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Effects of repeated fertilization and a straw application to the organic layers under Jack Pine and seedling… Kumi, Janna W. 1984

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EFFECTS OF REPEATED FERTILIZATION AND A STRAW APPLICATION TO THE ORGANIC LAYERS UNDER JACK PINE AND SEEDLING RESPONSE by JANNA W. KUMI B.A. (Hons), Concordia, 1972 Dip. Forst., U n i v e r s i t y of Munich, 1978 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Faculty of Forestry) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1984 © Janna W. Kumi, 1984 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r a n a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s m a y b e g r a n t e d b y t h e h e a d o f m y d e p a r t m e n t o r b y h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t b e a l l o w e d w i t h o u t m y w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a 1 9 5 6 M a i n M a l l V a n c o u v e r , C a n a d a V 6 T 1 Y 3 D a t e Oci. ^iq&H D E - 6 ( . 3 / 8 1 ) i i ABSTRACT In an optimum n u t r i t i o n experiment, a 45-year old jack pine (Pinus banksiana Lamb.) stand i n Quebec was repeatedly f e r t i l i z e d over a 10-year period with various l e v e l s of NPK f e r t i l i z e r s . In a separate experiment, a straw treatment was applied over snow to smother the ericaceous ground vegetation. Nitrogen f e r t i l i z e r additions for 10 years resulted i n increased humus biomass i n a l l treatments. The greatest gain was on plots receiving repeated low N doses. These organic layers also had the lowest decomposition rates. Heavier N applications increased humus decomposition s u b s t a n t i a l l y , but stand l i t t e r production was also increased. The straw-treated humus had decomposition rates approaching those found with high N additions. Repeated low N additions immobilized f e r t i l i z e r N within the humus. Most of the N applied at higher treatment levels appears to have been l o s t . Nitrogen mineralization rates were investigated i n an aerobic incubation study. N i t r i f i c a t i o n occurred i n spite of low pH (<4) on high N p l o t s . The straw addition increased humus nitrogen mineralization rates. The r e s u l t s of repeated additions of P and K were var i a b l e . Additions of P and K decreased nitrogen a v a i l a b i l i t y although decom-po s i t i o n rates were increased. It appeared that most of the P and K were l o s t from the organic layers due to leaching. Large N additions had l i t t l e e f f e c t on the humus C/N r a t i o . They increased the CEC and the pH but reduced the base saturation. i i i Straw additions lowered the humus C/N r a t i o but increased the CEC, pH and base saturation. Dramatic changes i n the ground vegetation occurred. With higher N additions the ericaceous vegetation was greatly reduced and increas-i n g l y replaced with Sambucus, Aster and other exotics. The straw app l i c a t i o n e f f e c t i v e l y smothered the ericaceous vegetation for 10 years. Seedling bioassay studies showed that both the straw treated humus and the sustained low additions of N plus P and K resulted i n the highest seedling biomass. This correlated w e l l with stand growth response. Seedling N n u t r i t i o n was adequate i n a l l treatments and r e f l e c t e d the a b i l i t y of mor humus to release immobilized N under improved environmental conditions i n the greenhouse. Some p o s s i b i -l i t i e s why f o l i a r N could not be correlated with seedling biomass are discussed. It i s concluded that i n jack pine stands with thin mor humus layers, repeated l i g h t nitrogen additions plus phosphorus and potassium r e s u l t i n s u f f i c i e n t nutrient turnover rates to ensure the highest stand response. i v TABLE OF CONTENTS Page ABSTRACT . i i TABLE OF CONTENTS i v LIST OF TABLES v i LIST OF FIGURES v i i ACKNOWLEDGEMENT i x 1.0 INTRODUCTION 1 2.0 LITERATURE REVIEW 3 2.1 The Nitrogen "Problem" 3 2.2 Mineral Nitrogen 4 2.3 Organic Matter Decomposition 11 2.4 Nitrogen Additions and Their E f f e c t on Nitrogen Cycling Rate 13 2.5 The Straw and Optimum Tree N u t r i t i o n Experiments . 