month later (Figure 5-9). Table 5-11. Effect of planting date, autumn soil mineral nitrogen content and cover crop species on biomass and cover crop Nafter winter leaching period (April 30, 1993). Treatment Biomass Cover crop N Treatment effects (P > F values) Planting date (D) NS 0.0297 Nitrogen (N) 0.0169 0.0006 D x N NS 0.0157 Cover crop (C) 0.0001 0.0001 C x D 0.0006 NS C x N NS NS C x D x N NS NS C V (%) 22 26 NS, not significant (P > 0.05); Chapter 5. Results 84 Biomass I I Spring Barley Winter Rye 120 Cover Crop N r — n 1 j 1 August/24/92 September/22/92 Aug/24/92 Sep/22/92 Planting date N-0 N-100 Autumn soil nitrogen content Figure 5-9. Spring barley and winter rye biomass (above) and nitrogen (below) in the spring of 1993 as influenced by planting date and autumn soil mineral N content. Error bars represent standard error of the mean (n = 8). Chapter 5. Results 85 In the spring of 1994, variations in cover crop biomass and N were independent of autumn soil mineral N content but were mainly affected by crop species (Table 5-12). Generally, cover crop biomass and N were greater than those of chickweed in fallow plots, although spring species residue biomass and N averaged only 1.4 t ha"' and 24 kg N ha"', respectively. A general comparison of winter species (winter rye and annual ryegrass) with autumn-planted spring species (barley, wheat and oat) showed that winter species had higher biomass and N relative to spring species. Table 5-12. Effect of autumn soil mineral nitrogen content and cover crop species on biomass and cover crop N after winter leaching period (April 14, 1994). Treatment Biomass Cover crop N Crop species Fallow/Chickweed (F) Spring barley (SB) Spring wheat (SW) Spring oat (SO) Winter rye (WR) Annual ryegrass (ARG) Nitrogen (N) Cover crop (C) C x N C V (%) F vs SB+SW+SO+WR+ARG SB vs SW+SO SW vs SO WR vs A R G SB+SW+SO vs WR+ARG Mean values^ 3.4 1.6 1.7 1.1 7.7 8.9 60 27 26 20 149 155 Treatment effects (P > F values) NS 0.0001 NS 16 NS 0.0001 NS 22 Orthogonal contrasts for cover crop 0.0001 0.0001 0.0001 0.0001 0.0005 NS NS NS 0.0001 0.0001 H Biomass is expressed as t ha and cover crop N as kg ha ; NS, not significant (P > 0.05); Chapter 5. Results 86 5-7.6. So/7 mineral nitrogen content after winter leaching period The influence of planting date, autumn soil mineral N content and crop species on residual mineral N in the 0-60 cm layer in spring of 1992 and 1993 varied with years. Variations in residual mineral N in the spring of 1992 (Table 5-13) were influenced by autumn planting date and crop species in the 0-20 cm layer and by crop species at all depth intervals. When spring barley was planted in August a greater increase in mineral N in the surface 20 cm was observed compared to fallow or winter rye (Figure 5-10). However, when cover crops were planted in September, mineral N was not considerably influenced by crop species. The main effect of crop species shows that growth of autumn-planted spring barley increased mineral N relative to fallow or winter rye at all depths (0-20, 0-40 and 0-60 cm). In contrast, variations in residual mineral N in the spring of 1993 were influenced by planting date and crop species at all depth intervals (Table 5-14). Residual mineral N in August-planted spring barley plots increased when compared to that under fallow but decreased under September-planted cover crops (Figure 5-11). Variations in residual mineral N in the 0-60 cm layer in the spring of 1994 were largely due to crop species, especially in the subsurface soil layers (Table 5-15). Similar to the previous two seasons, greater amounts of soil mineral N were measured under autumn-planted spring species (barley, wheat and oat) residues than in fallow/chickweed or winter species (winter rye and annual ryegrass) plots. Chapter 5. Results 87 Table 5-13. Effect of planting date, autumn soil nitrogen content and crop species on residual soil mineral N after winter leaching period (April 08, 1992). Soil sampling depth intervals (cm) Treatment 0-20 0-40 0-60 Mean values (kg ha"1) Crop species Fallow (F) 14 25 42 Spring Barley (SB) 19 40 59 Winter Rye (WR) 12 22 32 Treatment effects (P > F values)^ } Planting date (D) NS NS NS Nitrogen (N) NS NS NS D x N NS NS NS Cover crop (C) 0.0056 0.0001 0.0001 C x D 0.0040 NS NS C x N NS NS NS C x D x N NS NS NS CV(%) 11 5 4 Orthogonal contrasts for cover crop and interactions FvsSB+WR 0.0036 0.0001 0.0001 S B v s W R NS NS 0.0079 (FvsSB+WR)x(Dl vsD2) 0.0057 NS NS (SBvs W R ) x ( D l vsD2) 0.0382 NS NS ^ Analysis of variance performed on data transformed to natural logarithm scale; NS, not significant (P < 0.05); Chapter 5. Results Figure 5-10. Soil mineral N in spring of1992 in the 0-20 cm layer under fallow, spring barley and winter rye as influenced by planting date. Error bars represent standard error of the mean (n =8). Chapter 5. Results 89 Table 5-14. Effect of planting date, autumn soil mineral nitrogen content and crop species on residual soil mineral N after winter leaching period (April 30, 1993). Soil sampling depth intervals (cm) Treatment 0-20 0-40 0-60 Mean values (kg ha'1) Planting date August/24/1992 19 39 62 September/22/1992 13 25 45 Nitrogen level N-0 14 28 45 N-100 17 36 63 Crop species Fallow (F) 18 33 62 Spring Barley (SB) 18 40 65 Winter Rye (WR) 12 23 34 Treatment effects (P > F values)^ Planting date (D) NS 0.0330 0.0390 Nitrogen (N) NS 0.0355 0.0091 D x N NS NS NS Cover crop (C) 0.0001 0.0001 0.0001 C x D 0.0028 0.0001 0.0001 C x N NS NS NS C x D x N NS NS NS C V (%) 9 6 5 Orthogonal contrasts for cover crop and interactions F vs SB+WR 0.0172 0.0001 0.0001 SB vs WR 0.0001 0.0001 0.0001 (F vs SB+WR) x ( D l vs D2) 0.0116 0.0001 0.0001 (SB vs W R ) x ( D l vs D2) 0.0105 0.0053 0.0045 H Analysis of variance performed on data transformed to natural logarithm scale; NS, not significant (P < 0.05); Chapter 5. Results 90 Figure 5-11. Soil mineral N in spring of 1993 in the 0-60 cm layer under fallow, spring barley and winter rye as influenced by planting date. Error bars represent standard error of the mean (n =8). Chapter 5. Results 91 Table 5-15. Effect of autumn soil mineral nitrogen content and crop species on residual mineral N after winter leaching period (April 14, 1994). Soil sampling depth intervals (cm") Treatment . 0-20 0-40 0-60 Mean values (kg ha 1) Crop species Fallow/Chickweed (F) 18 34 54 Spring Barley (SB) 22 55 95 Spring Wheat (SW) 17 48 84 Spring oat (SO) 19 47 84 Winter Rye (WR) 19 33 45 Annual Ryegrass (ARG) 15 27 41 Treatment effects (P > F values)^ Nitrogen (N) NS NS NS Cover crop (C) NS 0.0001 0.0001 C x N NS NS NS CV(%) 10 5 5 Orthogonal contrasts for cover crop and interactions F vs SB+SW+SO+WR+ARG NS 0.0001 0.0001 SBvsSW+SO NS 0.0287 0.0045 S W v s S O NS 0.0113 NS WR vs A R G NS NS NS SB+SW+SO vs WR+ARG NS 0.0001 0.0001 ^Analysis of variance performed on data transformed to natural logarithm scale, NS, not significant (P > 0.05); Chapter 5. Results 92 5-1.7. Changes in cover crop biomass and nitrogen during winter A summary of statistical significance of biomass and cover crop N during the 1991-92, 1992-93 and 1993-94 winter cropping seasons is shown in Table 5-16. Table 5-16. Repeated measures analysis of variance (P > F values) for planting date (D), autumn soil nitrogen content (N), crop species (C) and time of sampling (T) effects on variations in biomass and cover crop N during the three cropping winters. Treatment Biomass Cover crop N (P > F values) 1991-92 T 0.0092 0.0002 T x D 0.0186 NS T x N NS NS T x D x N NS NS T x C 0.0001 0.0125 T x C x D 0.0214 0.0366 T x C x N NS NS T x C x D x N NS NS 1992- 93 T 0.0001 NS T x D 0.0001 0.0018 T x N NS 0.0200 T x D x N 0.0238 0.0109 T x C 0.0001 0.0024 T x C x D 0.0001 0.0307 T x C x N NS NS T x C x D x N NS NS 1993- 94 T NS 0.0001 T x N NS NS T x C 0.0001 0.0001 T x C x N NS NS Chapter 5. Results 93 The three-way interaction (1991-92 season) between time of sampling (T), planting date (D) and crop species (C) indicates that the biomass of spring barley decreased during winter but that of winter rye increased, and that the changes were dependent on planting date (Figure 5-12). The decrease in biomass for August-planted spring barley during winter was 41% and for the September-planted cover crop was 75%. In contrast, biomass increase during winter for August-and September-planted winter rye were 51 and 345%, respectively. Spring barley and August-planted winter rye N that was accumulated before late November, decreased during winter but that for September-planted cover crop increased. Similar variations in biomass and cover crop N with respect to time of sampling, planting date and crop species were observed in the 1992-93 winter cropping season (Table 5-16) and indicates the biomass for August-planted spring barley decreased by 34% during winter but that for September-planted crop increased by 459%. On the other hand, winter rye biomass increased during winter by 146 and 368% for the August- and September-planted crop, respectively (Figure 5-13). Cover crop N for August-planted spring barley decreased by 42% during winter but for the September-planted cover crop increased by only 22%. In contrast, N content of winter rye increased by 7 and 35% for the August- and September-planted cover crop, respectively. Variations in biomass and cover crop N during 1993-94 winter were affected by time of sampling and crop species (Table 5-16). The biomass for autumn-planted spring barley, spring wheat and spring oat decreased between November 24, 1993 and April 14, 1994 by 63, 69 and 80%o, respectively. In contrast, the biomass for chickweed (in fallow plots), winter rye and annual ryegrass increased by 24, 109 and 106%, respectively, within the 20-week period (Figure 5-14). Chapter 5. Results 94 Figure 5-12. Changes in biomass (above) and cover crop N (below) during 1991-92 winter as influenced by planting date and crop species. Error bars represent standard error of the mean (n = 8). Chapter 5. Results 95 Spring barley Spring barley T Planting dates | | August/24/92 September/22/92 Apr. 93 .1. 