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Soil organic components and aggregation as influenced by cover and ley crops Liu, Aiguo 1995

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Soil Organic Components and Aggregation as Influenced by Cover and Ley Crops by Aiguo L i u B . Sc., Shanxi Agricultural University, China, 1982 M . Sc., Graduate School of Chinese Academy of Agricultural Sciences, China, 1985 A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science 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 September 1995 ©Aiguo Liu , 1995 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 scholariy 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 Spi I SCj^flO^ The University of British Columbia Vancouver, Canada Date V-ece^vhr /f/ iftr DE-6 (2/88) Abstract Structural degradation of silty clay loam soils in Delta, British Columbia, has resulted from intensive cultivation for vegetable crops. Field and laboratory incubation studies were conducted to determine the changes in soil total C, total N and total and labile polysaccharides produced by cover crops in relation to aggregate stability of these soils. Mean weight diameter ( M W D ) was used as a measure of soil aggregate stability. The field experiment included two cover crop sites, a pasture site and a Reclamation Site with severe structural degradation. The results on the cover crop site in 1992-1993 were not significantly different between bare soils and winter cover crop treatments because of problems of variability and high precipitation in Apri l of 1993 when soils were sampled. In 1993-1994, cover crops which overwintered, annual ryegrass and fall rye, increased soil aggregate stability, and all of the cover crops significantly increased soil labile polysaccharides, but did not change total N during their growing period. Soil aggregate stability was significantly correlated with the content of labile polysaccharides and followed the order: annual ryegrass > fall rye > spring barley = bare soil. Long time pasture significantly increased the contents of total C, total and labile polysaccharides and total N , which effectively increased soil aggregate stability. A t the Reclamation Site, grass (tall fescue) ley treatments significantly increased M W D values and total carbon and labile polysaccharide contents in comparison with a cash crop/clover combination. However, M W D and total and labile polysaccharide contents did not show the differences between the drained and undrained soil in the sampling in May of 1993, and the total carbon content in the undrained soil was higher than in the drained soil. In a laboratory experiment, chopped shoots and coarse roots of green cover crops, fall rye and annual ryegrass, and starch were added to a subsoil from the 1993-1994 cover crop site and incubated for 2, 4 and 8 weeks. The added cover crop and starch treatments increased soil total C and N and labile polysaccharides, which significantly increased soil M W D . The starch amendment was more effective in forming larger soil aggregates than the double-dose fall rye (the amount added into the soil was twice as much as the fall rye amendment) and fall rye amendments for 2 and 4 weeks incubations, but less effective than the double-dose fall rye and fall rye amendments for the 8 weeks incubation. A l l amendments increased the proportion of 2-6 mm aggregates relative to the control at all sampling dates except the fall rye amendment after 4-weeks incubation. M W D values correlated with soil total and labile polysaccharides, total C and total N in the incubation samples. Macroaggregates contained higher contents of total C, total polysaccharides and labile polysaccharides and total N than microaggregates. i i i Table of Contents Page Abstract . ii Table of Contents iv List of Tables vii List of Figures viii Acknowledgments x 1.0 Background 1 2.0 Objectives 2 3.0 Literature Review 3 3.1 Hierarchy of Soil Aggregates and Some Organic Chemical Constituents 3 3.1.1 Hierarchy of Soil Aggregates 3 3.1.2 Aggregate Size and Some Chemical Properties 3 3.2 Nature of Organic Matter Related to Soil Aggregates 4 3.2.1 Decomposibility of Organic Matter and Soil Aggregation 4 3.2.2 Degradation of Soil Organic Matter and Aggregate Stability 5 3.3 Water-stable Aggregates Related to Polysaccharide Content and Other Binding agents 8 3.3.1 Correlation between Organic Matter and Soil Aggregate Stability 8 iv 3.3.2 Correlation between Particular Soil Organic Components and Aggregate Stability 9 3.3.2.1 Microbial Biomass C and Soil Aggregate Stability 9 3.3.2.2 Total Nitrogen and Soil Aggregate Stability 10 3.3.2.3 Acid-hydrolysable Polysaccharides and Soil Aggregate Stability.. 10 3.3.2.4 Water-extractable Polysaccharides and Soil Aggregate Stability.. 12 3.4 The Input of Organic C, Polysaccharides and Other Organic Binding Agents by Crops and Microorganisms 13 3.4.1 Direct Input by Crops 13 3.4.2 Syntheses by Microorganisms 14 3.5 Interaction of Polysaccharides with Clay or Oxides to Form Complexes 15 3.6 Cover and Ley Crops and Soil Aggregates 17 4.0 Materials and Methods 19 4.1 Site Description 19 4.2 Cover Crop Trials 19 4.3 Reclamation Trial 20 4.4 Incubation Trial 20 4.5 Laboratory Methods 21 4.5.1 Wet-sieving Methods 21 4.5.2 Chemical Analysis 22 4.6 Statistical Analysis 23 5.0 Results and Discussion 24 5.1 Cover Crop Trials 24 5.2 Reclamation Trial 35 5.3 Incubation Trial 39 6.0 Conclusion 50 7.0 Literature Cited 52 8.0 Appendices 64 vi List of Tables Page Table 4.1 The cropping history on the Reclamation Site 20 Table 5.1 Changes in water stable aggregates and some organic chemical properties induced by cover crops (1992-1993). Soils were sampled on May 6, 1993 24 Table 5.2 Comparison of coefficients of variation of winter cover crops at cover crop site between 1992-1993 and 1993-1994 25 Table 5.3. Comparison of M W D and contents of organic constituents between cover crop and pasture sites in 1992-1993. Soils were sampled on May 6, 1993 27 Table 5.4. Changes in water stable aggregates and some organic chemical properties induced by cover crops (1993-1994). Soils were sampled to a depth of 5 cm on Apri l 17, 1994 29 Table 5.5 Linear correlation coefficients (r) between M W D values and soil organic constituents for field and incubation samples 30 Table 5.6. The shoot biomass (kg/ha) of winter cover crops in 1992-1993 and 1993-1994 ..33 Table 5.7. Influence of cover crops and drainage system on soil aggregate stability, contents of total carbon, total polysaccharides and labile polysaccharides at Reclamation Site (soils sampled on May 6, 1993 to a depth of 5 cm) 35 Table 5.8. Comparison in contents of organic constituents between 1-2 mm and 0.25-1 mm water stable aggregates after 8-week incubation 48 vii List of Figures Page Figure 5.1 Comparison of monthly precipitation in 1992-1993, 1993-1994 and 1937-1990 .. 26 Figure 5.2 Contents of total carbon, labile and total polysaccharides and total nitrogen in different soil aggregated during cover crops' growing period. Soils were sampled on M a y 6, 1993 28 Figure 5.3 The relationship between mean weight diameter and soil labile polysaccharide content 31 Figure 5.4 Contents of total polysaccharides and labile polysaccharides in different soil aggregates at Reclamation Site 37 Figure 5.5 Influence of amendments on aggregate stability during different incubation times 40 Figure 5.6 Influence of amendments on 2-6 mm water- stable aggregates at different incubation times 41 Figure 5.7 Influence of amendments on soil total carbon contents averaged over incubation periods 42 Figure 5.8 Changes of total carbon, total and labile polysaccharide and total carbon contents with incubation times averaged over amendments 43 Figure 5.9 Influence of amendments on soil labile polysaccharide contents averaged over incubation periods 44 v i i i Figure 5.10 Influence of amendments on soil total nitrogen contents averaged over incubation periods 47 ix Acknowledgments First, I would like to thank to Dr. A . A . Bomke for his guidance. Thanks also to Dr. L . E . Lowe, Dr. T. M . Ballard and Dr. M . D . Novak for their input and advice. I am grateful to Dr. C. Chanway for his participating my defense. B . Hermawan was of great help for providing valuable field and laboratory assistance. I would like to thank M . Hilmer for providing help. I am grateful also to C. Dyck and especially for her much appreciated laboratory help and support. Thanks must also go to the graduate students of the department, for their support and good humor. And finally, to my family and my friends all of whom remained constant in their belief and encouragement - Thank you. x 1.0 Background Many soils in Delta, British Columbia, when subject to frequent and intensive cultivation suffer a deterioration in structure, compaction, low infiltration, ponding and a decrease in the stability of soil aggregates. The resistance of soil aggregates to the slaking and dispersive effects of water (aggregate stability) is important in maintaining a porous structure in arable soils. The long-term effects of soil management on aggregate stability are well documented (Tisdall & Oades, 1982). It is generally known that the formation of stable aggregates involves organic binding agents and that pasture increases soil aggregation and soil aggregate stability. However, changes in aggregate stability under different short-term (1-3 years) cover crops are still not well understood. Overwinter cover crops protect the soil surface from falling raindrops, reduce runoff, and improve subsurface structure through the penetrating function of roots. Another key role of cover crops for improving soil structure in the short-term is to bind soil particles into aggregates by polysaccharides and other binding agents. In general, the proportion of water-stable aggregates in soil is positively related to total carbon, total nitrogen, polysaccharide and other organic constituents. However, these relationships are not always good because of different soil types and climate conditions. 1 2.0 Objectives The objectives of this project are 1) to determine the effects of overwinter cover crops on organic binding agents and soil aggregate stability during their growing period, 2) to determine the effects of a short term (2-years) grass ley on organic binding agents and aggregate stability 3) to assess the effects of incorporating cover crop top and root material on soil organic binding agents and generation of aggregate stability. To meet objective 1, two field experiments were carried out in collaboration with Bandi Hermawan, who studied soil physical properties, and Leonard Nafuma, who evaluated overwinter cover crop effects on the soil N cycle. Objective 2 was met by sampling, the Reclamation Site, established by the Delta Farmers' Soil Conservation Groups and U B C Soil and Water Conservation Group to demonstrate the roles of subsurface drainage, cover cropping and grass ley on improving degraded soils. Sampling of the Reclamation Site was done in cooperation with Bandi Hermawan. The study of incorporating cover crop materials, objective 3, was conducted in the laboratory because the on-farm sites did not permit field sampling after cover crop incorporation. 