16 2.6 De f i n i t i o n s 19 3.0 MATERIALS AND METHODS 20 3.1 Site Description 20 3.2 Treatments 20 3.3 Sample Preparation and Laboratory Methods 24 3.4 S t a t i s t i c a l Analysis 32 4.0 RESULTS AND DISCUSSION 35 4(a) SOME PHYSICAL-CHEMICAL CHANGES IN THE GROUND VEGETATION, LITTER AND HUMUS 35 4.1 Ground Vegetation 35 4.2 Humus Weight and L i t t e r f a l l 43 4.2.1 Results 43 V Page A.2.1.1 Humus mass and depth 43 4.2.1.2 L i t t e r 48 4.2.2 Discussion 50 4.3 Humus pH 52 4.3.1 Results 52 4.3.2 Discussion 52 4.4 Nitrogen 56 4.4.1 Results 56 4.4.1.1 Total Nitrogen 56 4.4.1.2 Available Nitrogen 59 4.4.2 Discussion 66 4.5 Carbon and Decomposition 72 4.5.1 Results 72 4.5.2 Discussion 72 4.6 Cation Exchange Capacity (CEC) and the Cations K+, Ca++ and Mg++ 76 4.7 Phosphorus 80 4(b) SEEDLING GROWTH AND NUTRITION 82 4.8 Seedling Growth 82 4.8.1 Results 82 4.8.2 Discussion 85 4.9 Seedling N u t r i t i o n 88 4.9.1 Results 88 4.9.2 Discussion 92 5.0 SUMMARY 97 6.0 REFERENCES 1 Q 1 v i LIST OF TABLES Page Table 1 Design and treatments applied to the fo r e s t f l o o r of an immature jack pine stand 23 Table 2 Amount and time of f e r t i l i z e r applied to a jack pine stand during the 10-year period 25 Table 3 Analysis of variance table 34 Table 4 Ground vegetation analysis of the optimum n u t r i t i o n and straw treatment experiments 36 Table 5 Estimated three month l i t t e r weights (kg/ha) i n the treated jack pine stand and the i r corresponding above ground decomposition rate factor k, calculated on an annual basis 49 Table 6 Average nitrogen concentration, t o t a l nitrogen and ava i l a b l e nitrogen (NH4+-N + N03~-N) i n the treated humus (kg/ha) 58 Table 7 Available nitrogen (ppm) i n the humus layer following eight weeks of incubation at 20°C and 60% WHC 60 Table 8 Percent carbon, and the C/N r a t i o of the treated humus 73 Table 9 Exchangeable bases K+, Ca ++, Mg++ (me/100 g), CEC (me/100 g) and Base Saturation (%) of the treated humus 77 Table 10 Average concentrations (%) of phosphorus, potassium, magnesium and calcium i n the treated humus 79 Table 11 Average t o t a l phosphorus (kg/ha) i n the treated humus 81 Table 12 Root/shoot r a t i o s of seedlings grown on treated jack pine humus 84 Table 13 Average f o l i a r concentrations of nitrogen, phosphorus, potassium, magnesium and calcium of jack pine seedlings grown on treated humus 90 v i i LIST OF FIGURES Page Figure 1 Influence of the interactions of several processes on nitrogen a v a i l a b i l i t y i n low and high nitrogen s i t e s (Gosz, 1981) 14 Figure 2 S o i l p r o f i l e showing the humo-ferric podzol formed on deep outwash sand and covered with a thin mor humus layer 21 Figure 3 Vegetation consists of jack pine with a sparse understory of black spruce. The shrub layer i s composed of Kalmia and Vaccinium 22 Figure 4 Straw a p p l i c a t i o n on top of snow i n early March, 1970 26 Figure 5 Jack pine seedlings growing on treated humus i n the greenhouse 31 Figure 6 Reduction of the Kalmia-Vaccinium shrub layer i n a jack pine stand following N2 treatments 37 Figure 7 Appearance of exotic species i n a jack pine stand following N3 treatment 39 Figure 8 A straw a p p l i c a t i o n e f f e c t i v e l y reduced the Kalmia-Vaccinium shrub layer for almost 10 