2H Winter rye Winter rye Apr.93 Apr.93 Apr.93 Sampling date Sampling date Figure 5-13. Changes in biomass (above) and cover crop N (below) during 1992-93 winter season as influenced by planting date and crop species. Error bars represent standard error of the mean (n = 8). Chapter 5. Results 96 Nitrogen accumulated by spring barley, spring wheat and spring oat prior to winter declined by 76, 84 and 86%, respectively. The increase in cover crop N between late November 1993 and April 1994 averaged only 4% for winter rye and annual ryegrass (Figure 5-14.). Figure 5-14. Changes in biomass (above) and cover crop N (below) during 1993-94 winter season as influenced by crop species. Error bars represent standard error of the mean (n = 8). Chapter 5. Results 97 5-1.8. Changes in soil mineral nitrogen during winter During the 1991-92 winter, the influence of time of sampling and crop species (Table 5-17) indicates that while soil mineral N (0-40 and 0-60 cm layers) in fallow and winter rye plots decreased (by 65 and 40%, respectively), that in autumn-planted spring barley tended to increase (7%) between November 21, 1991 and April 08, 1992 (Figure 5-15). The effect of time of sampling, planting date and nitrogen treatment (Table 5-17) indicates that while there was little change in soil mineral N during winter under the low autumn-N treatment (N-0), greater decrease occurred under high autumn-N treatment (N-100), especially when planting was delayed by one month (Figure 5-15). During 1992-93 winter, soil mineral N (0-60 cm layer) was influenced by time of sampling, planting date and crop species (Table 5-17). The three-way interaction between time of sampling, planting date and crop species indicates that soil mineral N in the 0-60 cm layer in fallow and September-planted plots decreased during winter but that under August-planted spring barley and winter rye increased (Figure 5-16). The decrease in mineral N under fallow and September-planted cover crops averaged 60 and 69%o, respectively. Soil mineral N under August-planted spring barley increased by 48%> and 17% under winter rye. Chapter 5. Results 98 Table 5-17. Repeated measures analysis of variance (P > F values) for planting date (D), autumn soil nitrogen content (N), crop species (C) and time of sampling (T) effects on variations in mineral N in the 0-60 cm soil layer. Soil sampling depth intervals (cm) Treatment 0-20 0-40 0-60 (P > F values)^ } 1991-92 T NS 0.0001 0.0001 T x D NS 0.0001 0.0001 T x N 0.0054 0.0001 0.0001 T x D x N NS 0.0144 0.0193 T x C NS 0.0001 0.0001 T x C x D 0.0130 NS NS T x C x N NS NS NS T x C x D x N NS NS NS 1992-93 T 0.0001 0.0001 0.0001 T x D 0.0001 0.0001 0.0001 T x N 0.0001 0.0001 0.0001 T x D x N NS NS NS T x C 0.0037 0.0001 0.0001 T x C x D NS 0.0001 0.0001 T x C x N 0.0397 NS 0.0027 T x C x D x N NS NS NS 1993-94 T 0.0001 0.0001 0.0001 T x N 0.0001 0.0001 0.0019 T x C 0.0041 0.0001 0.0001 T x C x N 0.0037 NS NS H Analysis of variance was performed on data transformed to natural logarithm scale; NS, not significant (P > 0.05); Chapter 5. Results 99 140 Nov91 Apr92 Sampling date 140 Nov91 Apr92 Nov91 Apr92 Sampling date Sampling date Figure 5-15. Changes in soil mineral N in the 0-60 cm layer during 1991-92 winter as influenced by crop species (a) and by planting date and autumn soil mineral N content (b). Error bars represent standard error of the mean [n = 16 for (a) and n = 12 for (b)J. Chapter 5. Results 101 In the 1993-94 winter cropping season, soil mineral N in the 0-60 cm layer was influenced by time of sampling, autumn soil mineral N content and crop species (Table 5-17). There was greater decrease in soil mineral N in the N-100 than N-0 treatment (0-60 cm layer) during winter (Figure 5-17). Soil mineral N in the N-100 and N-0 treatments decreased by 51 and 40%, respectively, during winter. The interaction between time of sampling and crop species indicates that during the 1993-94 winter, soil mineral N in the 0-60 cm layer decreased drastically under fallow/chickweed, winter rye and annual ryegrass (72, 51 and 62%, respectively), but a lesser decrease of 22, 27 and 31% occurred under autumn-planted spring barley, spring wheat and spring oat, respectively (Figure 5-17). In fact more soil mineral N was measured under the residues of autumn-planted spring species than either under fallow/chickweed or winter species (rye and ryegrass) in the spring. Chapter 5. Results 102 J N-0 Nov.93 Apr.94 Sampling date Figure 5-17. Changes in soil mineral N in the 0-60 cm layer over the 1993-94 winter as affected by autumn soil mineral N content (above) and cover crop species (below). Error bars represent standard error of the mean (n = 24 for above and n = 8 for below). Chapter 5. Results 103 5-1.9. Fertilizer nitrogen balance At the start of each season (late August), half of the plots received 100 kg N ha"' applied as Ca (N0 3 ) 2 . When averaged over planting date and crop species, about 47% of applied fertilizer N was recovered as soil N (ANR S O IL ) in the 0-60 cm layer and 28% in the cover crop ( A N R C R O p ) in late November 1991 (Table 5-18), indicating that about 25% of applied fertilizer N was not accounted for. On average, 69% of applied fertilizer N was recovered in fallow soil and most of it was concentrated in the subsurface soil layers. The proportion of fertilizer N recovered under cover crops and distribution in the 0-60 cm layer varied with planting date and crop species. Fertilizer N recovered in August-planted spring barley was 41% and that accounted for in soil was 21%). Most of the fertilizer N under August-planted spring barley was concentrated in the 0-20 cm layer and decreased sharply with depth. In contrast, about 42% of applied fertilizer N was recovered in September-planted spring barley and 54% in soil with uniform distribution in the 0-60 cm layer. On the other hand, the proportions of fertilizer N recovered in August- and September-planted winter rye were 24 and 7%, respectively; apparent N recovery in soil accounted for 36 and 30% for August- and September-planted cover crop, respectively. Generally, apparent recovery of N under winter rye increased sharply with depth, indicating movement of N 0 3 " into the subsurface soil layers. Generally, low fertilizer N recoveries were observed in the crops and soil in the spring of 1992, except under August-planted spring barley which winter-killed in late November 1991. Most of the fertilizer N (82%) in August-planted spring barley was recovered in the soil. Chapter 5. Results 104 Table 5-18. Balance sheet for applied fertilizer NOf-N (100 kg N ha'1) for 1991-92 season as estimated by the difference method. August-planted (Aug/19/1991) September-planted (Sep/16/1991) N recovered in: Fallow Barley Rye Fallow Barley Rye Cover crop - 41 Soil 0-20 cm 4 18 20-40 cm 20 6 40-60 cm 28 -3 0-60 cm 52 21 Total recovered 52 62 Unaccounted for 48 38 Cover crop - 7 Soil 0-20 cm 0 7 20-40 cm 2 11 40-60 cm 9 13 0-60 cm 11 31 Total recovered 11 38 Unaccounted for 89 62 November 21, 1991 24 - 42 7 4 8 14 4 10 30 23 13 22 47 17 13 36 85 54 30 60 85 96 37 40 15 4 63 April 08, 1992 -5 - 3 11 3 - 4 2 0 4 - 1 - 8 1 0 3 10 -1 7 - 2 4 0 2 -2 7 11 98 102 93 89 Chapter 5. Results 105 Virtually all the fertilizer N was accounted for in the crops and soil before winter leaching period of 1992 (Table 5-19). When averaged over planting date and crop species, apparent fertilizer N recovered in soil was 76% of that applied and 36% was recovered in the cover crop. Comparatively, August-planted spring barley and winter rye recovered a considerably higher proportion of applied fertilizer N before winter leaching period than September-planted cover crops. When averaged over crop species, fertilizer N recovered in August- and September-planted cover crops (ANRCROP) were 69 and 4%, respectively. In contrast, the proportions recovered in soil (ANRSOIL) for August- and September-planted cover crops were 42 and 86%, respectively. Most of the fertilizer N on fallow plots was measured in the 0-20 cm layer and generally decreased sharply with depth, indicating limited movement into the subsurface layers (20-40 and 40-60 cm). On the other hand, fertilizer N recovered (ANR S OIL) under August-planted cover crops gradually decreased and that under September-planted cover crops increased with depth. Generally, a large proportion of applied fertilizer N was not accounted for in the spring of 1993 on fallow (78%) and September-planted (90%) plots. In contrast, about 50% of applied fertilizer N was recovered in crop and soil (0-60 cm layer) when the cover crops were planted in August. Nearly all the fertilizer N accounted for (89%) was in the soil on August-planted spring barley plots. On the other hand, most of the fertilizer N recovered on August-planted winter rye plots was in the cover crop (80%). The small proportion of applied fertilizer N recovered in winter-killed spring barley residues and the high proportion in soil was attributed to a combination of leaching of N from residues and, decomposition and mineralization of residue N. Lower N recovery in winter rye in spring than in late November 1992 was caused by partial freezing damage to the August-planted crop which received additional 100 kg N ha"1 (see Figure Chapter 5. Results 106 5-7). The small proportion of applied N under fallow and September-planted cover crops was likely caused by NCV leaching during winter. Table 5-19. Balance sheet for applied fertilizer NOi-N (100 kg N ha') for 1992-93 season as estimated by the difference method. August-planted (Aug/24/1992) September-planted (Sep/22/1992) N recovered in: Fallow Barley Rye Fallow Barley Rye Cover crop - 62 Soil 0-20 cm 68 19 20-40 cm 18 16 40-60 cm 13 14 0-60 cm 99 49 Total recovered 99 111 Unaccounted for 1 -11 Cover crop - 6 Soil 0-20 cm 3 7 20-40 cm 6 15 40-60 cm 14 27 0-60 cm 23 49 Total recovered 23 55 Unaccounted for 77 45 November 24.1992 75 - 3 5 20 63 8 19 9 26 46 47 5 13 27 23 34 102 81 89 109 102 84 94 -9 -2 16 6 April 30. 