2 3.0 Literature Review 3.1 Hierarchy of Soil Aggregates and Organic Chemical Constituents 3.1.1. Hierarchical Orders Soil structure is considered as a hierarchy. The basic units, the lowest hierarchical order, of soil structure are aggregates composed of coalesced primary particles and binding agents. Edwards and Bremner (1967) divided soil aggregates into two classes: macro aggregates >0.25 mm in diameter, and micro aggregates <0.25 mm in diameter. Dexter (1988) subdivided the micro aggregate fraction into three subclasses: quasi-crystals or domains (1-2 um in diameter) composed of combined primary (clay) particles. Clusters(2-20 u.m in diameter) composed of quasi-crystals, and micro aggregates (20-250 um in diameter) composed of clusters. Cluster formation occurs by flocculation or when various components are cemented together by inorganic or organic gels such as extracellular polysaccharides of roots and microorganisms (Foster, 1978). Compound particles of lower hierarchical order are more dense than those of higher order. This is because each order excludes the pore spaces between the particles of the next higher order. Compound particles of lower hierarchical order also have a higher internal strength than higher order particles. 3.1.2 Aggregate Size and Some Chemical Properties Different sized aggregates are bonded by different organic binding agents suggesting that different sizes contain different amounts of C, N , P and other elements. Although Baldock et al. (1987) found that total carbohydrate content significantly increased as aggregate size decreased for aggregates <0.50 mm in diameter, many other studies have shown that larger aggregates contain higher amounts of organic matter and polysaccharides, for example, Christensen (1986) found higher organic matter in 2-20 mm aggregates than in 0.25-1 mm fraction. Dormaar (1984) observed a reduction in 3 carbohydrate content with decreasing aggregate size when the carbohydrate content was expressed per unit weight of the total soil material collected in each aggregate size fraction. The average carbohydrate contents of >250 urn diameter fractions were much higher than those of the 100 (j.m diameter fractions for three different soils that contained relatively low organic matter. The carbohydrate contents of >250 u.m diameter fraction were also higher than those of 100 um diameter fraction for native prairie. For a more intensively cultivated wheat fallow rotation treatment, he also found that the total carbohydrate carbon decreased as aggregate size decreased. It would appear that the association between carbohydrate materials of different water-stable aggregate size fractions may differ in different soils. It is been hypothesized that a high proportion of the organic matter associated with microaggregates is highly processed or recalcitrant and the organic matter associated with macroaggregates is less highly processed and more decomposable (Elliott, 1986). Elliott, (1986) investigated ratios of C / N and C/P in microaggregate and macroaggregate size classes, and indicated that a slaked, cultivated soil had the highest C / N (12.2) for the largest macroaggregates (>4.7 mm) and lowest ratio (9.2) for 0.05 to 0.09 mm microaggregate size class. The slaked, native sod soils showed a similar trend but the C / N range was narrower (10.7-9.3). Trends in C/P ratios were similar to those of C / N ratios with values ranging from 113 to 75 for the largest size in the slaked, cultivated treatment. Because carbohydrates contain relatively higher C than average organic matter in soils, these results suggested that there was larger proportion of polysaccharides binding microaggregates into macroaggregates than that within microaggregates. 3.2. Nature and Amount of Organic Matter Related to Soil Aggregates 3.2.1. Decomposibility of Organic Matter and Soil Aggregation Early investigations by Browning and Milam (1944), Kroth and Page (1947). Martin and Waksman (1940, 1941), and Martin (1942) showed that, in general, materials 4 containing relatively large amounts of readily decomposable organic matter constituents exert the greatest and quickest aggregating effect. More resistant materials require a longer time to exert their maximum aggregating influence but continue to be effective over a longer period of time. Extremely resistant or relatively inert substances, such as well composted materials, certain lignified wood by-products, and some peats, have little or no influence on aggregation. For example, Martin (1942) investigated the effect of several compost materials upon the aggregation of silt and clay particles. The compost material varied with the type of plant material used, however, the larger the percentage of readily decomposable substances in the materials, the greater was their aggregating effect. The level of aggregation attained following organic matter applications is also dependent upon the amount of residue applied and the state of aggregation of the soil to which it is applied (Browning and Milam, 1941, 1944). 3.2.2. Degradation of Soil Organic Matter and Soil Aggregate Stability The organic binding agents involved in stabilizing aggregates can be considered in three main groups based on the age and degradation of the organic matter and not on the proportions of chemically defined components. The various binding agents are related to the age, size and stability of aggregates. The three groups of organic binding agents considered are transient, temporary and persistent.(Tisdall and Oades 1982 ) a) Transient Binding Agents: Transient binding agents are made of polysaccharides and gums which include (i) microbial polysaccharides produced when various organic materials are added to soil, and (ii) some of the polysaccharides associated with roots and the microbial biomass in the rhizosphere (Russell, 1973; Oades, 1978). These binding agents surround soil particles and hold them together through a cementing or encapsulation action. Living bacteria and other microorganisms have been shown in laboratory studies to bind soil particles together, and this is associated with large (>250 um diameter) transiently stable 5 aggregates (e.g. Guckert et al., 1975). Polysaccharides are produced rapidly (Harris et al., 1966; Aspiras et al., 1971) but are readily attacked by microorganisms, thus their effect is of short duration. b.)Temporary binding agents Temporary binding agents are roots and hyphae, particularly vesicular-arbuscular ( V A ) mycorrhizal hyphae (Hubbell and Chapman, 1946; Bond and Harris, 1964; Tisdall and Oades, 1979). Such binding agents build up in the soil within a few weeks or months as the root systems and associated hyphae grow. They persist for months or perhaps years and are affected by management of the soil (Tisdall and Oades, 1979, 1980a, b). The temporary binding agents are probably associated with young macroaggregates and can be equated with the organic skeleton grains described by Bal (1973). Roots. Crop roots not only supply decomposable organic residues to soil and support a large microbial population in the rhizosphere, but roots of some plants, especially grasses, themselves act as binding agents. They appear to enmesh fine particles of soil into stable macroaggregates, even when the root has died (Clarke et al., 1967; Coughlan et al., 1973; Forster, 1979). Residues released into the soil by roots are in the form of fine lateral roots, root hairs, sloughed - off cells from the root - cap, dead cells, mucilages, lysates and volatile and water-soluble materials (Soper, 1959; Rovira and McDougall , 1967; Shamoot et al., 1968; Martin, J. K . , 1971; Dickinson, 1974; Oades, 1978). The amount of organic carbon released by roots is related to the total length of root; Shamoot et al. (1968) found that, regardless of species, plants released 20-49g of organic material per lOOg harvested root. The root systems and associated fungal hyphae of pasture plants, especially grasses, are extensive and the upper layer of the soil under pasture is probably all rhizosphere (Thornton, 1958; Barley, 1970). Hyphae. Soil particles are held together through physical entanglement by fungal hyphae. The mycelia of fungi often extend throughout the soil and particles are entrapped 6 and tied together. Hyphae are sticky and encrusted with fine particles of clay and retain their strength when stable, wet aggregates from the field are dissected (Hubbell and Chapman, 1946; Bond and Harris , 1964; Tisdall and Oades, 1979). Small clumps of soil particles can be seen clinging to the mycelia. Water-stable aggregates in a sand-dune soils also may be held together by fungal hyphae (Koske et al., 1975; Forster, 1979). Although individual hyphae are not strong, the combined strength of all hyphae and fine roots , especially in a three-dimensional network, holds particles more or less equally in all directions so that aggregates do not slake when wetted rapidly. Temporary binding agents stabilize macroaggregates. i.e. >250 um diameter (Hubbell and Chapman, 1964; Harris et al., 1966; Tisdall and Oades, 1980b). This is probably because roots and fungal hyphae are relatively large and because they can grow in large pores in soil (Jackson, 1975; Marshall, 1976). Fungi have been shown to grow mainly in the outer parts of aggregates(Hattori, 1973). It is believed that stabilization of aggregates by fungi in the field is limited to periods when readily decomposable material has been added to the soil in large amounts leading to a flush of hyphal growth (Mar t in et al., 1955; L o w and Stuart, 1974). Fungal hyphae in a red-brown earth have been shown to be associated with water-stable aggregates with little seasonal variation, while unstable aggregates contained few hyphae (Bond and Harris, 1964). Most of the microbial filaments which have been reported to stabilize aggregates in the field in the presence of plants were probably from V A mycorrhizal fungi (Mosse, 1959; Koske et al., 1975; Tisdall and Oades, 1979). The water-stability of aggregates of a red-brown earth was related directly to the length of external hyphae of these fungi per unit weight of aggregates or soil.c) Persistent Binding Agents Persistent binding agents constituting 52-98% of total soil organic matter are made of aromatic humic substances complexed with amorphous iron and/or aluminum or aluminosilicates (Greenland, 1965; Hamblin, 1977: Tate and Churchman, 1978; Turchenek 7 and Oades, 1978; Tisdall and Oades 1982). The persistent binding agents probably include complexes of clay-polyvalent metal-organic matter within micro-aggregates. In addition to complexes of clay-polyvalent metal-organic matter, persistent binding agents probably include C - P - O M and ( C - P - O M ) x , both of which are <250 um diameter, as described by Edwards and Bremner (1967), and are probably included in the skeleton grains described by Bal (1973). Persistent binding agents may be derived from the resistant fragments of roots, hyphae , bacterial cells and colonies (i.e. temporary binding agents) developed in the rhizosphere; the organic matter is believed to be the center of the aggregate with particles of fine clay sorbed onto it (Marshall, 1976; Foster, 1978; Turchenek and Oades, 1978) rather than the organic matter sorbed onto clay surfaces (Emerson, 1959; Greenland, 1965). 3.3 Water-stable Aggregates Related to Organic Matter and Other Organic Constituents Factors such as, iron oxides, cation exchange capacity, base saturation of C E C , exchangeable sodium and calcium and clay content influence soil aggregation (Kemper & Koch , 1966; Williams, 1979), however, many studies have concentrated upon the influence of organic matter on soil aggregation (Harris et al., 1966; Allison, 1973; Tisdall & Oades, 1982). 3.3.1. Correlation between Organic Matter and Soil Aggregate Stability There have often been positive correlations between the content of organic matter in soils and water-stable aggregation (Allison, 1973; Kemper & Koch, 1966; Tisdall & Oades, 1982). Some investigators also have demonstrated an improvement in soil aggregation following organic matter applications, for example, In Britain, ley-fertility experiments have shown a relationship between the percentage of water-stable aggregates and soil organic matter level (Eagle, 1975; Clement, 1975). 