years i n a jack pine stand 40 Figure 9 Estimates of the average weight (kg/ha) of the combined L, F, H humus layers (+ standard deviation) of the Optimum N u t r i t i o n Experiment and Straw Treatment plots 44 Figure 10 Average forest f l o o r depth (cm) of the Optimum N u t r i t i o n Experiment and Straw Treatment plots (+ standard deviation) 45 Figure II Humus weight and depth increased dramatically following N^ treatments 46 Figure 12 With increased nitrogen additions, humus layers decreased i n weight and depth as shown here on N3 treated humus 47 Figure 13 Average pH of the humus (+ standard deviation) measured i n 1:8 d i l u t i o n d i s t i l l e d H2O and 0.01 M CaCl2 CaCl2 53 v i i i Page Figure 14 Average nitrogen concentration (+ standard deviation) of the treated humus 57 Figure 15 Total available nitrogen (NH4+-N and NC>3~-N) i n the humus of the con t r o l , and N2 treated humus following 0 , 2 , 4 and 8 weeks incubation at 20°C and a water content of 60% (WHC) 61 Figure 16 Total a v a i l a b l e nitrogen (NH4+-N and N03~-N) i n the humus of the N3 treated humus following 0 , 2 , 4 and 8 weeks incubation at 20°C and a water content of 60% WHC 62 Figure 17 Total available nitrogen (NH4+-N and NO3 -N) i n the humus of the N2PK and N3PK treated humus following 0 , 2 , 4 and 8 weeks incubation at 20°C and a water content of 60% WHC 63 Figure 18 Total available nitrogen (NH4+-N and N03~-N) i n the humus of the straw treated humus following 0 , 2 , 4 and 8 weeks incubation at 20°C and a water content of 60% WHC 64 Figure 19 Average dry matter production (grams) of seedlings grown on treated jack pine humus for s i x months ... 83 Figure 20 Gross basal area periodic increment (1970-1979) of the jack pine stand and t o t a l seedling dry weight (6 months) expressed as percent of the control NQ 87 Figure 21 Average f o l i a r nitrogen concentration (+ standard deviation) of the seedlings grown for s i x months on the treated humus 89 ACKNOWLEDGEMENT I would l i k e to thank a number of people who were involved i n this research project and without whose assistance the i n v e s t i g a t i o n and reporting of res u l t s could not have been accomplished. F i r s t and foremost, I thank my advisor, Professor G.F. Weetman for his help i n providing ideas, encouragement and f i n a n c i a l support. Min Tsze deserves s p e c i a l acknowledgement for providing excellent technical assistance with the laboratory analysis and i n taking the time to answer my many questions. Professor J.P. Kimmins generously allowed me the f u l l use of h i s laboratory and equipment i n spite of h i s lim i t e d space and provided enthusiasm and advice. I would also l i k e to thank my other committee member, Dr. T.M. B a l l a r d for the use of his s o i l s laboratory and for his suggestions. Dr. J . Otchere-Boeteng was very h e l p f u l with the mineralization studies which were incubated i n Dr. B. van der Kamp's laboratory. The s t a t i s t i c a l analyses were planned with the excellent help of Dr. Y. El-Kassaby, who made i t a l l appear so easy and the data processed by Barry Wong. And l a s t , but not least, a thank you to Rick Fournier, who shared the burden of carrying the many bags of humus across black f l y and mosquito infested country. 