1993 36 - -5 19 7 - 1 4 - 1 3 2 0 0 -1 20 4 0 9 21 8 -1 45 21 3 18 55 79 97 82 Chapter 5. Results 107 In late November 1993, about 50% of applied fertilizer N was accounted for in soil (ANRSOIL) and only 8% was recovered in the crop ( A N R C R O p ) . Most of the fertilizer N0 3 ' -N was recovered in the 0-20 cm soil layer and decreased sharply with depth (Table 5-20). In all the three seasons, a significant proportion of the fertilizer N was not accounted for in the spring by the difference method. Table 5-20. Balance sheet for appliedfertilizer NOi-N (100 kg N ha'1) for 1993-94 season as estimated by the difference method. N recovered in: Fallow Barley Wheat Oats Rye Ryegrass Cover crop 21 November 24. 1993 13 3 Soil 0-20 cm 38 20-40 cm 11 40-60 cm 2 0-60 cm 51 Total recovered 57 Unaccounted for 43 45 8 1 54 75 25 30 11 3 44 57 43 44 6 1 51 54 46 37 7 2 46 47 53 46 4 6 56 59 41 Cover crop April 14. 1994 5 1 Soil 0-20 cm 1 20-40 cm -1 40-60 cm 5 0-60 cm 5 Total recovered 2 Unaccounted for 98 -1 9 -7 1 3 97 -3 12 11 20 25 75 2 5 17 24 25 75 4 -2 1 3 7 93 -2 -1 6 3 10 90 \ Proportion of fertilizer N apparently recovered in chickweed; Chapter 5. Results 108 5-1.10. Cover crop carbon to nitrogen ratios In late November 1991, variations in the C/N ratios were largely due to planting date, autumn soil mineral N content and crop species (Table 5-21). The increase in autumn N supply by 100 kg ha"1 decreased the C/N ratios substantially in August-planted cover crops but had little influence on the C/N ratios when the cover crops were planted in September (Figure 5-18). Spring barley had higher C/N ratio than winter rye when the two cover crops were planted in August but species differences were not reflected in September-planted cover crops (Figure 5-18) . In the spring of 1992, winter rye C/N ratio was significantly higher than that for spring barley residues, despite loss of N from the residues due to leaching and decomposition (Figure 5-19) . Table 5-2 J. Analysis of variance for the effect ofplanting date, autumn soil mineral nitrogen content and crop species on C/N ratio during the 1991-92 and 1992-93 cover cropping seasons. Treatment Nov/21/1992 Apr/08/1992 Nov/24/1992 Apr/30/1993 (P > F values) Planting date (D) 0.0098 NS 0.0087 0.0065 Nitrogen (N) 0.0048 NS- 0.0050 NS D x N 0.0269 NS 0.0385 0.0012 Cover crop (C) 0.0006 0.0001 NS 0.0001 D x C 0.0010 NS 0.0001 0.0001 N x C NS NS 0.0064 NS D x N x C NS NS 0.0216 NS Chapter 5. Results 109 Aug/19/91 D x IN Planting date N-100 Sep/16/91 Aug/19/91 D x C C | Spring Barley Winter Rye Sep/16/91 Planting date Figure 5-18. Cover crop C/N ratios in late November 1991 as influenced by planting date and autumn soil N content (above) and, planting date and crop species (below). Error bars represent standard error of the mean (n = 8). D x N- planting date x nitrogen interaction; D x CC = planting date x crop species interaction. Chapter 5. Results 110 50 40 30 20 H Spring Barley Winter Rye Cover crop species Figure 5-19. The C/N ratios for spring barley residues and winter rye in the spring of 1992. Error bars represent standard error of the mean (n = 16). Variations in C/N ratios of cover crops in late November 1992 and spring of 1993 were largely due to planting date, autumn soil N content and crop species (Table 5-21). The increase in autumn N supply by 100 kg ha"1 decreased the C/N ratios in August-planted spring barley and winter rye by 31 and 9%, respectively, but had no influence on the C/N ratios of the cover crops that were planted in September (Figure 5-20). In the spring of 1993, increase in autumn N supply by 100 kg ha"1 caused a 15% decrease in the C/N ratios of August-planted cover crops but slightly increased the C/N ratios by 6% when the cover crops were planted in September (Figure 5-21). § •3 s: J 3 V) K 3 • f t i Os <5 N/3 z z s °W& HJD •2 C3 § § Q •S "C3 I « co S -s: o cu § e C3 o 3^ K o CJ a ^ 2 Chapter 5. Results 112 D x N Aug/24/92 I I N-0 N-100 Planting date Sep/22/92 .2 25 H U 20 Aug/24/92 j | Spring Barley I Winter Rye Sep/22/92 Planting date Figure 5-21. Cover crop C/N ratios in the spring of 1993 as influenced by planting date and autumn soil N content (above) and, planting date and crop species (below). Error bars represent standard error of the mean (n = 8). D x N = planting date x nitrogen interaction; D x CC = planting date x crop species interaction Chapter 5. Results 113 Winter rye had a significantly higher C/N ratio than spring barley residues when the cover crops were planted in August but the opposite was true for September-planted cover crops (Figure 5-In late November 1993 and spring of 1994, variations in the C/N ratios were mainly influenced by crop species (Table 5-22). The C/N ratios of autumn-planted spring species in late November 1993 were generally higher than those of winter species including chickweed in fallow plots but the opposite was true in the spring of 1994 (Figure 5-22). Table 5-22. Analysis of variance for the effect of autumn soil mineral nitrogen content and crop species on C/N ratio in the 1993-94 cover cropping season. 21). Treatment Nov/24/1993 Apr/14/1994 (P > F values) Nitrogen (N) NS NS Cover crop (C) 0.0001 0.0077 C x N 0.0090 NS Chapter 5. Results 114 50 -r 45 -40 -35 -30 -o Chickweed Spring Barley Spring Wheat Spring Oats Winter Rye Annual Ryegrass Cover Crop Species Figure 5-22. Cover crop C/N ratios in late November 1993 (above) and in the spring of 1994 (below). Error bars represent standard error of the mean (n = 8). Chapter 5. Results 115 5-2. NITROGEN DYNAMICS OF SPRING GRAMINACEOUS COVER CROPS 5-2.1. Nitrogen retention by spring species during winter The statistical significance of planting date and autumn soil mineral N content effects on spring barley residue biomass remaining (% of D M in November) and N retained (% of T N in November) over the 1991-92 and 1992-93 winter cropping seasons are summarized in Table 5-23. Biomass remaining and N retained in the residues of autumn-planted spring barley at spring time were independent of autumn soil mineral N content but were largely influenced by planting date. In both seasons, planting of spring barley in August increased both the biomass remaining and N retained in the residue over the winter as compared to planting one month later (Figure 5-23). Nitrogen retention values for the 1991-92 winter cropping season were generally lower than those for 1992-93 season. Table 5-23. Analysis of variance (P > F values) for effects of planting date (D) and autumn soil mineral nitrogen content (N) on residue biomass remaining and nitrogen retention by spring barley over the 1991-92 and 1992-93 winter seasons. Biomass remaining Nitrogen retained Treatment 1991-92 1992-93 1991-92 1992-93 Planting date (D) 0.0154 0.0222 0.0066 0.0077 Nitrogen (N) NS 1 NS NS NS D x N NS NS NS NS 1 NS, not significant (P > 0.05); Chapter 5. Results 116 6 Planting dates August September 1991-92 1992-93 Winter cropping season 80 70 Nitrogen E 60 50 40 30 20 10 H 1991-92 1992-93 Winter cropping season Figure 5-23. Influence of planting date on biomass remaining (above) and N retention (below) by spring barley over the 1991-92 and 1992-93 cover cropping seasons. Error bars represent standard error of the mean (n - 8). Chapter 5. Results 117 Biomass remaining and N retention over the 1993-94 winter season were independent of autumn soil mineral N content but were influenced by crop species (Table 5-24). The proportions of spring barley residue biomass remaining (%DM in November) and N retained (%TN in November) in residue at spring time were relatively greater than that of spring wheat or oat regardless of whether the residues were in contact with soil (Figure 5-24). Generally, the proportions of biomass remaining and N retained in the residues for the three spring species were higher for samples not in contact with soil than for residues on the soil surface. This was consistent with the 1991-92 and 1992-93 season results for spring barley (see Figure 5-23). In the 1991-92 season, the residue in meshbags was in contact with the soil surface but in the 1992-93 season the residue was placed on a wire mesh table to avoid soil contact and the results indicate that in addition to leaching following rainfall events, microbial decomposition likely contributed to loss of N from meshbag residues that were on the soil surface. Table 5-24. Analysis of variance (P > F values) for effects of autumn soil mineral nitrogen content (N) and crop species (C) on winter-killed crop residue biomass remaining and nitrogen retention over the 1993-94 winter. Biomass remaining Nitrogen retained Treatment Surface residue Meshbag residue Surface residue Meshbag residue Nitrogen (N) NS 1 NS NS NS Cover crop (C) 0.0317 0.0023 0.0326 0.0057 N x C NS NS NS NS 1 NS, not significant (P > 0.05); Chapter 5. Results 118 Spring Burloy Spring Wheat Spring Outs Cover Crop Species Figure 5-24. Residue biomass remaining (above) and nitrogen retention (below) in the spring of 1994 for surface and meshbag residues not in contact with soil. Error bars represent standard error of the mean (n = 8). MSDo.m is the minimum significant difference according to Tukey's (HSD) test at P < 0.05. Chapter 5. Results 119 5-2.2. Spring species nitrogen fractions at winter-kill A summary of analyses of variance for the effect of planting date and autumn soil mineral N content on the various spring barley N fractions at winter-kill (late November 1992) is shown in Table 5-25. The proportion of original biomass remaining after cold-water extraction was comparatively greater for August- than September-planted spring barley. Autumn soil mineral N content had no effect on the proportion of biomass remaining after extraction. Table 5-25. Analysis of variance (P > F-values) for effects of planting date (D) and autumn soil mineral nitrogen content (N) on the various nitrogen fractions of spring barley at winter-kill during the 1992-93 winter cropping season.