8 When correlations have not been good, one or all of the following reasons have been cited: (a) only part of the organic matter is responsible for water-stable aggregation, (b) there is a content of organic carbon above which there is no further increase in water-stable aggregation, (c) organic materials are not the major binding agents, (d) it is the disposition rather than the type or amount of organic matter which is important, and (e) some of the water stability in virgin soils is related to physical factors such that the particle reorganization associated with the first disturbance of virgin soil reduces water stability (Heinonen, 1955; Mal ik et al., 1965, Greenland, 1971; Low, 1972; Tisdall and Oades, 1980b). 3.3.2. Correlation between Particular Soil Organic Components and Aggregate Stability The stability of soil aggregates is sometimes related better to free organic materials than to total organic carbon. Some investigations (Greenland, 1971; Hamblin & Davies, 1977; Hamblin & Greenland, 1977) have shown significant correlations between particular organic matter fractions and aggregate stability. These suggest that total organic matter levels alone may not be sufficient to explain variation in aggregate stability but certain fractions of the soil organic matter may be particularly active because they act as a substrate for microbial production of organic glues (Oades, 1967) and /or because this fraction is a measure of roots and hyphae. Some particular organic matter fractions related to soil aggregation are discussed below: 3.3.2.1 Microbial Biomass C and Soil Aggregate Stability Microbial biomass C can be used as an indicator of early changes in soil organic matter brought about by management practices (Carter 1986; Powlson et al. 1987). A close relationship between microbial biomass and aggregate stability values has been observed in several recent studies. Drury et al. (1991) found that biomass C was significantly correlated with 9 aggregate stability. Microbial biomass was highest under 3 years of grass or lucerne and lowest under 3 years of soybean or maize yet organic C content remained relatively unchanged. Similarly, Robertson et al. (1991) observed that aggregate stability and biomass C were increased by 2 years of grass cover although organic C remained unchanged. Haynes and Francis (1993) indicated that large increases in microbial biomass in rhizosphere soil occurred though there were no changes in organic C or acid-hydrolysable carbohydrate content. 3.3.2.2. Total Nitrogen and Soil Aggregate Stability Chaney and Swift (1984 ) indicated total soil N content gave a highly significant correlation with aggregate stability (r=0.895), which is in agreement with the findings of Kemper and K o c h (1966) and Williams (1970). This is based on the idea that a total organic matter value includes all the substances: inactive and active, such as coarse lignified particles of low nitrogen content (Williams 1970), but the organic substances that contain relative high nitrogen are relatively active. Thus, soil aggregate stability would be better correlated with total nitrogen than with total organic matter. 3.3.2.3 Acid-hydrolysable Polysaccharides and Soil Aggregate Stability Statistical correlation between acid hydrolysable polysaccharide levels and degree of aggregation has been reported (Acton et al. 1963; Lowe 1978; Toogood 1959; Webber, L . R. 1965; Greenland et al. 1962). Evidence has long been available to indicate that polysaccharides may play a major role in the development of stable aggregates. Greenland et al. (1962) reported that soil polysaccharides are more effective in aggregate stabilization than the humic or fulvic fractions of the soil, which may be due to the stronger adsorption by clay particles of the polysaccharides than of humic or fulvic acides. Early studies showed polysaccharides to be very effective binding agents (Martin 1946). Tisdall and Oades (1982) found that in soils with low organic matter contents (1%), soil particles 1 0 were held together by polysaccharides. The addition to soil of polysaccharides derived from plant and soil microorganisms has been shown to improve aggregation (Clapp et al. 1962;). Baldock et al (1987) suggested that some subset of the total soil carbohydrates might be responsible for the management-induced changes in aggregate stability. M u c h work has been done on the effect of microbial polysaccharides on soil aggregation. Concentrations of microbial polysaccharides in the 0.2-2 g/kg range exerted a marked binding action on soil particles. Polysaccharides, especially those of microbial origin, have been considered the most important aggregate stabilizing agents(Theng 1979). The carbohydrate fraction of soil organic matter, composed largely of extracellular polysaccharides (EPS) produced by soil microorganisms, has been shown to promote aggregate stability (Griffiths, 1965; Cheshire, 1979; Molope et al. 1987). Metting (1986) and Cheshire (1979) reported that polysaccharide-producing organisms improved soil structure and increased infiltration in agricultural soil. Burns and Davies (1986) reviewed the role of filamentous fungi, both free-living and mycorrhizal, and microbial polymers (particularly polysaccharides) in the formation of aggregates. They indicated the improvement of soil stability by amendment with O M , addition of microbial polysaccharides and other metabolites. Skinner (1986) by studying the nature of soil particles and aggregates also found that aggregation was affected by microbial polysaccharides and root exudates. Martens and Frankenberger (1992) determined the effectiveness of selected microbial polymers in stabilizing soil aggregates. Glucose content of the polymer added was significantly correlated with soil aggregation. They also found that the stabilization of soil aggregates was a result of the metabolites from microbial processes rather than the direct binding effects of the added polysaccharides. Chaney and Swift (1986) studied re-formation of soil aggregates. When natural soil aggregates were destroyed by crushing, techniques traditionally used for re-forming 11 aggregates, such as wetting/drying and freezing/thawing cycles did not produce any stable re-formed aggregates. Incubation without amendment, was'similarly unsuccessful, whereas incubation with glucose amendment did produce stable aggregates, and their stability was related both to the natural soil organic matter levels and to the original stability of the natural aggregates. Although the stability induced by incubation with glucose was of a transient nature, this behavior was attributed to the production of microbial polysaccharides of known structure confirming that such polymers are capable of producing stable re-formed aggregates without the assistance of further microbial activity. Longer term incubation showed that stability of the re-formed aggregates also declined as soil microorganisms broke down the polysaccharide materials. Interest in the use of polysaccharide-producing organisms to improve structure and infiltration in agricultural soils has been increasing (Metting, 1986, Cheshire, 1979). Researchers in this area have generally added presynthesized polysaccharides or extracellular polysaccharides (EPS)-producing organisms to soils. The periodate anion is believed to oxidize soil polysaccharides and therefore any reduction in the stability of aggregates after treatment with periodate may be due to the removal of polysaccharides. Baldock and Kay (1987) found that periodate treatment reduced the stability of aggregates although it did not appear to account for the large differences in aggregate stability observed across cropping treatments. Molope et al. (1987) also showed that destruction of polysaccharides by periodate oxidation greatly diminished aggregate stability. 3.3.2.4 Water-extractable Polysaccharides and Soil Aggregate Stability Water has been used as mild agent to extract a polysaccharide fraction involved in the short term binding of aggregates. It was suggested that this fraction represented polysaccharides mainly of microbial origin that are involved in aggregating and / or stabilizing soil aggregates (Haynes and Swift 1990). Similarly, studying aggregation of soils amended 12 with sewage sludge, both Metzger et al. (1987) and Kinsbursky et al. (1989) observed that the hot water-extractable substances in soils represent extracellular polysaccharides related to soil aggregation. 3.4. The Input of Organic C, Polysaccharides and Other Organic Binding Agents by Crop and Microorganisms Soil polysaccharides are mainly the product of two continuous processes: the degradation of plant and animal polysaccharides within the soil, and the synthesis of polysaccharides by soil organisms using carbon compounds from various sources including polysaccharides undergoing degradation. 3.4.1 Direct Input by Crops It has been recognized for a long time that crops input organic carbon, polysaccharides and other organic constituents into the soil by their root systems and residues. For example, Clark et al (1967) indicated that soil organic matter built up under pasture, owing to decomposition of plant tops and roots, deposition of animal excreta and accumulation of mucigels in the crop rhizosphere (Clark et al. 1967, Tisdall and Oades, 1982). Lowe (1978) and Theng (1979) indicated that of the large number and variety of natural occurring polysaccharides, a majority were introduced into the soil system by plants, animals and as a result of microbial synthesis. Angers and Mehuys (1989) observed that cropping a clay soil for 2 years to barley and alfalfa resulted in greater (15-20%) carbohydrate content compared to fallow or corn. Changes in carbohydrate content were partially correlated with aggregate stability. Oades (1967) has shown that a soil under a four-course rotation including pasture, had a carbohydrate content nearly twice as high as the same soil under a continuous wheat-fallow system. 1 3 Oades (1984) also indicated that polysaccharides and other mucilage were produced by crop roots. Macroaggregates are enmeshed by plant roots, both living and decomposing, and they increase in number because of input of organic materials and polysaccharides by grass . Sparling and Cheshire (1985) examined the relationship between the stability of soil macroaggregates in water and the soil polysaccharide content in rhizosphere. They found that total C, N and polysaccharide contents of rhizosphere soil were greater than those of the bulk soil. 3.4.2. Syntheses by Microorganisms Microbial polysaccharides play an important role in crop induced changes in soil water-stable aggregates (Griffiths 1965, Rennie et al. 1954). Certain fungi as well as bacteria are capable of producing aggregate-stabilizing gums. In an early study designed to determine the nature of soil-binding substances synthesized by soil organisms, polysaccharides produced by Bacillus subtilis were found to be an effective binding material (Martin, 1945). In a continuation of this study (Martin, 1946), several bacterial polysaccharides were found to be very effective binding agents and it was concluded that during the decomposition of organic matter in the soil there is an accumulation of microbially synthesized substances which bring about the binding of soil particles into aggregates. Studies with pure and mixed cultures of microorganisms indicate that a range of fungi, actinomyces, yeast, and bacteria effect the aggregation of soil particles in the presence of a suitable energy material. This aggregation is transitional when microflora is present since the aggregating agents per se are subject to microbial degradation in the absence of more readily available carbon sources. The temporary increase in aggregation frequently observed following incubation of soils amended with organic materials is related closely to microbial activity; the more favorable the incubation conditions are to microbiological 14 decomposition of organic matter, the more rapid, but more short-lived, is the ameliorative effect. Oades (1984) showed that polysaccharides produced by bacteria, fungi and actinomycetes increased soil macroaggregates. Studies (Chapman and Lynch 1985) using cocultures of a cellulotytic fungus with capsular organisms show that microbial polysaccharides can be synthesized during the degradation of wheat straw. Several other cocultures also produced microbial polysaccharides. These studies indicated that crops produce polysaccharides directly by root exudates, by crop residues and/or indirectly by microbial activity. 