1 1.0 INTRODUCTION Growth of many coniferous crops i n northern boreal forests i s often limited by the supply of nutrients, e s p e c i a l l y nitrogen. At the same time, the mor humus layers which commonly characterize these s i t e s have large reserves of nitrogen unavailable for tree growth (Viro, 1963; Weetman and Webber, 1972). As an adaptation to these low l e v e l s of available nutrients, boreal tree species conserve what l i t t l e nutrients they have by maintaining very 'tight' nutrient cycles from which losses are small (Tamm, 1979). In order to improve tree growth on these s i t e s , considerable research has been ca r r i e d out to determine the growth response of boreal forests to nitrogen (N) f e r t i l i z e r s (Tamm, 1974; Krause et a l . , 1983). Research has also shown that chemical f e r t i l i z e r s have an ef f e c t on nutrient turnover rates of the humus (Roberge and Knowles, 1966; Mahendrappa, 1978). Added nutrients can stimulate microbial a c t i v i t y d i r e c t l y by adding nutrients i n short supply or i n d i r e c t l y by ameliorating t h e i r environment (Williams, 1972). Increased mineraliza-t i o n can mobilize the organic nitrogen and other nutrient reserves contained within the humus and make them available for tree use. Rapid increases i n humus decomposition rates can release nutrients i n a short time which would normally be cycled over a much longer period (Kelly and Henderson, 1978). These nutrient releases may occur so rapidly that tree uptake cannot absorb them and they are e f f e c t i v e l y l o s t from the s i t e . In Canada, one of the e a r l i e s t works i n forest f e r t i l i z a t i o n was the establishment of an optimum n u t r i t i o n experiment i n a jack pine 2 (Pinus banksiana Lamb.) stand i n Quebec (Weetman and Algar, 1974). This experiment attempts to e s t a b l i s h the possible upper l i m i t s of tree growth by manipulating nutrient concentrations and maintaining defined f o l i a r nutrient regimes over an extended period of time with the a p p l i -cation of N, P and K f e r t i l i z e r . In the same stand, but unrelated to the optimum n u t r i t i o n experiment, straw was used to smother the ground vegetation i n order to reduce competition for a v a i l a b l e N and increase N uptake by the trees. The jack pine responded with substantial volume growth for both the sustained low nitrogen additions and the straw a p p l i c a t i o n (Weetman and Fournier, 1984). Neither experiment was intended to be a f u l l y integrated eco-system study. However, both experiments presented the opportunity to investigate the effects repeated N, P and K applications had on the humus layers and ground vegetation under a coniferous stand. The sub-s t a n t i a l response of the trees to both the sustained low nitrogen addi-tions and straw a p p l i c a t i o n obviously c a l l e d for studies of changes i n forest humus layer. The objective of the study was to test the hypo-thesis that the physical and chemical c h a r a c t e r i s t i c s of the humus and the ground vegetation were s i g n i f i c a n t l y d i f f e r e n t from the untreated humus laye r s . Further, that these changes i n the humus would s i g n i f i -cantly influence the nutrient supplying capacity of the humus to the jack pine seedlings i n a bioassay experiment. Although no one can seriously imagine large-scale applications of straw to the forest f l o o r , repeated applications of f e r t i l i z e r on i n t e n s i v e l y managed forests w i l l l i k e l y increase. Studies such as these can add awareness of the p o t e n t i a l and magnitude of these changes to the humus c a p i t a l of a forest ecosystem. 