^ Treatment D M WIN PN NPN N N A N Date(D) 0.0099 0.0001 0.0001 0.0261 0.0168 0.0001 Nitrogen (N) NS 0.0101 0.0144 0.0266 0.0013 0.0001 D x N NS 0.0131 0.0109 NS 0.0052 0.0001 H Analysis of variance was performed on data expressed on a kg ha"' basis, except for DM; NS, not significant (P > 0.05); DM, residue biomass remaining after extraction (% of original); WIN, cold-water insoluble N; PN, TCA-precipitable protein N; NPN, organic nonprotein N; NN, nitrate N; AN, ammonium N; Cold-water insoluble N (WIN), protein N (PN), NCV-N (NN) and N H / - N (AN) for spring barley increased with additional N supply in autumn when the cover crop was planted in August but changed little for the September-planted crop (Figure 5-25). In contrast, organic nonprotein N (NPN) increased when the cover crop was either planted in August relative to September or when autumn soil mineral N content was increased (Table 5-26). Chapter 5. Results 120 90 80 CWIN •S 50 10 0 90 80 70 60 m 50 z NN 40 • 30 • 20 • 10 • Planting dates | | Aug/24/92 Sep/22/92 N-0 N-100 Autumn soil mineral nitrogen content 90 80 70 60 J> 50 Z | 4 0 g a. 30 20 10 0 90 80 70 _p 60 j= 60 * 50 Z E 1 40 o < 30 20 10 0 PN AN i la N-0 N-100 Autumn soil mineral nitrogen content Figure 5-25. Effect of planting date and autumn soil mineral nitrogen content on cold-water insoluble-, protein-, NOi- and NH4+-Nfractions of spring barley at winter-kill in late November 1992. Error bars represent standard error of the mean (n = 4). CWIN, cold-water insoluble N; PN, TCA-precipitable protein N; NPN, organic nonprotein N; NN, nitrate N; AN, ammonium N; Chapter 5. Results 121 Planting of spring barley in August as compared with September, increased organic NPN by 127% and increasing autumn soil mineral content by 100 kg ha"1 caused a 94%> increase.! Table 5-26. Effect of planting date and autumn soil mineral nitrogen content on organic nonprotein N (NPN) fraction (kg ha'1) at winter-kill in late November 1992. Planting date main effect Nitrogen main effect August/24/92 11.8±2.64 N-0 5.812.81 September/22/92 5.2±0.44 N-100 11.2±0.54 The effects of autumn soil mineral N content and crop species on the various spring species N fractions at winter-kill (late November 1993) are shown in Table 5-27. Biomass remaining (% of biomass before extraction) after cold-water extraction was not influenced by autumn soil mineral N content but was significantly affected by crop species. Spring barley and wheat residue biomass remaining after extraction were comparable but significantly greater than that of spring oat (Table 5-28). At winter-kill in late November 1993, none of the N fractions were influenced by autumn soil mineral N content but were largely dependent on crop species (Tables 5-27). Spring barley and oat contained similar amounts of cold-water insoluble N but significantly less than that of spring wheat (Table 5-28). Spring wheat and oat had the same amount of N associated with soluble proteins but considerably greater than the amount in spring barley. The three species had comparable amounts of organic nonprotein N (NPN), NOy-N and NH 4 + -N. Chapter 5. Results 122 Table 5-27. Analysis of variance (P > F-values) for effects of autumn soil mineral N content (N) and cover crop species (C) on the various spring species nitrogen fractions at winter-kill in late November 1993.^ Treatment D M WIN PN NPN N N A N Nitrogen (N) NS NS NS NS NS NS Cover crop (C) 0.0001 0.0001 0.0012 NS NS NS N x C NS NS NS NS NS NS H Analysis of variance was performed on data expressed on a kg ha"' basis, except for DM; DM, residue biomass remaining after extraction (% of original);W!N, cold-water insoluble N; PN, TCA-precipitable protein N; NPN, organic nonprotein N; NN, nitrate N; AN, ammonium N; NS, not significant (P > 0.05); Table 5-28. Effect of cover crop species (C) on the various spring species nitrogen fractions at winter-kill in late November 1993. \ Cover crop D M WIN PN NPN N N A N (kg ha') Spring barley 70a 30b 37b 25a 19a 3a Spring wheat 68a 47a 57a 31a 18a 3a Spring oat 60b 30b 58a 29a 22a 3a K DM, residue biomass remaining after extraction (%of original DM); WIN, cold-water insoluble N; PN, TCA-precipitable protein N; NPN, organic nonprotein N; NN, nitrate N; AN, ammonium N; values in columns followed by the same letter(s) are not significantly different according to Tukey's (HSD) test (P > 0.05); Chapter 5. Results 123 5-2.3. Protein nitrogen fractions and nitrogen retained in meshbag residues in the field Generally, most of the N accumulated by the cover crops was in the form of organic N . In 1992-93 and 1993-94, spring species in meshbags were placed on wire mesh tables to estimate N retention without contamination from soil. The proportion of N retained (NR) in the residues of autumn-planted spring species during winter was compared with cold-water insoluble N (WIN) and with total protein N (WIN + PN) as paired observations using a two-tailed T test (Table 5-29). Table 5-29. Comparison of cold-water insoluble N (WIN) and total protein N fractions with N retained in meshbag residues in the field in 1992-93 and 1993-94 seasons.^ Autumn planting dates Nitrogen fractions August/24/1992 September/22/1992 August/24/1993 (%TN) WIN 22(±1.2) 25(±1.7) 26(±1.9) WIN + PN 79(±1.4) 72(+1.8) 62(±2.0) NR 48(±1.2) 28(±1.8) 48(±2.1) Comparisons (P > T) WINvsNR 0.0001 0.2904 0.0007 (WIN + PN)vsNR 0.0001 0.0001 0.0001 t WIN and PN, cold-water insoluble- and soluble protein N fractions, respectively (% of total N for all fractions); NR, N retained in meshbags placed on mesh tables to avoid soil contact (% of total N in November 1992 or 1993); values in brackets are standard errors of the means (n = 8 for 1992-93 and n = 24 for 1993-94); Chapter 5. Results 124 The proportion of N retained (NR) in the residues following winter-kill was greater than that of cold-water insoluble N (WIN) but less than that of total protein N (WIN + PN) when the cover crops were planted in August. When cover crops were planted in September, the proportion of N retained in the residue was similar to that of cold-water insoluble N but significantly less than that of total protein N. It appears that for August-planted spring barley, in addition to structural N (cold-water insoluble N), a proportion of the N associated with cold-water soluble protein N was retained in the residue following winter-kill. In contrast, when spring barley was planted in September, only the N associated with cold-water insoluble fraction (WIN) was retained in the residues. Chapter 6. Discussion 125 Chapter 6: DISCUSSION 6-1. Cover crop and soil nitrogen before winter leaching period August- relative to September-planting of spring barley and winter rye increased N uptake before winter leaching period of 1991 and 1992. Although increased N uptake was accompanied by reduction in residual mineral N (0-60 cm layer), this was only significant in late November 1992. This suggested that, apart from changes in soil mineral N due to N uptake by cover crops in 1991, other processes, most likely N0 3"-N leaching from the 0-60 cm soil layer, denitrification and immobilization contributed to variations in residual mineral N. Displacement of N0 3~ from the surface 20 cm to the subsurface soil layers in 1991 started in the last week of August due to intense rainfall events (see Figure 4-2) and was evident at the late planting date (see Table 5-1). This early displacement of N0 3~ may have resulted in movement of some of the N0 3~ below the 0-60 cm soil layer when intense rainfall resumed in November. The differences in soil mineral N due to N uptake were likely offset by losses due to a combination of leaching of N0 3~, denitrification and immobilization. The water table rose to within 20 and 60 cm of the soil surface in late November 1991 and 1992, respectively. This may also have influenced soil N measurements in 1991 through a dilution effect. Widdowson et al. (1987) in Great Britain, and Sorensen (1992) in Denmark have reported an inverse relationship between N uptake and residual mineral N with winter wheat and annual ryegrass, when the cover crops were planted early. Winter wheat was planted in September and October, while annual ryegrass was planted in the middle of July, beginning of August and middle of August. Increased growth period, warmer soil temperatures and longer daylength associated with early planting of winter crops stimulates tillering. Development of greater numbers of tillers per unit area and higher total biomass production