3.5. Interaction of Polysaccharides with Clay or Oxides to Form Complexes A number of studies of polysaccharide-clay interaction have been reported. Various plant polysaccharides, such as starch, cellulose dextrin, inulin and water-soluble corn polysaccharides or animal polysaccharides, have been found to be strongly adsorbed by kaolinite and montmorillonite clays (Lynch et al. 1956; Parfitt and Greenland 1970; Chenu and Guerif 1991). Adsorption occurs on both surface and interlamellar space (Cheshire 1979). Clapp et al. (1968) showed that a rhizobial polysaccharide was sorbed between the expanding layers of montmorillonite. Tiessen and Stewart (1988) studied organo-mineral and microbial associations in microaggregates, and indicated the importance of polysaccharides and microbes in the mechanisms of aggregate formation and structure stabilization in soil. The binding concept for polysaccharides and hence, enhanced interparticle bonding has recently been deduced from tensile-strength measurements on clay minerals. Investigating the ability of polysaccharides to increase the strength of interparticle bonds, Chenu and Guerif (1991) measured the mechanical strength of clay-polysaccharide complexes. The polysaccharide was found to increase the mechanical strength of both kaolinite and montmorillonite minerals. 15 Complexes of clay minerals with polysaccharides are conveniently classified in terms of the charge characteristics of the polymers concerned. Polysaccharides possessing no charged groups in their structure are adsorbed by ion-dipole and Van der Waals interactions. Again, the driving force of the adsorption process would be provided by entropy effects arising from a net desorption of water molecules and the resulting gain in translational entropy by the system (Greenland, 1972). The presence of hydroxyl and other polar groups in the molecule also offers scope for hydrogen bonding of polysaccharides to surface oxygen of the silicate layer. These factors and the large number of clay surface to polysaccharide segment contacts that may be established generally lead to strong interactions between uncharged polysaccharides and clays. Since positive sites mostly exist on aluminum and hydroxides , negatively charged polysaccharides can be associated with the oxides by simple Coulombic attraction. The adsorption of negatively charged polysaccharides is then readily reversible by exchange with other anions. This type of adsorption is pH-dependent. In addition to simple negative adsorption, the exchangeable polyvalent cations act as a "bridge" between the carboxyl groups of the polysaccharides and the negatively charged clay surface. Since monovalent cations would only show a very limited, i f any, tendency to function in this way, a cation-bridge mechanism could not adequately account for the appreciable uptake by hydrogen clay systems. We must therefore presume that some A l ions (or their polyhydroxy derivatives) were 'present at the exchange sites of the H +montmorillonite, used by Lynch et al.(1956, 1957), such ions being released from their positions in the octahedral sheet of the silicate layer by proton attack. This acid-induced transformation of hydrogen clays into the corresponding H + / A 1 3 + forms may even lead to a reversal in sorption capacity, that is, the hydrogen clay becoming a better adsorbent for anionic polysaccharides as compared with its calcium saturated counterpart. Polysaccharides containing positively charged amino sugar units in their structure would be complexed with negatively charged clays. The adsorption of cationic 16 polysaccharides by layer silicates is primarily controlled by an exchange reaction between the cationic groups of the polymer and cations occupying exchange sites at the mineral surface (Parfitt, 1972). Further support for such a mechanism is provided by the relative strength of their adsorption. A positively charged segment is adsorbed onto a negatively charged site at the clay surface more strongly than an uncharged polymer segment due to Van der Waals or hydrogen bonding. The cationic property of weakly basic polysaccharides is strongly p H dependent. Thus adsorption by this mechanism is related to both the nature of the polysaccharides and the p H of system. Adsorption is also influenced by the type and quantity of cations on the exchange complex and type of clay. 3.6 Cover and Ley Crops and Soil Aggregate Stability Soil aggregation can be increased by the use of cover crops or ley crops and reduced tillage (Weill et al. 1988; Dormaar and Lindwall, 1989; Meek et al. 1990). Haynes and Swift (1990) found that, two years after an arable soil was converted to pasture, soil aggregate stability significantly increased though the amounts of organic C and total acid-hydrolysable carbohydrate did not. Emily et al. (1991) showed that cover crops can have rapid and significant effects on the stability of soil macroaggregates, even when the total amount of organic C in soils is apparently not affected. They have identified heavy fraction (HF) carbohydrate as a fraction that responds quickly to increases in C input by cover crops and can be an important force in macroaggregate stabilization. Their results are consistent with the hypothesis that some fraction within soil carbohydrate is involved with short-term improvements in soil structure by cover crops. Microbial processes play an important role in ley or cover-crop-induced changes in soil structure, composed largely of extracellular polysaccharides (EPS) produced by soil 17 microorganisms, has been shown to promote aggregate stability (Griffiths, 1965; Cheshire, 1979; Molope etal., 1987). 18 4.0 Materials and Methods This study consisted of three experiments: 1) cover crop trials; 2) reclamation trial; 3) incubation trial. 4.1 Site Description The cover crop and reclamation trials were conducted at three sites located in Delta Municipality about 30 km south of Vancouver, British Columbia. The average annual precipitation of the area is 1100 mm. The average mean daily temperature is 17.3 °C during July and 3 °C during January. The soil at both cover crop sites is a Westham silty clay loam and is classified as a Rego Humic Gleysol (Luttmerding 1981). The soil at the Reclamation Trial Site is a Ladner silty clay loam and is classified as a Humic Luvic Gleysol. Soil texture and some chemical characteristics of soils from the study sites are shown in Appendices 1, and 2. 4.2 Cover Crop Trials In 1992-1993, three treatments: control (bare soil), spring barley (Hordeum vulgare L . , Virden), and fall rye (Secale cereale L . , Danko), were included; in 1993-1994, in addition to the above three treatments, another cover crop, annual ryegrass {Lolium multiflorum, Aubade) was added. The cover crop trial was a randomized complete block design with four replicates on 6 m x 9 m plots with 10 cm row width in two winter seasons, 1992-1993 and 1993-1994. Plots were seeded on August 20 in both experimental years. Spring barley and fall rye were seeded at 100 kg/ha and annual ryegrass was at 25 kg/ha (1993-1994 only). Soils were sampled at 0-5 cm on M a y 6, 1993 for the 1992-1993 trial and Apri l 17, 1994 for the 1993-1994 trial, respectively. In addition to the two cover crop trials, a long-term pasture field adjacent to the cover crop site in 1992-1993 was selected to compare to the cultivated, cover cropped sites. 19 4.3. Reclamation Trial The reclamation trial was established in 1991. The field was divided into two parts: the southern part in which filter wrapped continuous plastic drain tiles were installed and the northern part with no drainage facilities. The drain tiles were placed at depths ranging from 0.8 to 1.2 m below the surface and at 9.14 m spacing. Two kinds of cropping were conducted on half of the each part of the field. The cropping history is summarized in Table 4.1. Thus, this experiment was a two-factor and two-level design, drainage status (drained vs undrained soils) and crop species (grass vs cash crop-clover [Trifolium pratense L . , Pacific doublecut]), which constituted four treatments: cash crop- clover on undrained soil; cash crop-clover on drained soil, grass on undrained soil; grass on drained soil. The grass consisted of 95% Tall fescue (Festuca arundinacea Schreb) and 5% Timothy (Phleum pratense L . ) . Soils were sampled on May 6, 1993. Table 4.1 The cropping history on the Reclamation Site Cropping program Year Cash crop/clover Grass 1991 sweet corn + fall rye cover crop peas + winter barley cover crop 1992 peas + red clover cover crop spring barley + grass 1993 barley + alsike clover cover crop grass 1994 alsike clover hay grass 20 4.4 Incubation Trial In the incubation experiment, a bulk sample was collected from the 30-50 cm depth at the cover crop site in 1993-1994 and sieved at field moisture content through a 6 mm sieve. The study included five treatments: control (no amendment), annual ryegrass (12 g/kg), fall rye (9g/kg), double-dose fall rye (18 g/kg), starch, equal to 50 % of the average standing biomass of the three field cover crop treatments (6.5 g/kg). The fresh green shoot and coarse root materials of cover crops were chopped to approximately 1 mm length. The application rates, which are expressed as amounts of dry matter per kilogram soil, o f the cover crop amendments were similar to mixing standing biomass of shoots and coarse roots found in the field plots to a depth of 10 cm in the soil. Each amendment was mixed with 400 g soil in a 1000 mL plastic container, which was covered with a lid. The soil moisture content was adjusted to field capacity (250 g/kg) gravimetrically with tap water. Incubation was conducted under aerobic conditions with three replicates at the room temperature ( 1 9 - 2 2 °C) in the laboratory for 2, 4 and 8 weeks. The container lids were opened three times a day for three hours for aeration. The containers were weighed three times weekly to determine water loss and appropriate amounts of water were added. The five amendments and three incubation periods formed a factorial design. After the soil from each container was sampled, the remainder was discarded. 