3 2.0 LITERATURE REVIEW 2.1 The Nitrogen "Problem" Nitrogen i s known to be one of the most important l i m i t i n g factors a f f e c t i n g growth and y i e l d of fo r e s t s . Although i t i s a common element of the earth's atmosphere, only a small part of this nitrogen i s contained i n the active biogeochemical cycle. Here i t plays a very important r o l e , since most of the available nitrogen i s obtained from transformation processes occurring i n s o i l organic matter. Within the boreal forest ecosystem, nitrogen release i s slow, despite often large reserves held i n the raw humus horizons. The German s c i e n t i s t Themlitz (1954) writes, "Although r i c h i n nitrogen, mor humus a c t u a l l y hungers a f t e r nitrogen". This inadequate release of plant nutrients has been the object of much research since the early 1900's. Much of the nitrogen present i n mor humus occurs as organic nitrogen - nitrogen unavailable to plants u n t i l i t i s mineralized by s o i l microorganisms to inorganic forms. These organic nitrogen complexes are very r e s i s t a n t to decomposition i n spite of the fact that 40% are i n the form of e a s i l y mineralizable organo-nitrogen complexes (Bremner, 1965). Handley (1961) suggested that tannins present i n the leaves of some plant species react with cytoplasmic proteins. These protect the mesophyll tissue from rapid decomposition and may be of fundamental importance i n the formation of mor humus found under coniferous f o r e s t s . Broadbent and Norman (1947) and Broadbent (1948) concluded that the s t a b i l i t y of organic nitrogen complexes i s more 4 apparent than r e a l . Due to the absence of enough energy material, r e a d i l y available for decomposition, a vigorous microbial population cannot be supported, with the res u l t that decomposition i s slow. 2.2 Mineral Nitrogen Increasing use has been made of nitrogen f e r t i l i z e r s to augment nitrogen a v a i l a b i l i t y and thereby improve forest growth. F e r t i l i z e r s can be applied to forests i n either inorganic or organic forms. When nitrogen i s applied to the moist forest f l o o r i n an inorganic form such as ammonium n i t r a t e , the nitrogen i s quickly incorporated into the s o i l s o l u t i o n or exchange complex. In this form i t i s immediately available to plants and s o i l organisms. In contrast urea, an organic nitrogen source, must f i r s t be converted to the inorganic ammonium (Ntty"1") form before i t can be absorbed by plants. Additions of ammonium s a l t s to an a l k a l i n e aqueous solution, r e s u l t s i n the escape of free ammonia according to the following reaction: NH4+ + 0H~ — > NH3 + H2O Thus, i f ammonium nitrogen f e r t i l i z e r s are added to a l k a l i n e s o i l s , free ammonia may be l o s t . Under f o r e s t s , the quantity of ammonia v o l a t i l i z e d depends on s o i l conditions, such as temperature, moisture, pH and thickness of the humus layer. Since additions of urea can re s u l t i n a r i s e i n pH (Knowles, 1964; Roberge and Knowles, 1966; Overrein, 1968; Weetman et a l . , 1972; Foster, 1979), applying urea on 5 an acid humus can r e s u l t i n ammonia v o l a t i l i z a t i o n (Bernier et a l . , 1969; Overrein, 1969; and Bhure, 1970). The use of ammonium n i t r a t e , which does not rai s e the pH of the s o i l , w i l l reduce v o l a t i l i z a t i o n losses often associated with urea additions. Overrein (1969) found v o l a t i l i z a t i o n losses from NH4+ or NO3 - treated podzol s o i l s to be non-significant. Leaching constitutes one of the main pathways whereby nitrogen i s lo s t from s o i l s . The quantity of nitrogen leached i s dependent on many variables ( A l l i s o n , 1965): (a) form and amount of soluble and unadsorbed nitrogen present; (b) amount and d i s t r i b u t i o n of r a i n f a l l ; (c) i n f i l t r a t i o n and percolation rates, which are markedly affected by s o i l composition, texture, structure, p r o f i l e depth and surface treatment; (d) s o i l water-holding capacity and moisture content; (e) cation exchange capacity as influenced by the amount of organic matter, clay content and pH; (f) c h a r a c