increases the potential for Chapter 6. Discussion 126 more cover crop N accumulation. Thus, N uptake by cover crops is related to the length of time the crop is allowed to grow. When cover crops are primarily planted to capture NOV, extending the vegetative growth period will maximize N uptake. But the N taken up by cover crops during autumn and early winter months is the most critical in terms of reducing NOV leaching into groundwater (Bergstrom and Brink, 1986). Spring barley was not different from winter rye in terms of biomass production and N uptake. The difference in growth stages of spring barley and winter rye (especially for August-planted crops) at the time of sampling in late November is large (see Table 3-5.), but winter rye produces more tillers compared to spring barley. Despite winter rye and spring barley absorbing similar amounts of mineral N by late November 1991 and 1992, less mineral N was measured in winter rye than spring barley plots in the 0-60 cm soil layer. In fact planting of winter rye in August in 1992 caused greater reduction in soil mineral N (0-60 cm layer) than spring barley but the two cover crops had little influence when planted a month later (see Figure 5-3). The data also indicated that under low autumn N supply, winter rye resulted in greater soil mineral N reduction than spring barley but the two cover crops were comparable when an additional 100 kg N ha"1 was applied. This was likely due to species differences in the root system of winter rye and spring barley; for example, differences in root biomass and consequently amount of N accumulated in roots. Mitchell and Teel (1977) in their evaluation of winter annual cover crops for no-tillage corn production have reported a 4-fold increase in root biomass and N accumulated in roots of winter rye relative to that of spring oat. These parameters were not determined in this study. Also, the rhizosphere plays a major role in terms of plant nutrient turnover and availability; among other factors the type and quantity of root exudates that influence rhizosphere microbial activity depend on plant species (Rovira and Davey, 1974). The difference thus may suggest greater immobilization of mineral N in the root zone (Jansson and Persson, 1982) of Chapter 6. Discussion 127 winter rye than spring barley. Also, root exudates stimulate microbial growth and activity in the rhizosphere of plants (Rovira, 1965). Woldendorp (1963) has suggested that roots and root exudates of plants may contribute sufficient organic matter to stimulate denitrification. The difference in type and quantity of root exudates of spring barley and winter rye may have resulted in different denitrification losses under the two cover crops. 6-2. Impact of cover cropping on soil N03' before winter leaching period Cover crops minimize the problem of N0 3" leaching by assimilation of mineral N, thus converting the most readily teachable N0 3" to organic forms. Thus, planting of cover crops can decrease N0 3 " leaching by reducing the amount of the available form of N for leaching during winter. I made assessment of cover cropping effect on soil N0 3"-N by calculating percent N0 3~-N reduction (PR) due to the cover crop. This method has been used by Meisinger et al. (1991) and Brandi-Dohrn et al. (1997) to estimate the impact of cover crops on soil N0 3~-N. However, one weakness of the method may arise on particular soils, especially on fine-textured or sloping soils where cover cropping can reduce runoff and increase water infiltration in cover crop relative to fallow plots (Langdale et al., 1979; McVay et al., 1989; Hermawan, 1995); and thus overestimate cover crop influence on soil N0 3"-N content. In this study, the decrease of N0 3"-N in the 0-60 cm layer due to cover cropping (PR) was variable among the years, and generally ranged from 33 to 91%. The values reported in this study represent potential reduction in N0 3"-N leaching as actual leaching measurements were not obtained. Similar field plot studies in other regions have shown variable results. Staver and Brinsfield (1990) in a 1-year study have reported a 77%> reduction in N0 3"-N (0-30 cm layer) due to winter rye (planted in first week October) cropping on silty loam soil in Maryland. In Denmark, Nielsen and Jensen (1985) in a 2-year study using annual ryegrass Chapter 6. Discussion 128 on a sandy loam soil found that the cover crop reduced N0 3"-N by 62% in the 0-100 cm layer. In France, a 1-year field plot study with rape, radish and winter rye (planted in October) found reduction in N0 3"-N due to cover cropping of 35, 44 and 59%, respectively, in the 0-100 cm layer (Muller et al., 1989). Brandi-Dohrn et al. (1997) in a 3-year study have reported percent N0 3"-N reduction due to cover cropping with winter rye ranging from 25 to 62% on Willamette loam in Oregon. A 1-year 1 5 N lysimeter study in France found that annual ryegrass cover crop reduced N0 3"-N leaching by 64% (Martinez and Guiraud, 1990). While all the studies reviewed indicate that cover cropping reduces N0 3"-N and thus the potential to leach out of the root zone, it is difficult to compare the results due to inadequate reporting of such factors as soil mineral levels at planting, winter soil and air temperatures, winter precipitation, cover crop biomass production, cover crop N uptake and N0 3"-N leaching losses. Further, as indicated above, estimates of the impact of cover crops on N0 3"-N in the studies were based on different depth intervals. When planted in August, winter rye was more efficient than spring barley in reducing soil N0 3" levels in the 0-60 cm layer before winter leaching period of 1992 but when planted one month later the two cover crops were equally less effective (see Figure 5-5). This was consistent with soil mineral N data (see Figure 5-3) and was attributed to species differences in the root systems of the cover crops, for example root biomass and N content. Measurements of the N accumulated in cover crop roots are usually difficult to make but are equally as important as those for N in the aboveground portions. Despite differences due to growth stages and soil N contents at sampling time, literature estimates of root N for winter rye and annual ryegrass are 25 and 33%) of total N (root N + shoot N), respectively (Pieters, 1927; McVickar et al., 1946; Mitchell and Teel, 1977). Root biomass and total N were not determined in this study. The inclusion of root contribution to total cover crop biomass and N measurements may likely Chapter 6. Discussion 129 improve the relationship between biomass production and residual mineral N in late autumn as soil N mineralization may be negligible due to cold temperatures. 6-3. Cover crop and soil nitrogen after winter leaching period In all the three winter cropping seasons, spring species that were planted in August winter-killed in late November (start of winter). Their biomass and N decreased considerably during winter and the decrease was accompanied by a significant increase in soil mineral N in the 0-60 cm layer relative to the amount measured in fallow plots in spring (see Figures 5-10 and 5-11). This observation is the first to be reported on Lower Fraser Valley soils and indicates that August planting of spring species may be beneficial in terms of conserving autumn mineral N (N03~ in particular). August planting of spring species results in greater biomass production and N uptake before winter leaching period (Bomke and Temple, 1994). There are also indications that August-planted spring species have a greater potential to retain accumulated N than September-planted cover crops by spring time (see Figure 5-23). In addition to cold-water insoluble N (structural N), August-planted spring species may retain some of the water-soluble N, most likely N associated with soluble proteins (see Table 5-29). This may be explained as follows; plants contain very diverse secondary products (phenolic compounds) and these products, especially tannins and lignins accumulate with plant age (Albersheim, 1965; Swain, 1965; Mengel and Kirkby, 1987; Marschner, 1995; Sorenson, 1992; Walton, 1983). Low molecular weight phenolic compounds combine with proteins reversibly by hydrogen bonding, and irreversibly by oxidation to quinones followed by covalent condensations (Loomis and Battaile, 1966; Loomis, 1974). Higher molecular weight phenolics (tannins) form insoluble complexes with proteins, removing them Chapter 6. Discussion 130 from solution (Swain, 1965; Loomis, 1974; Gagenheimer, 1990). The interaction of plant phenolics with proteins at the time of winter-kill thus may not only limit the chances of enzymatic protein breakdown but also alter their physical properties and confine protein N within the residues. Accumulation of fibre content (lignin in particular) with plant age also increases the mechanical strength of the residues and may not only reduce microbial accessibility but also cause resistance of the residues to raindrop impact. The effect of August-planted spring species mulch on soil mineral N may indicate that residue N readily mineralizes in the spring thus increasing soil mineral N content and may be particularly significant with respect to main season crop production as N uptake and N availability may be synchronized. This can influence the amount of supplemental N fertilizer required for subsequent summer crop production. In the Netherlands, it has been determined that optimum fertilizer N application rate decreases with increasing amounts of soil mineral N (Neeteson et al., 1989). The processes of organic matter decomposition are largely controlled by soil microorganisms and are therefore influenced by weather (temperature and moisture), pH and soil aeration (Jenkinson, 1981). Another factor that influences the breakdown of organic matter is the chemical make-up. Generally, crop residues contain about the same amount of carbon (approx. 