4.5 Laboratory Methods 4.5.1 Wet-sieving Methods A modification of the wet-sieving method of Yoder (1936) was used. Two wet-sieving methods, direct wet-sieving and vapor prewetting of samples , were used. The direct wet-sieving method was used for the 1992-1993 cover crop trial, reclamation trial and pasture samples. For the direct wet-sieving, the equivalent of 100 g of bulk soil was transferred to the uppermost of a set of two sieves having 2.0 and 0.25 mm aperture mesh respectively. The water level in the tub was adjusted so that the aggregates on the 2.0 mm sieve were 21 just submerged at the highest point of the oscillation. The oscillation rate was 31 cycles per minute, the amplitude of the sieving action was 3.5 cm and the period of sieving was 15 min. The results are expressed as a mean weight diameter ( M W D ) which is the sum of the fraction of soil remaining on each sieve after sieving for the standard time multiplied by the mean diameter of the adjacent sieve meshes, i.e. M W D = (fraction of sample on sieve x mean intersieve size). The upper and lower limits of M W D in this case were 4.0 and 0.125 mm, respectively. The prewetted samples were used for 1993-1994 cover crop trial and the incubation trial. The prewetting wet-sieving was the same as directly wet-sieving except that 1) 2-6 mm aggregate soil samples were wetted to saturation in a vaporizer before wet-sieving; 2) also a 1 mm aperture sieve was added into the middle of the two-sieve set. 4.5.2 Chemical Analysis Chemical composition of the bulk soil and three sizes of aggregates, 2.0-6.0, 0.25-2.0 and <0.25 mm diameter for 1992-1993 field experiment, the bulk soil for 1993-1994 cover crop trial and bulk and 1.0-2.0 and 0.25-1.0 mm diameter aggregates for the incubation experiment was determined as follows. The total and labile polysaccharides were analyzed by the modification, which was proposed by Lowe (1994), of Whistler and Wolfrom's (1962) technique. The acid-hydrolysis procedure for total polysaccharides was: 1) add 4.2 mL 72 % H2SO4 to 0.5 g soil in a 250 mL Erlenmeyer flask and moisten all of the sample by the acid; 2) after one hour, add 100 m L distilled water and autoclave at 100 kPa for 1 hour. For labile polysaccharides, the procedure was the same as for the total polysaccharide hydrolysis except that 100 mL of distilled water was added first, immediately followed with 4.2 m L ' of 72 % H2SO4. After autoclaving, cooled samples were filtered and a spectrophotometer was used to read absorbances at 480 nm. Results were determined as glucose equivalents. 22 The total nitrogen was determined by a semi-micro-Kjeldahl apparatus. Under a condenser, soil samples were acid-digested; and then steam-distilled with l O M N a O H . The resulting NH3-N was trapped in 5 % boric acid and titrated. Total carbon was analyzed with the L E C O Induction Furnace and Carbon Analyzer (Furnace Model 521 and Analyzer Model 572-200). 4.6 Statistical Analysis Data were subjected to variance analysis and correlation analysis. In variance analysis, the A N O V A procedure of a computer SAS program was used (Little and Hills 1978). Statistical significance was determined at the probability level of 5 %. Least Significant Different (LSD) test was used to compare means following a significant F-value test. Comparison of the 1992 block means (four) to the four samples from the adjacent pasture site were made using a t-test. 2 3 5.0 Results and Discussion 5.1. Cover Crop Trials In 1992-1993, mean weight diameter ( M W D ) , the contents of total organic C, total and labile polysaccharides and total N were not significantly different among the two cover crops and bare control (Table 5.1). The higher variability, coefficient of variation (Table 5.2) may have resulted from the absence of a pre-wetting treatment when the amount of entrapped air within soil aggregates was high. Dickson et al (1991) showed that the effect of entrapped air, which led to aggregate explosion when soil was directly immersed in water, was most effectively reduced by slow wetting. Also, in Apri l o f 1993, there was more precipitation than usual (Fig. 5.1). The wet soil made soil aggregate stability weak and may have reduced M W D ' s and the differences between them. Table 5.1. Changes in water stable aggregates and some organic chemical properties induced by cover crops (1992-1993). Soils were sampled on M a y 6, 1993. Total Labile Total N Treatment M W D Total C polysac- polysac- (g/kg) (mm) (g/kg) charides charides (g/kg) (g/kg) Fa l l 0.59 a* 22.03 a 11.26 a 10.36 a 1.82 a rye Spring 0.55 a 19.66 a 10.64 a 9.78 a 1.76 a barley Bare 0.50 a 19.29 a 10.49 a 9 .31a 1.92 a soil (Control) * Means followed by the same letter are not significantly different at p=0.05. 24 Table 5.2. Comparison of coefficients of variation of winter cover crops at cover crop site between 1992-1993 and 1993-1994. Coefficient of variation ( C . V . ) Analyzed properties 1992-1993 1993-1994 M W D 20.86 14.88 Total Carbon 16.84 4.90 Total polysaccharides 17.19 6.09 Labile polysaccharides 14.49 7.85 Total nitrogen 3.13 3.97 Table 5.3 showed that the pasture site had very significantly higher M W D value, total carbon, labile and total polysaccharides and total nitrogen than the adjacent cropped site. Many workers have compared long-term cultivated sites with long-term pasture.(Kemper and Koch , 1966; Clement, 1975; Chaney and Swift, 1984). Their results were consistent with this experiment. Long-term pasture samples have a considerably higher total carbon and aggregate stability than their long-term arable counterparts. Although humified binding agents under long-term pasture play a major role, polysaccharide contents and other temporary organic binding agents are much higher under long-term pasture than under long-term tillage. Haynes et al. (1991) showed that pasture had high aggregate stability, organic C, acid-hydrolyzable carbohydrate, hot water-extractable carbohydrate and biomass C. The contents of total carbon, total nitrogen, total polysaccharides and labile polysaccharides in macroaggregates at the cover crop site were significantly higher than 25 Figure 5.1. Comparison of monthly precipitation in 1992-1993,1993-1994 and 1937-1990. 180 m 1992-1993 El 1993-1994 BNormal (1937-1990) 26 Table 5.3. Comparison of M W D and contents of organic constituents between cover crop and pasture sites in 1992-1993. Soils were sampled on May 6, 1993. Soil Properties Site Cover crop Pasture t value Probability M W D (mm) 0.550 Total carbon 18.70 (g/kg) Total polysac- 8.975 charides (g/kg) Labile polysac- 8.325 charides (g/kg) Total nitrogen 1.750 (g/kg) 1.425 100.6 43.53 38.23 8.428 •14.18 -14.65 -11.96 •10.68 -21.01 0.0001 0.0001 *** * * * 0.0011' 0.0014 0.0001 *** those in microaggregates (Fig. 5.2). The total carbon, total polysaccharide and total nitrogen contents followed the order: (2-6 mm) = (0.25-2 mm) > (<0.25 mm), and the labile polysaccharide content followed a similar order: (2-6 mm) > (0.25-2 mm) > (<0.25 mm) After analyzing C, N and P in different aggregates, Elliott (1986) showed that macroaggregates had considerably higher organic C, N and P than microaggregates. According to Tisdall and Oade's experiment, extracellular polysaccharides and fungal hyphae produced and /or induced by plants were responsible for holding macroaggregates together (Tisdall and Oades 1979; Oades 1984). Thus the contents of total carbon and polysaccharides in larger aggregates were higher than those in smaller aggregates. 27 Figure 5.2 Contents of total carbon, labile and total polysaccharides and total nitrogen in different soil aggregates during cover crop's growing period. Soils were sampled on May 6, 1993 25 - r 20 _ 15 o ~5> c c o o 10 4-5 4 Carbon Total Labile polysaccharides polysaccharides Soil constituents Total nitrogen • 2-6 mm aggregates S 0.25-2 mm aggregates U <0 .25 mm aggregates 28 In 1993-1994, the soils under annual ryegrass and fall rye had significantly higher M W D values and percentages of water stable 2-6 mm aggregates than the control in the cover crop site, but the spring barley treatment was similar to the control (Table 5.4). The M W D value followed the order: annual ryegrass > fall rye > spring barley = control. The annual ryegrass treatment had significantly higher total carbon content than control, but fall rye and barley did not. However, the total carbon contents followed the order similar to M W D values. Correlation between carbon content and M W D value was not significant (Table 5.5). Table 5.4. Changes in water stable aggregates and some organic chemical properties induced by cover crops (1993-1994). Soils were sampled to a depth of 5 cm on Apri l 17, 1994. Treatment M W D (mm) 2-6 mm water-stable aggregates (%) Total C (g/kg) Total polysac-charides (g/kg) Labile polysac-charides (g/kg) Total N (g/kg) Annual 1.985 a* 42.5 a 19.85 a 10.75 a 10.21 a 1.75 a ryegrass Fall 1.665 ab 33.5 b 18.75 ab 9.40 b 8.76 b 1.73 a rye Spring barley 1.350 be 25.5 be 18.18b 9.38 b 8.44 b 1.65 a Bare 1.237 c 20.5 c 17.43 b 8.95 b 7.34 c 1.65 a soil (Control) * Means followed by the same letter are not significantly different at p=0.05. 29 A l l the cover crop treatments had significantly higher labile polysaccharide contents than the control. Soils under annual ryegrass had greater content of labile polysaccharides than under fall rye and spring barley. For total polysaccharides, only the annual ryegrass treatment was significantly higher than the control. M W D value was highly correlated with labile polysaccharide content and moderately correlated with total polysaccharides (Table 5.5 and Fig. 5.3). Table 5.5 Linear correlation coefficients (r) between M W D values and soil organic constituents for field and incubation samples. Incubation Samples 1993-1994 Field Samples soil N o . of Correlation No . of Correlation organic observations coefficients observations coefficients constituents Total C 45 0.445 ** 16 ns Total N 45 0.304 * 16 0.520 * Total 45 0.513 *** 16 0.543 ** Polysaccharides Labile 45 0.801 *** 16 0.660 *** Polysaccharides Statistical significance shown: *p<0.05, **p<0.01, ***p< 0.001, NS=not significant Total N contents were not affected by cover crops, although M W D values were weakly correlated with total N contents. Angers and Mehuys (1988) showed that, similar to their field results, after 9 weeks in a growth chamber, soil cropped to both barley and alfalfa had larger M W D values than 30 Figure 5.3. The relationship between mean weight diameter and soil labile polysaccharide content 2.5 T 2.3 + 1.1 + 0.9 -f 0.7 + 0.5 -I 1 \— 1 • 1 1 1 1 1 5 6 7 8 9 10 11 12 13 Labile polysaccharide content (g/kg) 31 fallow. Short-term increase of aggregate stability is in the top layers, where most of the fine roots, organic matter and fungi are found (White et al., 1989). Spring barley was winter-killed, therefore, it stopped growth prior to the next spring. However, annual ryegrass and fall rye remained vegetative over the winter months and continued growth in early spring. They developed densely ramified root systems and their root systems remained active. Large quantities of organic materials are supplied to soils from the roots o f cover crops during their growing periods. The major source o f this is dieback of root segments and root hairs, but roots also exude a range of organic substances (Shamoot et al., 1968). The exudates and other organic constituents result in the production of large amounts of active polysaccharide binding agents in the surface soil (Haynes et al., 1991). Because organic C is supplied to the soil by the cover crop roots, a dense microbial population proliferates in the rhizosphere and this also produces exocellular mucilaginous polysaccharide materials that have capacity to stabilize soil aggregates (Lynch and Bragg, 1985). Another possible role of growing plants is to retard the decomposition of organic matter in soils (Jenkinson 1977). Annual ryegrass had higher shoot biomass than fall rye (Table 5.6) and has been shown to have notably higher root mass and length densities than other crops in both the 0-10 and 10-20 cm soil layers (Haynes and Francis 1993), thus, annual ryegrass produced more metabolic organic products including transient binding agents by its root system than fall rye (Shamoot et al., 1968). Biological activity of soil microorganisms was likely more vigorous under annual ryegrass than under fall rye because there was more substrate. Soil microorganisms under annual ryegrass could, therefore, synthesize more polysaccharides and increase soil aggregate stability and 2-6 mm water-stable aggregates. Also, the larger root system under annual ryegrass likely increased soil aggregate stability by directly entangling soil particles to form aggregates. 