t e r i s t i c s of the surface vegetation; (g) evapotranspiration; (h) rate of nitrogen uptake by plants. Boreal f o r e s t ecosystems are able to completely or p a r t i a l l y accumulate and r e t a i n nutrients added as f e r t i l i z e r . Those nutrients not retained are leached out of the upper s o i l horizons and are e f f e c -t i v e l y l o s t from the rooting zone. The leaching losses of nitrogen a f t e r addition of urea-N, NH4-N and NO3-N i n a mixed Norway spruce - Scots 6 pine stand growing on a well-drained sandy s o i l was reported by Overrein (1968, 1969). Total leaching losses of nitrogen increased l i n e a r l y with increasing rates of f e r t i l i z e r a p p l i c a t i o n s. Leaching losses at 250 kg N/ha l e v e l were n e g l i g i b l e with urea (1.6%), high for NH4+ (21.5%) and very high for N03~ (91.8%). Studies on the retention of added ammonium and n i t r a t e ions i n the s o i l have shown that n i t r a t e i s not strongly retained i n the s o i l (Nommik and Popovic, 1971; Overrein, 1971). Not only may the added nitrogen be e f f e c t i v e l y moved below the rooting depth of forest trees (Tamm, 1973), but there i s also the threat of n i t r a t e contamination of ground water. When nitrogenous f e r t i l i z e r s are added to the s o i l , the ammonium ion pool i s d i r e c t l y increased. Part of th i s added nitrogen can be absorbed or "fixed " by organic matter ( c h i e f l y l i g n i n ) and clays ( A l l i s o n , 1973). Ammonium can also be ph y s i c a l l y adsorbed to organic matter, but i t i s a very weak bond which can be read i l y removed when the ammonium concentration i s lowered. Under certain conditions, the ammonia can react chemically with the organic matter to form insoluble compounds r e s i s t a n t to decomposition (Mortland and Wolcott, 1965; Nommik, 1965). Conditions found i n most raw humus layers however, r e s t r i c t t h i s process as reported by Knowles and Chu (1969) who found low f i x a t i o n rates of 15JJH^ +-DJ i n black spruce raw humus. Nommik (1970) also reported that the f i x a t i o n of added ammonium was low i n acid Norway spruce humus but increased rapidly as the pH was raised. N i t r i f i c a t i o n i s the reaction by which certain chemo-autotrophic organisms and some heterotrophs oxidize ammonium ions to n i t r i t e and then to n i t r a t e . Each stage i s carried out by s p e c i f i c organisms, with 7 autotrophic bacteria responsible for most n i t r a t e production. In c o n i -ferous forest ecosystems with t y p i c a l acid humus layers, autotrophic n i t r i f i e r s can be i n e f f e c t i v e or absent. Here, heterotrophic n i t r i -f y i ng fungi may be more important (Keeney and Gardner, 1970). The f i r s t stage of the n i t r i f i c a t i o n process i s carried out by various bacteria: Nitrosomonas, Nitrosococcus, N i t r o s o c y s t i s , N i t r o - sospira and Nitrosogloea (Alexander, 1965). The effects of inorganic N f e r t i l i z e r s on these species are limited to the ammonia they supply. Growth i s generally speeded up since ammonia i s t h e i r chief food ( A l l i s o n , 1973). However, when f e r t i l i z e r concentrations are high (immediately upon a p p l i c a t i o n , or around the p e l l e t ) some or most of the organisms can be k i l l e d . The second stage of oxidation i s carried on by Nitrobacter and N i t r o c y s t i s but due to the highly a c i d i c conditions i n boreal forest s o i l s , Nitrobacter species are not numerous (Lutz and Chandler, 1959) and may explain the low l e v e l of n i t r i f i c a t i o n i n some forest s o i l s . In eastern Canada, Roberge and Knowles (1966) detected only n e g l i g i b l e