40% on dry-weight basis) and their N contents are usually compared on the basis of C/N ratios. In this study carbon content (%DM) of cover crops ranged from 37 to 44%. Although not always, the C/N ratio can be useful in predicting residue decomposition rates. A negative correlation exists between decomposition rate constants and the C/N ratio of plant residues (Tian et al., 1992; Christensen, 1986; Keya, 1975; Nyamai, 1992). Plant materials that are low in lignin and other polyphenols, and high in N and soluble carbohydrates, generally undergo relatively rapid Chapter 6. Discussion 131 decomposition (Tian et al., 1995). Thus the rate of initial breakdown is dependent on the age of the tissues as well as species. Three possible factors may be responsible for rapid release of N (see Figures 5-10 and 5-11) from the residues of August-planted spring species in spring. Firstly, the residues provide a favourable habitat for soil fauna, notably earthworms {Lumbricus rubellus) which may accelerate decomposition. Hermawan (1995), while working on the same plots (1993-94 season) reported a higher earthworm population in the spring on spring barley residue than on bare (fallow) plots. Secondly, mineralization of N from plant residues is dependent on the quality of the residue in terms of lignin and polyphenol contents, and C/N ratio. An inverse relationship exists between the rate of plant residue decomposition and the three residue quality factors (Tian et al., 1992). Parnas (1975) studied the relationship between N immobilization and C/N ratio, and concluded that N immobilization occurs if the residue has a C/N ratio greater than 30 because all the N is utilized by microorganisms. Biochemical composition (lignin and polyphenols) of the cover crops were not determined in this study but the quality of August-planted spring species at winter-kill in terms of the C/N ratio was in the range that favours net N mineralization (see Sec. 5-1.10). The C/N ratios for August-planted spring species in all the three seasons ranged from 13 to 25 under low autumn soil N (N-0) and 9 to 20 under high autumn soil N (N-100). It is also possible that, readily available C from cover crop mulch can have a priming effect on soil organic matter, enhancing mineralization of soil organic N (Stevenson, 1982). August-planted winter rye was susceptible to freezing damage in late November, especially when autumn soil N content was increased by 100 kg ha"1 (see Figure 5-7). Several factors are known to influence hardening of plants. Susceptibility of August-planted winter rye to freezing damage may be explained as follows. August planting exposes winter rye to long days and high temperatures, both of which enhance growth rate and developmental stages of the plants. Chapter 6. Discussion 132 Freezing tolerance in winter annuals is inversely related to both growth rate and developmental stages of plants (Levitt, 1980). Many reports (Levitt, 1956) indicate that full hardening of plants is not achieved in the presence of excess N. This is attributable to the fact that N also promotes growth rate through increase in leaf area index and therefore the potential for photosynthesis; consequently root growth and biomass production increase (Marschner, 1995: Wild, 1988). Nitrogen also increases the size of plant cells and reduces the thickness of their walls and thus makes plants more susceptible to freezing damage (Wild, 1988). Despite the fact that August-planted winter rye may be as effective as spring species in accumulating and retaining significant amounts of N during winter, other studies indicate that high C/N ratios due to rapid spring growth result in N immobilization, increasing fertilizer needs for summer crops (Hargrove and Frye, 1987; Wagger and Mengel, 1988; Holderbaum et al., 1990). The potential for winter rye to release N during summer in these studies was indirectly determined using response of summer crops (biomass and N uptake) as an estimator of N contribution by the cover crop either after incorporation or herbicide-kill in the spring. In this study, C/N ratios for winter rye in the spring varied among the years and ranged from 31 to 44 (see sec. 5-1.10). 6-4. Fertilizer nitrogen balance Before winter leaching period of 1993-94, about 42% of the fertilizer N was not accounted for in soil and crop (see Table 5-20) despite little indication of leaching. Total precipitation between August and November 1993 was only 40% of the long-term (1937-90) normal, while that between August and November in 1991 and 1992 was 103 and 90% of the long-term normal, respectively (see Table 3-1). Lack of complete fertilizer N recovery may be explained in three Chapter 6. Discussion 133 ways. Firstly, N was not limiting at the study location (see Table 5-1) and it is evident from statistical analysis (see Table 5-4) that cover crop biomass and N uptake were not influenced by the addition of 100 kg N ha"1; thus the difference method may have resulted in small apparent fertilizer N recovery by the cover crops. Secondly, there is evidence that although cropping (especially with grasses) reduces soil mineral N content through uptake, rhizosphere microorganisms can utilize root exudates as a source of energy to cause immobilization of soil mineral N (Goring and Clark, 1948; Jansson and Persson. 1982). Thirdly, decomposing potatoes (Solanum tuberosum L.) from the previous crop at the 1993 location (L93) may have supplied a readily available source of energy for microorganisms to cause immobilization of fertilizer NOV. The difference method relies on the assumption that contribution of soil N to crop uptake is the same for the fertilized and unfertilized treatments. But many studies have shown that application of fertilizer N enhances uptake in the fertilized crop. This phenomenon has been explained in two ways. Firstly, fertilizer N application stimulates root development and enhances N uptake due to rhizosphere effects (Kissel and Smith, 1978; Fried and Broeshart, 1974; Aleksic et. al., 1968). Secondly, fertilizer N stimulates the mineralization of soil N, which leads to increased N consumption by the fertilized crop (Westerman and Kurtz, 1974; Chichester and Smith, 1978; Filimonov and Rudelev, 1977; Kissel and Smith, 1978). In order to achieve meaningful results, there must be crop response to N application which sets the precondition that soil N must be at suboptimal levels. Many experiments with 1 5 N have shown that plant uptake efficiency of fertilizer N does not often exceed 50 to 60% of that applied (Herron et al., 1968; Hauck, 1971; Westerman, 1972; Powlson et al., 1986; Ditsch et al., 1993), and under certain conditions may be much less. The remaining N is not necessarily subject to loss or leaching, as some will be incorporated into soil organic matter by microbial means. For example, Smith and Power (1985) reported that 20 to Chapter 6. Discussion 134 50% of the fertilizer N not initially utilized by perennial grass in the semiarid northern Great Plains is incorporated into soil organic matter. When leaching and denitrification losses are negligible during the main season, winter cover crop biomass and N uptake increase with increasing rates of fertilizer N application to the main season crop (Ditsch, et al., 1993; Shipley etal., 1992). Low apparent fertilizer N recovery in the cover crop and soil in the spring of all the three seasons was evident. The low apparent N recovery in spring species residues was attributed to a combination of surface decomposition of the residues and leaching of organic N compounds from the residues due to winter rainfall events. The low apparent N recovery in the living cover crops (winter species) was mainly due to the fact that there was generally little additional N uptake in the spring and that the August-planted cover crop which received 100 kg N ha"' was susceptible to freezing damage in late November (see Figure 5-7). This resulted in a net loss of N during winter in August-planted winter rye in the first two seasons (1991-92 and 1992-93). Generally, low apparent fertilizer N03" recoveries in soil may have been caused by NGy leaching during winter (Kowalenko, 1987b and 1989), denitrification and incorporation of fertilizer NGy into soil organic matter (Smith and Power, 1985). Although the three processes may account for N0 3" loss during winter, a significant proportion of the loss is most likely due to leaching. Bomke et al. (1994) reported the consistent appearance in spring of N deficiency symptoms on unfertilized winter wheat in south coastal B.C. and attributed it NOV leaching during winter. Paul and Zebarth (1997) partitioned N losses to denitrification and leaching from autumn-applied manure and reported that most of the loss (83 to 95%) was due to N03" leaching. Chapter 6. Discussion 135 6-5. Recommendations When spring species (barley, wheat and oat) were planted in the third or fourth week of August, the cover crops winter-killed and soil mineral N increased in the root zone relative to that under fallow plots in the spring. Under western Lower Fraser Valley farming system, winter cropping with spring species particularly in situations where summer crops (early potatoes, peas and beans) are harvested in July, may be beneficial not only in protecting soil NOV from leaching during winter but also synchronization