32 Table 5.6. The shoot biomass (kg/ha) of winter cover crops in 1992-1993 and 1993-1994. Cover crops Sample times Apri l 30, 1993 Nov. 24, 1993 Apri l 14, 1994 Spring barley 4135 4205 1563 Fall rye 5615 3636 7598 Annual ryegrass - 4310 8870 Tisdall and Oades (1979) observed that cropping to annual ryegrass increased the proportion of aggregates larger than 2 mm. From these results, the authors postulated that aggregates between 0.05 and 0.25 mm were bound into units 2-6 mm. Applying the same reasoning to our data would suggest an aggregation model in which <2 mm aggregates were bound into large aggregates 2-6 mm, i.e. the relative increase of larger aggregates was at the expense of smaller aggregates, which is in agreement with other work (Angers and Mehuys 1988, 1989). Although total polysaccharides are considered to be extremely important in relation to soil aggregate stability, total polysaccharide content was sometimes not significantly better correlated with aggregate stability than organic C content (Chaney and Swift, 1984; Baldock et al., 1987; Haynes et al., 1991). Baldock et al. (1987) observed that both total acid-hydrolyzable carbohydrate content and content of its constituent individual monosaccharides was not significantly correlated with the aggregate stability of an Ontario soil under different cropping histories. Total polysaccharide content does not differentiate between active and inactive carbohydrate binding agents. They concluded that i f carbohydrate materials are involved in changes in aggregate stability that occur over 33 relatively short time periods, then a specific pool of carbohydrate must be involved. Haynes and Swift (1990) showed that the hot water-extractable carbohydrate fraction approximated the active binding agents. The labile polysaccharide fraction measured in this study during crop growing periods appears to represent such a pool. Labile polysaccharides accounted for about 75-95 % of total polysaccharides and this fraction represented the easily hydrolyzable carbohydrates, originating from soil microorganisms and crop exudates, which are involved in aggregation. Another explanation for the low correlation coefficient between M W D value and content of total polysaccharides was the model hypothesized by Molope et al. (1987), in which there were a limited number of microsites between soil particles where polysaccharides could act as adhesives in aggregate formation. Any polysaccharide in excess of that occupying all the available sites would add little or nothing to aggregate stability. That cover crops increased soil polysaccharide content but did not increase N suggested that cover crops input less N - than C-compounds by their root systems during their growing periods. Because polysaccharides contain less N and soil aggregate stability closely correlated with labile polysaccharide content, it is also suggested that labile polysaccharides are the main binding agents for aggregation during the crop growing period. This study supports Chaney and Swift (1984 ) results that soil aggregate stability was highly correlated with soil total nitrogen though the correlation coefficient (r=0.52 ) in this study was smaller than in theirs (r=0.895), because of different soil type, base exchange capacity, crop system and organic matter content. Also, they sampled soils in fall when crops were harvested and all the crop residues were incorporated into soils, while we sampled in late spring when fall rye and annual ryegrass grew vigorously and organic substances were being supplied to the soil only by the root system. 34 5.2. Reclamation Trial M W D values of grass treatments was significantly higher than that of clover treatments; but drainage did not affect aggregate stability (Table 5.7). Table 5.7. Influence of cover crops and drainage system on soil aggregate stability, contents of total carbon, total polysaccharides and labile polysaccharides at Reclamation Site (soils sampled on May 6, 1993 to a depth of 5 cm). Crops Drainage status Soil properties Grass Cash crop/clover Drained soils Undrained soils M W D (mm) 0.6125 a* 0.5183 b 0.5825 A 0.5483 A Total carbon (g/kg) 28.52 a 26.33 b 25.80 B 2 9 . 0 4 A Total polysac-charides (g/kg) 13.59 a 12.40 a 12.50 A 13.50 A Labile polysac-charides (g/kg) 11.54a 10.38 b 11.16 A 10.76 A * Means followed by the same letter are not significantly different at p=0.05. The total carbon content under grass treatments was significantly higher than under cash crop/clover treatments, and the total carbon content in the undrained soil was significantly higher than that in the drainage. Total polysaccharide contents were not significantly affected by either crop or drainage. The labile polysaccharide contents under grass treatments were significantly higher than 35 that under cash crop/clover treatments, but drainage did not change labile polysaccharide content. Total carbon, total polysaccharide and labile polysaccharide contents among 2-6 mm, 0.25-2 mm and <0.25 mm aggregates were significantly different and followed the similar order to those at the cover crop site in 1992-1993: (2-6 mm ) > (0.25-2 mm) > (<0.25 mm) (Fig. 5.4). Grass, tall fescue, has a larger root system than clover, thus, the entanglement by root and hyphae was more effective under grass than under cash crop/clover and organic binding agents exuded by the grass root system were higher than by cash crop/clover roots. Soil water content also physically influences soil aggregation and aggregate stability. It has been shown that drainage reduces soil water content. This drying process can result in an increase in the effective stress (Croney et al., 1958) or intergranular contact pressure (Groenevelt and Kay, 1981). In saturated soils that are not under a load or confining pressure, the effective stress is equal to the swelling pressure. In unsaturated soils the effective stress is proportional to the pressure drop across the menisci of water films surrounding the particles, that is , soil water suction. On the other hand, drainage increases the number of wetting and dying cycles, which result in a progressive decrease in aggregate strength (Dexter et al., 1984) and can result in progressive decrease in aggregate size (Shiel et al., 1988). Large aggregates would be expected to have a large number of failure zones with low strength and would be expected to be more susceptible to wetting and drying. The two opposite physical processes for the soil aggregate stability in this experiment may have counteracted each other. Also the drainage system had only been installed for 2 years when soils were sampled. It may be too little time to improve soil aggregate stability under the drained soils. Thus, soil aggregate stability under drainage condition did not significantly increase in this study. 36 Figure 5.4. Contents of total C, total polysaccharides and labile polysaccharides in different soil aggregate at. Reclamation Site 37 The total amount of organic substances deposited below ground by grass and the resistance of this material to complete mineralization is larger than that by other crops. Davenport and Thomas (1988) indicated that the total amount of photosynthate deposited below ground by grass was twice as large as that by corn, and carbon originating from bromegrass showed greater persistence than that from corn (Davenport et al., 1988). Although those studies did not compare grass with a combination of cash crop and a clover cover crop, they suggested that the organic carbon and other organic binding agents produced by grass were greater and resisted decomposition more than under annual crops. Webber (1965) showed that the polysaccharide content under continuous grass (7.24 g/kg) was higher than under a rotation of corn, oats, winter wheat and red clover (5.53 g/kg). Although the contents of labile polysaccharide under grass and other crops in our study were higher than those in the above studies, they followed similar trends. Accumulation of organic materials in soils was influenced not only by quality and quantity of organic matter deposited by crops, but also by water regime, which in turn affected biological activities including crops and microorganisms. At the Reclamation Site, the clover grew more vigorously, thus, supplied more organic carbon and other binding agents into the soils under drained than under undrained conditions; the grass growth did not show a big difference between the drained and the undrained soils. On the other hand, aeration in drained soils is generally better than in undrained soils, therefore, activities of microorganisms in the drained soils may have been faster than in the undrained soils. Soil organic matter in the undrained soils probably decomposed more slowly than in the drained soils. The total results of the above complex biological processes determined whether or not soil total C and total and labile polysaccharides, which influence soil aggregate stability, increased. In this experiment, the decomposition of organic matter may be more influenced by the different water regimes than by the different total quantities of organic 38 matter supplied by the crops, thus, total carbon content in the undrained site was higher than in the drained site. 5.3 Incubation Tria l A l l the amendments significantly increased soil M W D values at all incubation periods except the fall rye treatment after 4-weeks incubation (Fig. 5.5). Figure 5.5 also shows that after 2-weeks incubation, the starch treatment had a higher M W D than the double-dose fall rye and fall rye amendments. After the 8-week incubation, the double-dose fall rye and fall rye amendments had higher M W D values than the starch amendment. A l l the amendments significantly increased percentages of 2-6 mm water-stable aggregates at all incubation periods (Fig. 5.6). This trend was similar to that with M W D values for the treatments. After 4-weeks incubation, the 2-6 mm water-stable aggregate percentage of the double-dose fall rye amendment was significantly higher than those of the other amendments. After 8-weeks incubation, the 2-6 mm water-stable aggregate percentages of all the amendments reached similar levels. Fig. 5.7 shows that, averaged over incubation periods, all amendments significantly increased total carbon over the control. Total carbon contents of the double-dose fall rye amendment were higher than those of the other amendments. The content of carbon averaged over all the amendments was reduced after 4-weeks incubation (Fig. 5.8). M W D values were moderately correlated with soil total carbon contents (Table 5.5). Averaged over incubation periods, the amendments did not significantly increase total polysaccharide contents during the experiment. However, all o f the amendments significantly increased labile polysaccharide contents in comparison with the control (Fig. 5.9). The labile polysaccharide contents of the annual ryegrass, double-dose fall rye and starch amendments were significantly higher than that of the fall rye amendment. Averaged over all amendments, both total and labile polysaccharide contents did not 39 Figure 5.5. Influence of amendments on aggregate stability during different incubation times 2-week incubation 4-week incubation 8-week incubation Incubation time a Starch DD Annual ryegrass H double-dose fall rye H Fall rye S Control 40 Figure 5.6. Influence of amendments on 2-6 mm water-stable aggregates at different incubation times. 2-week 4-week 8-week incubation incubation incubation Incubation time S Starch • Annual ryegrass.• Double-dose fall rye U Fall rye B Control 41 Figure 5.7. Influence of amendments on soil total carbon contents averaged over incubation periods Annual Double-dose Starch Fall rye Control (no ryegrass fall rye amendment) Soil amendments 42 Figure 5.8. Changes of total carbon, total and labile polysaccharide and total carbon contents with incubation times averaged over amendments I • limiiiiiinHiiiiim | total C total labile polysaccharides polysaccharides Soil constituents total N 12 weeks HO 4 weeks H 8 weeks 43 Figure 5.9. Influence of amendments on soil labile polysaccharide contents averaged over incubation periods 9 T Annual Double-dose Starch Fall rye Control (no ryegrass fall rye amendment) Soil amendments 44 change during the incubation periods (Fig. 5.8). Soil M W D values were highly correlated with both total polysaccharide and labile polysaccharide contents (Table 5.5). The effectiveness of added organic materials in increasing the stability of soil aggregates is determined by the amount and decomposibility of the organic amendment. Because starch is the easiest decomposed material of the three kinds of amendments, it should stabilize soil aggregates or glue soil particles together in the short incubation period (2-weeks). Although the green cover crops are rapidly decomposable materials, they are not so easily decomposable as starch. Thus, after 2-weeks incubation, the M W D value of the starch amended soil was the highest of all the treatments and was significantly higher than that of the double-dose fall rye and fall rye amendments. The results also agree with other studies, for example, Rennie et al. (1954) found as little as 0.2 g/kg of bacterial polysaccharide synthesized by Agrobacterium diobacter added to a silt loam soil increased the number of aggregates greater than 0.1 mm diameter by about 50 %. Polysaccharides, however, are transient binding agents, i.e. they are easily attacked by microorganisms. After 8-weeks incubation, M W D and the labile polysaccharide content of starch amended soils were reduced due to its decomposition, but the green cover crop amendments still released polysaccharides and other organic binding agents into soils, which increased labile polysaccharide content, although soil polysaccharides were decomposed at same time. Thus, M W D values and labile polysaccharide contents under double-dose fall rye and fall rye amendments were higher than in starch amended soil after 8-weeks incubation. The balance between the input and decomposition of organic carbon, polysaccharides and other binding agents determine whether soil aggregate stability is increased or decreased by amendments. After 8-weeks incubation, the double-dose fall rye amendment reached the highest M W D value and proportion of 2-6 mm water-stable aggregates of all the treatments because of the larger quantities of organic materials supplied to the soil by 45 this treatment. This further showed that the more amendment added, the more effective it is for increasing soil aggregate stability. Annual ryegrass materials applied in this study were more tender and probably more easily decomposable material than the fall rye amendment. Also the annual ryegrass amendment added more total organic matter to soil because of its higher biomass and produced more polysaccharides and other organic binding agents by decomposition. Thus, the annual ryegrass amendment was more effective at producing potential binding agents than the fall rye amendment and tended to produce higher M W D values (N.S. at probability = 0.05) after the 2 and 4-week incubation periods. On the other hand, these added organic binding agents were, similar to starch, decomposed by microorganism. This process reduced soil organic binding agents and soil aggregate stability. After 8-weeks incubation, the annual ryegrass amendment may have resulted in less organic binding agents in the soil and a tendency to lower M W D value as compared to the fall rye amendment. That the amendments significantly increased 2-6 mm water-stable aggregates was consistent with the field results. This further showed that organic amendments tend to build up larger aggregates. The contents of total N for all the green cover crop amendments were significantly higher than for the starch amendment and control (Fig. 5.10). Average contents of total N for all treatments were stable during the incubation periods (Fig. 5.8). M W D values were weakly correlated with total nitrogen contents (Table 5.5). It is easily understood that the green cover crop amendments increased soil total nitrogen because they contained large quantities of N-compounds, but pure starch did not. There were no significant differences in contents in total carbon, labile polysaccharides and total N between 1-2 mm and 0.25- 1 mm aggregates (Table 5.8). Total polysaccharides were higher in the larger aggregate fraction. 46 Figure 5.10. Influence of amendments on soil total nitrogen content averaged over incubation periods Table 5.8. Comparison in contents of organic constituents between 1-2 mm and 0.25-1 mm water stable aggregates after 8-week incubation. Aggregate size Organic Difference t value Proba Constituents 0.25-1 mm 1-2 mm of content bility Total carbon 24.51 25.94 -0.095 -1.727 0.106 (g/kg) Total polysac- 9.560 10.23 -0.217 -3.08 0.0081** charides (g/kg) Labile polysac- 7.620 7.810 -0.190 -0.988 .0.340 charides (g/kg) Total nitrogen 1.380 1.450 -0.070 -1.394 0.185 (g/kg) Table 5.5. showed that the significant correlation between M W D and total carbon content in the incubation study differed from the lack of relationship between organic carbon and M W D in the field study, however, the correlations between M W D values and labile polysaccharide contents were at the same significance levels. In the incubation experiment, all o f the parts of cover crops including tops and coarse roots, which contained a variety of carbon materials including labile polysaccharides, were mixed with the soil. The content of added labile polysaccharides was approximately proportional to total carbon, thus, i f labile polysaccharide content was better index for soil aggregate stability, M W D values would be correlated with both total carbon and labile polysaccharide contents. However, during the crops' growing period, carbon materials came only from root exudates and root dieback materials, which resulted in lower quantities of total carbon supply in the field studies than after the incorporation of cover 48 crop tops into the soil. The root exudates and other microorganism substrates contained a large proportion of labile polysaccharides, which were soil binding agents. Thus, M W D correlated with labile polysaccharides and did not correlate with total carbon. Comparison of the results between the cover crop field trial and the incubation trial suggests that incorporating cover crop tops into soil was more effective for soil aggregate stability than effects measured during their growing period. In the 1992-1993 cover crop trial, cover crops did not show significant benefit for soil aggregate stability because of higher variability and more precipitation when soils were sampled. In the 1993-1994 cover crop trial, overwintered cover crops, annual ryegrass and fall rye, increased M W D values, but winter-killed spring barley did not. A l l the cover crop amendments in the incubation study increased M W D values (Fig. 5.5). The correlation between soil total polysaccharides and M W D of the incubation samples also was better than that in the field (Table 5.5). In addition to the different amounts of organic substances supplied to the field and incubated soils, the total organic matter and other organic constituents of the incubated soil was lower than those of the field soils before these experiments were conducted (Appendix 1). Martin at al. (1955) concluded that soil aggregate stability was increased more in a soil which was poorly aggregated owing to lower organic matter than in one which had higher organic matter and was relatively well aggregated. The results of this study supported their conclusion. 49 6.0 Conclusions 1) During the winter cover crop growing period in 1993-1994, annual ryegrass significantly increased the contents of total carbon and total and labile polysaccharides, which significantly increased soil M W D value and 2-6 mm aggregate percentage. Fall rye significantly increased the content of labile polysaccharides and soil M W D value. However, the spring barley treatment did not improve soil aggregate stability because it was winter-killed and produced lower biomass. 2) Long-term pasture had significantly higher M W D , total carbon, total nitrogen and total and labile polysaccharide contents than an adjacent cultivated cover crop site. 3) M W D value, total carbon and labile polysaccharide contents under grass (tall fescue) were higher than under a cash crop/clover treatment. However, drainage after 2 years did not affect M W D values and total and labile polysaccharide contents. The drained soils had less total carbon contents than undrained soils in this study at the Reclamation Site. 4) . The incubation experiment indicated that incorporation of green cover crops into soil increased the contents of soil total carbon, total nitrogen and labile polysaccharides which significantly increased soil M W D values. The starch amendment was more effective than the double-dose fall rye and fall rye treatments in the 2-week incubation for increasing soil aggregate stability, but less effective than the double-dose fall rye and fall rye treatments after 8-weeks. 5) . M W D values were highly correlated with labile polysaccharides and total polysaccharides, moderately correlated with total carbon and weakly correlated with total nitrogen in the incubation samples; M W D values were highly correlated with labile polysaccharides, moderately correlated with total polysaccharides, not significantly correlated with total carbon and weakly correlated with total nitrogen in the field samples. 50 6). The contents of total carbon, total nitrogen, total and labile polysaccharides in macroaggregates (>0.25 mm) were significantly higher than those in microaggregates (<0.25 mm). 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Organo-mineral fractions of a climosequence of soils in New Zealand tussock grasslands. J. Soil Sci.. 29:331-339. Theng, B . K . G. 1979. Formation and properties of clay-polymer complexes. Elsevier. Thornton, R. H . 1958. Biological studies of some tussock-grassland soils. II. Fungi. New Zealand Journal of Agricultural Research 1:922-93 8. Tiessen, H . and Stewart, J. W. B . 1988. Light and electron microscopy of stained microaggregates: the role of organic matter and microbes in soil aggregation. 5:312-322. Tisdall, J. M . and Oades, J. M . 1979. Stabilization of soil aggregates by the root systems of ryegrass. Aust. J. Soil Res. 17:429-441. Tisdall, J. M . and Oades, J. M . 1980a. The management of ryegrass to stabilize aggregates of a red-brown earth. Aust. J. Soil Res. 18:415-422. Tisdall, J. M . and Oades, J. M . 1980b. The effect of crop rotation on aggregation in a red-brown earth. Aust. J. Soil Res. 18: 423-434. Tisdall, J. M . and Oades, J. M . 1982. 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J. B . 1970. Relationships between the composition of soils and physical measurements made on them. Rothamsted Experimental Station Report, Part 2, pp. 5-53. Williams, R. J. B . 1979. Rothamsted studies of soil structure V . Comparative measurements of density, stability, strength, and water-holding capacity of soils from the Denchworth, Evesham, Ragdale, Hanslope, Salop and Flint series. J. Soil Sci. 30:453-461. Yoder, R. E . 1936. A direct method of aggregate analysis of soils and a study of the physical nature of soil erosion losses. Am. Soc. Agr. J. 28:337-351. 