of N availability with N uptake for summer crops. It was evident from this study that when winter rye was planted in the third week of August (especially under high autumn soil N content), it was highly susceptible to freezing damage. This resulted in decrease during winter of the N taken up before winter leaching period without significantly affecting spring soil mineral N. Thus, despite the fact that August-planting increases N uptake relative to September-planting, the role of winter species in conserving autumn soil N during winter may be limited to situations where summer crops (for example corn) are harvested late in the season. Chapter 7. Conclusions and Future Research 136 Chapter 7: CONCLUSIONS AND FUTURE RESEARCH 7-1. Conclusions 7-1.1. Cover crop and soil mineral nitrogen before winter leaching (1) Cover crops that were planted in the third or fourth week of August showed greater potential to produce higher biomass, accumulate larger amounts of N and reduce soil mineral N compared to those that were planted in the third or fourth week of September. Cover crops that were planted in August were also more efficient in absorbing fertilizer N. (2) In the first two seasons, spring barley and winter rye were similar in terms of biomass production and N uptake, despite great differences in growth stages. Crop species differences in productivity and N accumulation were evident in the 1993-94 season. Cover crops produced higher biomass and accumulated greater amounts of N compared with chickweed (Stellaria media L.) in fallow plots. Spring wheat and spring oat were superior to spring barley in terms of biomass production and N uptake. Winter rye and annual ryegrass were equally effective in productivity and N uptake. Generally, winter relative to spring species absorbed greater amounts of autumn residual mineral N, despite less biomass production than by spring species. 7-1.2. Cover crop and soil mineral nitrogen after winter leaching (l) Despite winter-killing at the start of winter, August-planted spring species may play a positive role in the cycling of autumn residual mineral N. Planting of spring species in the third or fourth week of August results not only in greater biomass production and N accumulation before winter leaching period relative to planting one month later but also in an increase (relative to amount in fallow plots) of root zone soil mineral N in the spring. This was attributed to Chapter 7. Conclusions and Future Research 137 mineralization of residue N in early spring. The mineralized N may be readily available and be synchronized with N uptake by the next crop. (2) The data consistently shows that while winter species growth is vigorous in early spring, only a small amount of net N uptake occurs between late November and spring. This may be attributed to a combination of N0 3~ leaching, denitrification and immobilization during winter and may imply that high mineral N content in the root zone at the start of winter under cover cropping may still lead to great losses of mineral N (mainly N03") during winter under western Lower Fraser Valley mild weather and fine-textured soils. (3) August-planting of winter rye may not always be beneficial in terms of N conservation depending on late summer weather conditions. When winter rye is planted in the third or fourth week of August, its growth is greatly enhanced by long days and warmer temperatures in late summer, rendering the crop susceptible to freezing damage. This reduces plant density and hence the amount of N retained during the winter, without significantly affecting spring soil mineral N in the 0-60 cm layer. Winter rye materials in this study that were damaged by freezing, decomposed rapidly in a similar manner to September-planted spring barley that winter-killed at the beginning of winter. This observation was based on visual disappearance of residues and may indicate loss through N0 3" leaching, denitrification and immobilization during winter. (4) The biomass increase of winter cover crops between late November and spring of the next year was accompanied by a small increase in N accumulation. Winter-killed spring species residue greatly declined in biomass and N that was assimilated prior to winter, most likely through leaching of soluble organic compounds and decomposition of the residue. For August-planted crops, this decrease over the winter was accompanied by an increase in soil mineral N (0-60 cm) content relative to that in fallow plots in the spring. Chapter 7. Conclusions and Future Research 138 (5) While drastic decreases in accumulated N of spring species that were planted in the third or fourth week of August was associated with an increase in soil mineral N in the spring in 0-60 cm layer, the small increase in cover crop N uptake of winter species does not explain the decrease in soil mineral N over the 20-week period, starting late November. This indicates that the main N0 3~ leaching period precedes both spring species residue N mineralization and N uptake by winter species in the spring, suggesting retention of N accumulated by spring species. The small increase in N uptake by winter species may be caused by low soil mineral N concentrations in the spring. (6) Planting of spring species in August as compared to a month later significantly increased the potential of the cover crops to retain accumulated N during winter. 7-1.3. Spring species nitrogen fractions at winter-kill (1) Most of the N in autumn-planted spring species was in the organic form indicating most of the mineral N taken up by the cover crops prior to winter was assimilated. A significant proportion of the assimilated N was associated with proteins (cold-water insoluble and soluble). There were some indications of retention of the soluble protein N fraction by the winter-killed spring species. Cover crop N0 3" content, which can readily leach out of the crop residues after winter-kill, represented a small fraction of the total N (~ 15%) and at maximum amounted to about 20 kg N ha"1 when the cover crops were planted in August and autumn soil mineral N content in the 0-60 cm layer was 200 kg ha"1. The loss of N accumulated by spring species through leaching as N0 3"-N following winter-kill may contribute little to the N leached out of the plant residues and ultimately to the overall leaching out of the root zone during winter. The proportion of NFJV-N in the spring species was small and averaged only 3% of total N . Chapter 7. Conclusions and Future Research 139 The role of winter species, especially winter rye and annual ryegrass in conserving soil mineral N during winter, has been widely researched. It is evident from this study that autumn-planted spring species can be integrated into winter cropping systems on western Lower Fraser Valley soils. When planted in August, it is apparent that spring species can effectively retain the N taken up before winter-kill and release it to the next crop through mineralization. 7 - 2 . Future research In three consecutive winter cropping seasons, I found that on the fine-textured soils of western Lower Fraser Valley, spring barley that was planted in the third or fourth week of August can accumulate between 105 and 138 kg N ha"1 prior to winter at high (N-100) autumn soil mineral N content. Spring wheat and oat in the third year accumulated about 163 and 143 kg N ha'1, respectively. In all the three seasons, these amounts were reflected in soil mineral N measurements made in spring. This suggests that the residues of these cover crops can retain accumulated N following winter-kill and readily release it in plant available form (NH« + and N0 3 " ) in spring through decomposition and mineralization. This result can be confirmed by using l 5 N labelled plant materials at the start of winter with more frequent sampling for biomass and total N remaining, and soil mineral N content following winter-kill. Alternatively, adequate soil and plant data base can be generated before and after winter leaching on different soil types over several years, and correlation analysis used to associate either biomass production or N accumulation at winter-kill with increase in soil mineral N due to crop residue in the spring. The experiments can be designed to include indirect measurements of N0 3" leaching (with ' 5N). In this study residue biomass remaining and N retained in the residue by spring time were estimated using meshbags, but more accurate laboratory techniques have been developed recently (Nyamai, Chapter 7. Conclusions and Future Research 140 1992; Lefroy et al., 1995) that can be used to accurately study residue breakdown rates and N release from different plant species. This may provide more information on the role of cover cropping with autumn-planted spring species in protecting autumn soil N0 3" from leaching during winter. Planting dates of spring species should be studied from as early as mid July to represent earliest summer crops removed from the fields in the western Fraser Valley, for example, early potatoes, peas and beans. The main aim of cover cropping is to maximize mineral N uptake between summer crop harvest and start of winter leaching period. 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Resource Analysis Branch, B.C. Min. Environ. Victoria, B.C. pp. 3-10 Schomberg, H.H., J.L. Steiner and P.W. Unger. 1994. Decomposition and nitrogen dynamics of crop residues: Residue quality and water effects. Soil Sci. Soc. Am. J. 58: 372-381. Schuman, G.E. , T . M . McCalla, K . E . Saxton and H.T. Knox. 1975. Nitrate and its distribution in the soil profile of differentially fertilized corn watersheds. Soil Sci. Soc. Am. Proc. 39: 1192-1197. Shipley, P.R., J.J. Meisinger and A . M . Decker. 