63 8.0 Appendices Appendix 1. Some chemical properties of composite soil samples taken from 0-20 cm on September 18, 1993 for the field study and a bulk soil sample taken from 30-50 cm on the cover crop site for the incubation study Soil properties Cover crop site Reclamation site Incubation sample p H 5.40 5.60 5.30 Organic matter (g/kg) 33.5 44.6 25.5 Total carbon (g/kg) 17.8 26.3 14.3 Total nitrogen (g/kg) 1.64 2.05 1.21 Total polysaccharides (g/kg) 8.80 11.8 7.50 64 Appendix 2. Soil texture at cover crop sites and Reclamation Site Site 1 ) Depth Sand Silt Clay Class % .... Cover crops 0-20 cm 1.7 58.7 39.6 silty clay loam (1992-1993) 20-40 cm 1.7 57.4 40.9 silty clay 40-60 cm 3.2 64.2 32.6 silty clay loam Cover crops 0-5 cm 3.1 63.2 33.7 silty clay loam (1993-1994) 0-20 cm 3.5 66.5 29.5 silty clay loam 20-40 cm 1.7 71.4 26.9 silt loam 40-60 cm 3.8 71.5 24.7 silt loam Reclamation 0-5 cm 3.6 61.6 34.8 silty clay loam Site 0-20 cm 4.3 65.2 30.5 silty clay loam 20-40 cm 5.2 71.5 23.3 silt loam 40-60 cm 10.5 69.3 20.2 silt loam 1) The site in 1992-1993 was different from that in 1993-1994. 65 Appendix 3. Analysis of variance for soil mean weight diameter at cover crop site in 1992-1993. Soils were sampled on May 6, 1993. Source Sum of D F Squares Mean Square F Value Pr > F Model Block Treatment Error Corrected Total 0.06375833 0.01275167 0.97 0.5022 0.04749167 0.01583056 1.21 0.3849 0.01626667 0.00813333 0.62 0.5693 0.07873333 0.01312222 11 0.14249167 Dependent Variable: M W D Appendix 4. Analysis of variance for soil total carbon content in water-stable aggregates under winter cover crops in 1992-1993. Soils were sampled on M a y 6, 1993. Source D F Anova SS Mean Square F Value Pr > F R E P C R O P R E P * C R O P (error 1) A G G R R E P * A G G R C R O P * A G G R R E P * C R O P * A G G R (error 2) Corrected total 3 2 6 2 6 4 12 2.43816667 0.49067222 1.27481667 2.61570556 0.98685000 1.70879444 2.30971667 0.81272222 0.24533611 0.21246944 1.15 0.3765 1.30785278 6.79 0.0106 0.16447500 0.85 0.5534 0.42719861 2.22 0.128 0.19247639 35 11.8247222 Dependent Variable: Carbon 66 Appendix 5. Analysis of variance for total polysaccharide content in water-stable aggregates under winter cover crops in 1992-1993. Soils were sampled on M a y 6, 1993. Source D F Anova SS Mean Square F Value Pr > F R E P 3 0.19603056 0.06534352 C R O P 2 0.03962222 0.01981111 0.27 0.7703 R E P * C R O P (error 1) 6 0.43597778 0.07266296 A G G R 2 1.61420556 0.80710278 14.67 0.0006 R E P * A G G R 6 0.12906111 0.02151019 0.39 0.8710 C R O P * A G G R 4 0.07429444 0.01857361 0.34 0.8474 R E P * C R O P * A G G R 12 0.66010556 0.05500880 (error 2) Corrected total 35 3.14929722 Dependent Variable: Total polysaccharides Appendix 6. Analysis of variance for soil labile polysaccharide contents in water-stable aggregate size under winter cover crops in 1992-1993. Soils were sampled on M a y 6, 1993. Source D F Anova SS Mean Square F Value Pr > F R E P 3 0.16267778 0.05422593 C R O P 2 0.06635000 0.03317500 1.27 0.3472 R E P * C R O P (error 1) 6 0.15693889 0.02615648 A G G R 2 0.72455000 0.36227500 15.43 0.0005 R E P * A G G R 6 0.01167222 0.00194537 0.08 0.9970 C R O P * A G G R 4 0.08750000 0.02187500 0.93 0.4781 R E P * C R O P * A G G R 12 0.28181111 0.02348426 (error 2) Corrected total 35 1.49150000 Dependent Variable: Labile polysaccharides 67 Appendix 7. Analysis of variance for total nitrogen content in water-stable aggregates under winter cover crops in 1992-1993. Soils were sampled on M a y 6, 1993. Source D F Anova SS Mean Square F Value Pr > F R E P 3 0 01351919 0.00450640 C R O P 2 0 00168022 0.00084011 0.93 0.4452 R E P * C R O P (error 2) 6 0 00542689 0.00090448 A G G R 2 0 01471239 0.00735619 6.51 0.0122 R E P * A G G R 6 0 00738072 0.00123012 1.09 0.4220 C R O P * A G G R 4 0 00452428 0.00113107 1.00 0.4446 R E P * C R O P * A G G R 12 0 01356194 0.00113016 (error 2) Corrected total 35 Dependent Variable: Nitrogen Appendix 8. Analysis of variance for soil mean weight diameter under winter cover crops in 1993-1994. Soils were sampled on A p r i l 17, 1994. Source Sum of D F Squares Mean Square F Value Pr > F Model 1.60758750 0.26793125 4.98 0.0163 Block Treatment 3 0.24856875 3 1.35901875 0.08285625 0.45300625 1.54 0.2703 8.42 0.0056 Error 0.48430625 0.05381181 Corrected Total 15 2.09189375 Dependent Variable: M W D 68 Appendix 9. Analysis of variance for 2-6 mm water-stable aggregates under winter cover crops in 1993-1994. Soils were sampled on A p r i l 17, 1994. Source Sum of D F Squares Mean Square F Value Pr > F Mode l Block Treatment Error Corrected Total 6 0.12260000 0.02043333 6.43 0.0071 3 0.01140000 0.00380000 1.20 0.3654 3 0.11120000 0.03706667 11.66 0.0019 9 0.02860000 0.00317778 15 0.15120000 Dependent Variable: > 2 mm aggregates Appendix 10. Analysis of variance for soil total carbon content under winter cover crops in 1993-1994. Soils were sampled on A p r i l 17, 1994. Source Sum of D F Squares Mean Square F Value Pr > F Model Block Treatment Error Corrected Total 6 0.51355000 0.08559167 10.35 0.0013 3 0.38810000 0.12936667 15.64 0.0006 3 0.12545000 0.04181667 5.06 0.0253 9 0.07445000 0.00827222 15 0.58800000 Dependent Variable: Total Carbon 69 Appendix 11. Analysis of variance for soil total polysaccharide content under winter cover crops in 1993-1994. Soils were sampled on A p r i l 17, 1994. Source D F Sum of Mean Squares Square F Value Pr > F Model 0.34960337 0.05826723 17.33 0.0002 Block Treatment 3 0.27647119 0.09215706 27.40 0.0001 3 0.07313219 0.02437740 7.25 0.0090 Error 0.03026506 0.00336278 Corrected Total 15 0.37986844 Dependent Variable: Total polysaccharides Appendix 12. Analysis of variance for soil labile polysaccharide content under winter cover crops in 1993-1994. Soils were sampled on A p r i l 17, 1994. Source D F Sum of Squares Mean Square F Value Pr > F Model 0.31626638 0.05271106 11.34 0.0009 Block Treatment 3 0.14858819 0.04952940 10.66 0.0026 3 0.16767819 0.05589273 12.03 0.0017 Error 0.04183206 0.00464801 Corrected Total 15 0.35809844 Dependent Variable: Labile polysaccharides 70 Appendix 13. Analysis of variance for soil total nitrogen content under winter cover crops in 1993-1994. Soils were sampled on A p r i l 17, 1994. Source Sum of D F Squares Mean Square F Value Pr > F Model 0.00168750 0.00028125 6.23 0.0079 Block Treatment 3 0.00136875 0.00045625 10.11 0.0031 3 0.00031875 0.00010625 2.35 0.1401 Error 0.00040625 0.00004514 Corrected Total 15 0.00209375 Dependent Variable: Nitrogen Appendix 14. Analysis of variance for soil mean weight diameter under cover and ley crops at Reclamation Site. Soils were sampled on May 6, 1993. Source Sum of D F Squares Mean Square F Value Pr > F Model 0.06591250 0.02197083 6.23 0.0037 C R O P 1 0.05320417 0.05320417 15.10 0.0009 D R A I N A G E 1 0.00700417 0.00700417 1.99 0.1740 C R O P * D R A I N A G E 1 0.00570417 0.00570417 1.62 0.2179 Error 20 0.07048333 0.00352417 Corrected Total 23 0.13639583 Dependent Variable: M W D 71 Appendix 15. Analysis of variance for soil total carbon under cover and ley crops at Reclamation Site. Soils were sampled on May 6, 1993. Sum of Mean Source D F Squares Square F Value Pr > Model 11 29.85198194 2.71381654 14.32 0.0001 C R O P 1 0.86900139 0.86900139 4.59 0.0363 D R A I N 1 1.89151250 1.89151250 9.98 0.0025 C R O P * D R A I N 1 0.04253472 0.04253472 0.22 0.6373 A G G R 2 22.8542194 11.4271097 60.3 0.0001 C R O P * A G G R 2 2.30120278 1.15060139 6.07 0.0040 D R A I N * A G G R 2 1.69247500 0.84623750 4.47 0.0155 C R O P * D R A I N * A G G R 2 0.20103611 0.10051806 0.53 0.5910 Error 60 11.36711667 0.18945194 Corrected Total 71 41.21909861 Dependent Variable: Total carbon Appendix 16. Analysis of variance for soil total polysaccharide contents under cover and ley crops at Reclamation Site. Soils were sampled on M a y 6, 1993. Sum of Mean Source D F Squares Square F Value Pr > F Mode l 11 11.21326111 1.01938737 13.70 0.0001 C R O P 1 0.25442222 0.25442222 3.42 0.0694 D R A I N 1 0.18000000 0.18000000 2.42 0.1251 C R O P * D R A I N 1 0.13347222 0.13347222 1.79 0.1855 A G G R 2 9.92295278 4.96147639 66.67 0.0001 C R O P * A G G R 2 0.3430027.8 0.17150139 2.30 0.1086 D R A I N * A G G R 2 0.30517500 0.15258750 2.05 0.1376 C R O P * D R A I N * A G G R 2 0.07423611 0.03711806 0.50 0.6098 Error 60 4.46493333 0.07441556 Corrected Total 71 15.67819444 Dependent Variable: Total polysaccharides 73. Appendix 17. Analysis of variance for soil labile polysaccharide contents under cover and ley crops at Reclamation Site. Soils were sampled on M a y 6, 1993. Sum of Mean Source D F Squares Square F Value Pr > F Model 11 3.40598194 0 30963472 11.37 0.0001 C R O P 1 0.23920139 0 23920139 8.78 0.0044 D R A I N 1 0.03001250 0 03001250 1.10 0.2981 C R O P * D R A I N 1 0.05723472 0 05723472 2.10 0.1524 A G G R 2 2.83801111 1 41900556 52.09 0.0001 C R O P * A G G R 2 0.07924444 0 03962222 1.45 0.2416 D R A I N * A G G R 2 0.08410000 0 04205000 1.54 0.2220 C R O P * D R A I N * A G G R 2 0.07817778 0 03908889 1.43 0.2462 Error 60 1.63455000 0 02724250 Corrected Total 71 5.04053194 Dependent Variable: Labile polysaccharides 74 Appendix 18. Analysis of variance for soil mean weight diameter of incubation experiment. Source D F Sum of Squares Mean Square F Value Pr > F Mode l C R O P T I M E C R O P * T I M E Error Corrected Total 14 0.69760787 0.04982913 11.83 0.0001 4 0.44029276 0.11007319 26.13 0.0001 2 0.10181373 0.05090687 12.09 0.0001 8 0.15550138 0.01943767 4.61 0.0009 30 0.12635933 0.00421198 44 0.82396720 Dependent Variable: M W D Appendix 19. Analysis of variance for 2-6 mm water-stable aggregates of incubation experiment. Source D F Sum of Squares Mean Square F Value Pr > F Mode l C R O P T I M E C R O P * T I M E Error Corrected Total 14 1274.400698 91.028621 6.49 0.0001 4 2 724.9874311 7.1463244 8 542.2669422 30 420.847467 44 1695.248164 181.2468578 12.92 0.0001 3.5731622 0.25 0.7768 67.7833678 4.83 0.0007 14.028249 Dependent Variable: 2-6 mm water-stable aggregates 75 Appendix 20. Analysis of variance for total carbon content of incubation experiment. Source Sum of D F Squares Mean Square F Value Pr > F Model C R O P T I M E C R O P * T I M E Error Corrected Total 14 1.27109013 0.09079215 9.41 0.0001 4 0.97531769 0.24382942 25.27 0.0001 2 0.21374413 0.10687207 11.08 0.0002 8 0.08202831 0.01025354 1.06 0.4143 30 0.28946467 0.00964882 44 1.56055480 Dependent Variable: Total carbon Appendix 21. Analysis of variance for total polysaccharide contents of incubation experiment. Source Sum of D F Squares Mean Square F Value Pr > F Model C R O P T I M E C R O P * T I M E Error Corrected Total 14 0.28762947 0.02054496 1.10 0.3947 4 0.11828636 0.02957159 1.59 0.2035 2 0.03114973 0.01557487 0.84 0.4435 8 0.13819338 0.01727417 0.93 0.5090 30 0.55920533 0.01864018 44 0.84683480 Dependent Variable: Total polysaccharides 76 Appendix 22. Analysis of variance for labile polysaccharide content of incubation experiment. Source Sum of D F Squares Mean Square F Value Pr > F Model 14 0.31580013 0.02255715 7.11 0.0001 C R O P T I M E C R O P * T I M E 4 0.28774591 0.07193648 22.69 0.0001 2 0.00152013 0.00076007 0.24 0.7883 8 0.02653409 0.00331676 1.05 0.4251 Error 30 0.09511667 0.00317056 Corrected Total 44 0.41091680 Dependent Variable: Labile polysaccharides Appendix 23. Analysis of variance for total nitrogen content of incubation experiment. Sum of Mean Source D F Squares Square F Value Pr > F Model 14 0.00854658 0.00061047 10.41 0.0001 C R O P T I M E C R O P * T I M E 4 0.00803858 0.00200964 34.27 0.0001 2 0.00034698 0.00017349 2.96 0.0672 8 0.00016102 0.00002013 0.34 0.9416 Error 30 0.00175933 0.00005864 Corrected Total 44 0.01030591 Dependent Variable: Total nitrogen 77 

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