1992. Conserving residual corn fertilizer nitrogen with winter cover. Agron. J. 84: 869-876. Soil Science Society of America, Terminology Committees. 1979. Glossary of soil science terms. Revised Ed. Soil Sci. Soc Am., Madison, WI. References 150 Smettem, K.R.J. , S.T. Trudgill and A . M . Pickles. 1983. Nitrate loss in soil drainage waters in relation to by-passing flow and discharge on an arable site. J Soil Sci. 34: 499-509. Smith, S.J. and J.F. Power. 1985. 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Production and management of cultivated forages. Reston Publishing Company, inc. A Prentice-Hall Company. Reston Virginia. Westerman, R.L. and L.T. Kurtz. 1974. Isotopic and nonisotopic estimation of fertilizer nitrogen uptake by sudangrass in field experiments. Soil Sci. Soc. Am. Proc. 38:107-109. Westerman, R.L., L .T. Kurtz and R.D. Hauck. 1972. Recovery of 15N-labeled fertil izer in field experiments. Soil Sci. Soc. Am. J. 36: 82-86. Widdowson, F.V. , A. Penny, R.J. Darby, E. Bird and M.V. Hewitt. 1987. Amounts of N03"-N and N H 4 + - N in soil, from autumn to spring, under winter wheat and their relationship to soil type, sowing date, previous crop and N uptake at Rothamsted, Woburn and Saxmundham, 1979-85. J. Agr. Sci., Cambridge 108: 73-95. Wild, A. 1988. Russell's soil conditions and plant growth. 11th edition. Longman, London, pp. 652-694. Woldendorp, J.W. 1963. The influence of living plants on denitrification. Meded. Land-bouwhogesch. Wageningen. 63: 1-100. Wong, E. 1973. Plant phenolics. In G.W. Butler and R.W. Bailey (eds.) Chemistry and biochemistry of herbage. Vol. 1. Academic Press. London and New York. pp. 265-322. Wright, M.J. and K . L . Davison. 1964. Nitrate accumulation in crops and nitrate poisoning in animals. Ad van. Agron. 16: 197-247. Zadoks, J . C , T.T. Chang and C F . Konzak. 1974. A decimal code for the growth stages of cereals. Weed Res. 14: 415-421. Tables of Appendices 152 TABLES OF APPENDICES Table A-l. List of common terms and abbreviations N Nitrogen N H 4 + Ammonium N0 3" Nitrate T C A Trichloroacetic acid Residual Amount remaining in soil after main cropping season D Planting date N-0 Autumn residual mineral N only N-100 Autumn residual mineral N + 100 kgN ha" applied PR Percent reduction in soil mineral N or NOV -N ANR Apparent fertilizer N recovery C Cover crop PVP Polyvinylpyrrolidone PMSF Phenylmetlrylsulfonylfluoride E D T A Ethylenediaminetetraacetic acid WIN Plant cold-water insoluble N fraction PN Plant cold-water soluble N fraction NPN Plant organic nonprotein N fraction N N Plant nitrate N fraction A N Plant ammonium N fraction D M Dry matter TN Total nitrogen T Time of sampling Soil mineral N N H 4 + + NCV Tables of Appendices 153 Table A-2. Cold-water, hot-water and KCl extractable ammonium and nitrate Nfor spring barley in late November 1992. Ammonium N Nitrate N Planting date SOILN C W 1 HW K C L CW HW K C L kg ha 1 August/24/92 0 1.7(±0.1) 2.1 0.6 1.6(±0.2) 2.1 1.7 August/24/92 100 4.3(±0.2) 7.2 2.2 19.2(±3.9) 25.2 17.9 September/22/92 0 1.0(±0.1) 1.7 0.5 3.1(±0.5) 1.7 3.4 September/22/92100 1.2(±0.1) 0.7 0.5 5.6(±0.4) 5.4 5.8 K CW, HW and KCL are cold-water, hot-water and KCl extractable fractions, respectively; Table AS. Cold-water, hot-water and KCl extractable ammonium and nitrate Nfor spring species in late November 1993. Ammonium N Nitrate N Nitrogen Cover Content Crop C W 1 HW K C L CW HW K C L kg ha"1 0 Barley 2.9(±0.1) 3.8 1.3 18.1 (±2.6) 19.6 19.5 0 Wheat 2.9(±0.1) 2.7 1.0 19.0(±0.9) 21.7 21.2 0 Oat 2.9(±0.4) 3.7 1.1 18.4(±4.8) 22.7 18.8 100 Barley 3.3(±0.3) 3.5 1.2 21.7(±2.1) 30.3 21.3 100 Wheat 2.9(±0.3) 3.2 0.7 17.1 (±3.2) 20.1 18.5 100 Oat 3.7(±0.3) 3.9 1.1 29.2(±4.2) 33.0 26.9 H CW, HW and KCL are cold-water, hot-water and KCl extractable fractions, respectively. Tables of Appendices 154 Table A-4. Soil ammonium nitrogen content in late autumn 1991 and in spring 1992. November/21/91 April/08/1992 Autumn Depth intervals (cm) Depth intervals (cm) Planting soil N Cover date content crop 0-20 0-40 0-60 0-20 0-40 0-60 kg ha' Aug/24/92 N-0 Fallow 4 8 11 6 13 20 Aug/24/92 N-0 Barley 4 7 11 10 16 23 Aug/24/92 N-0 Rye 4 7 10 7 14 22 Aug/24/92 N-100 Fallow 4 9 12 7 16 24 Aug/24/92 N-100 Barley 6 12 16 8 16 25 Aug/24/92 N-100 Rye 5 8 13 7 15 24 Sep/22/92 N-0 Fallow 2 4 7 9 15 20 Sep/22/92 N-0 Barley 4 6 9 5 21 26 Sep/22/92 N-0 Rye 4 8 11 6 12 18 Sep/22/92 N-100 Fallow 5 10 14 7 13 20 Sep/22/92 N-100 Barley 4 9 13 5 11 18 Sep/22/92 N-100 Rye 5 7 11 5 11 16 Table A-5. Soil ammonium nitrogen content in late autumn 1992 and in spring 1993. November/24/92 April/30/1993 Autumn Depth intervals (cm) Depth intervals (cm) Planting soil N Cover date content crop 0-20 0-40 0-60 0-20 0-40 0-60 kg ha'1 Aug/24/92 N-0 Fallow 3 7 12 9 17 24 Aug/24/92 N-0 Barley 6 10 13 10 22 29 Aug/24/92 N-0 Rye 3 5 8 10 21 33 Aug/24/92 N-100 Fallow 3 5 7 12 21 30 Aug/24/92 N-100 Barley 6 10 13 8 15 23 Aug/24/92 N-100 Rye 3 6 8 11 23 32 Sep/22/92 N-0 Fallow 5 10 16 12 21 29 Sep/22/92 N-0 Barley 8 16 20 8 17 27 Sep/22/92 N-0 Rye 5 12 18 8 16 25 Sep/22/92 N-100 Fallow 6 11 16 9 15 22 Sep/22/92 N-100 Barley 7 13 17 11 19 26 Sep/22/92 N-100 Rye 6 14 18 7 16 23 Tables of Appendices 155 Table A-6. Soil ammonium nitrogen content in late autumn 1993 and in spring 1994. November/24/93 April/14/1994 Autumn Depth intervals (cm) Depth intervals (cm) soil N Cover content crop 0-20 0-40 0-60 0-20 0-40 0-60 k g h a ' N-0 Fallow 4 9 13 6 11 17 N-0 Barley 6 13 20 4 9 14 N-0 Wheat 6 11 18 4 8 11 N-0 Oat 6 10 15 4 10 19 N-0 Rye 5 10 17 6 12 18 N-0 Ryegrass 6 11 18 6 10 17 N-100 Fallow 6 12 19 8 12 16 N-100 Barley 6 11 18 4 7 12 N-100 Wheat 4 10 15 4 9 14 N-100 Oat 5 9 14 3 7 13 N-100 Rye 6 11 16 4 7 12 N-100 Ryegrass 7 12 20 4 8 14 Table A-7. Soil NO3-N exprressed as a proportion (%) of total mineral N (NH4+ + NOf-N) before winter leaching period in late November 1991. August planting (Aug/19/91) September planting (Sep/16/91 Cover crop N-0 N-100 N-0 N-100 Fallow 86 91 91 92 Spring barley 72 73 73 85 Winter rye 64 80 75 85 Tables of Appendices 156 Table A-8. Soil NOf-N expressed as a proportion winter leaching period in late November 1992. (%) of total mineral N (NH4+ + NOf-N) before August planting f Aug/24/92) September planting (Sep/22/92) Cover crop N-0 N-100 N-0 N-100 Fallow 90 97 88 93 Spring barley 63 85 76 90 Winter rye 53 84 75 89 Table A-9. Soil NOf-N expressed as a proportion winter leaching period in late November 1993. (%) of total mineral N (NH4+ + N03'-N) before Cover crop N-0 N-100 Fallow/chickweed 92 91 Spring barley 79 88 Spring wheat 81 89 Spring oat 85 91 Winter rye 78 87 Annual ryegrass 77 85 k. I to Ik > o Ik 1k I tr o § k ^3 2 a § to R •2 •2 5 5 k S t - I -5 PH PH o 00 00 DH T3 O © 4J_, O 4 J , vo O O -fl o © -H CN © © © o 41^ rr; CN O O 4 J , o o CN g\ CN "5b o 41, cK o 41 o © 41, CN' o o 4 J , © o 4 J , © CN o 4J^ CN CN O © 41 o o CN CN 3 < © 4j_, o 4 j , r<-> O © 41 m o o 4 J , o 00 o o 41, © ' vo o 41, CN o o 41 CN ON CN CN "a. 00 CN © 41, 00 I o 41 o 4 J , o 0 41, 00 © 0 41, r- I o o 4 T ^ I CN O 0 41 CN I o o CN as CN CN ~Q. <U 00 S I o o !< CJ >* CJ s « 50 R O C .3 o "a c a to C •2 cj a . ON v. ct> £ cu t H 5 § a o ° •s § oo o Q o 4j, cn oo p o 4 j , cn cn p O -H o o © 00 o o 4j, o o o 41, ON' o o o 4^ cK o CQ o o o 4j_ NO CO o cn cn p O 41 o © © NO o © 4j_ 00 © o ON o o 41 cd <u o 4J, cn cn © 4j, oo cn © © ' 41 o 4j, O o 4^ oo o © 41 m o © ' 41, 00 © O p © 4J, \£> cn © 4H, o o -H © o 41, oo © o 00 o C N © 4J, NO' o o -H cd CQ o o ON O © 4J, cn ON o © cn © o 4H ON o © 4H, o ON O © 4H, © ' o 4j_ NO o -H CO o o © 4 T o'l ON o o 4J, C N o © -H ON o o 41, 00 I © ON o o 4H, ON'I ©' © 41 o © 41, © o o o Tables of Appendices 159 Table A-12. Comparison of cold-water (CW) and hot-water (HW) for extraction ofprotein- and organic nonprotein-Nfractions from plant materials. • November/24/1992 November/24/1993 Extraction method Protein-N Nonprotein-N Protein-N Nonprotein-N %TN CW 75(±4.1) 12(±1.1) 60(±2.1) 22(±2.7) HW 63(±3.6) 24(±1.3) 58(±1.8) 22(±2.2) P > T 0.0066 0.0042 0.5119 0.9440 H Protein-N was calculated as the sum of the insoluble (WIN) and soluble fraction (PN); Table A-l 3. Comparison of cold-water (CW), hot-water (HW) and 2MKCI (KCL) for extraction ofNOf-N and NHf-N fractions from plant materials. November/24/1992 November/24/1993 Extraction method NCY-N N H / - N NCV-N N H / - N o / o T N CW 10(±2.9) 3(±0.2) 15(±1.4) 3(±0.2) HW 10(±2.7) 4(±0.3) 18(±2.0) 3(±0.3) P > T 0.9138 0.4136 0.1172 0.1747 C W 10(±2.9) 3(±0.2) 15(±1.4) 3(±0.2) K C L 10(±2.8) 1(±0.2) 16(±1.2) 1(±0.1) P > T 0.7565 0.0001 0.2566 0.0001 Tables of Appendices 160 Table A-14. Carbon content (%DM) of cover crops for the 1991-92 and 1992-93 winter cropping seasons. Planting Nitrogen date Level Crop species Nov/21/91 Apr/08/92 Nov/24/92 Apr/30/93 Aug/19/91 N-0 Barley 44 42 37 40 Aug/19/91 N-0 Rye 42 42 39 42 Aug/19/91 N-100 Barley 43 36 35 41 Aug/19/91 N-100 Rye 39 43 38 42 Sep/16/91 N-0 Barley 40 32 37 40 Sep/16/91 N-0 Rye 39 43 38 41 Sep/16/91 N-100 Barley 40 33 36 40 Sep/16/91 N-100 Rye 39 43 38 42 Table A-15. Carbon content (%DM) of cover crops for the 1993-94 winter cropping seasons. Nitrogen level Crop species Nov/24/93 Apr/14/94 N-0 Chickweed (fallow) 32 35 N-0 Spring barley 38 39 N-0 Spring wheat 39 40 N-0 spring oat 38 40 N-0 Winter rye 39 40 N-0 Annual ryegrass 38 38 N-100 Chickweed (fallow) 30 37 N-100 Spring barley 37 39 N-100 Spring wheat 39 40 N-100 spring oat 37 39 N-100 Winter rye 39 39 N-100 Annual ryegrass 37 38 

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