amounts of n i t r i f i c a t i o n i n t h e i r incubation studies with an acid podzolic humus formed under a highly productive black spruce following f e r t i l i z a t i o n with urea at 448 kg N/ha. N i t r i f i c a t i o n was detected only when the pH rose above 5.6 a f t e r an a p p l i c a t i o n of 3500 ppm urea-N i n the laboratory to the humus previously f e r t i l i z e d i n the f i e l d . Overrein (1967) made a s i m i l a r observation. He found an accumulation of nitrate-N only a f t e r urea was applied to a Norway spruce mor humus. Contrary to the above r e s u l t s , some investigators have reported s i g n i f i c a n t n i t r i f i c a t i o n i n forest s o i l s , but these have been 8 r e s t r i c t e d mainly to P a c i f i c West Coast s o i l s (Bollen and Lu, 1968; Likens ejt^  a l . 1969; Heilman, 1974). According to Heilman (1974), i t has been shown that i n applying urea on poor Douglas-fir s i t e s , no n i t r a t e was produced. On better s i t e s , n i t r a t e levels doubled a f t e r f i v e months. Thus, li m i t e d n i t r i f i c a t i o n may be due to the absence of ammonium on poor s i t e s . It has been reported (Tyler and Broadbent, 1960) that Nitrobacter i s s e n s i t i v e to even low ammonia concentrations. Considerable n i t r i t e may be formed, but l i t t l e n i t r a t e . In acid s o i l s , n i t r i t e s are unstable and some of the nitrogen may be l o s t ( A l l i s o n , 1973). D e n i t r i f i c a t i o n i s the reduction of n i t r i t e and n i t r a t e to v o l a t i l e gases, usually nitrous oxides and molecular nitrogen (Broadbent and Clark, 1965). In the wider sense, i t refers to the gaseous loss of nitrogen either by b i o l o g i c a l or chemical means - exclusive of ammonia v o l a t i l i z a t i o n . D e n i t r i f i c a t i o n as a b i o l o g i c a l process i s accomplished by f a c u l t a t i v e l y anaerobic bacteria which are capable of using n i t r a t e or n i t r i t e i n place of oxygen as an electron acceptor i n o x i d i z i n g various kinds of organic matter, including carbohydrates: C 6 H 1 2 ° 6 + 4 N 0 _ 3 > 6 C ° 2 + 6 H 2 ° + 2 N2 D e n i t r i f i c a t i o n i s influenced by the s o i l s pH, being very slow i n acid s o i l s and rapid i n s o i l s of high pH (Bremner and Shaw, 1958). Nommik (1956) reported that d e n i t r i f i c a t i o n rates are f a i r l y constant 9 at pH 6. L i t t l e d e n i t r i f i c a t i o n occurs when the moisture content i s less than 60% of the s o i l ' s water holding capacity (Bremner and Shaw, 1958). Using lysimeter studies, Overrein (1968, 1969) noted that no d e n i t r i f I c a t i o n occurred i n mor humus. S o i l r e ceiving the higher rates of urea a p p l i c a t i o n , l o s t only small amounts of N2. Weetman (1962) found i n greenhouse studies, that half the n i t r a t e applied as ammonium n i t r a t e was l o s t . However, the a r t i f i c i a l hot humid conditions of the greenhouse was credited i n having stimulated n i t r i f i c a t i o n and d e n i t r i -f i c a t i o n processes. Chemo-denitrification, on the other hand, involves various chemi-c a l or non-enzymatic reactions which break down n i t r a t e s . Broadbent and Clark (1965), Nommik and Thorin (1972) discussed some of the processes involved i n chemo-denitrification. 1. Self-decomposition of nitrous acid under acid forest conditions to y i e l d n i t r i c oxide: 3 HN02 > 2 NO + HNO3 + H 20 The n i t r i c oxide can i n turn, be oxidized to nitrogen dioxide: 2 NO + 0 2 > 2 N0 2 which, i n turn, can react with water to form n i t r i c acid: 3 N0 2 + H 20 •> 2 HNO3 + NO 2 N0 2 + H 20 •> HNO3 + HN0 2 Large losses of nitrogen from s o i l s v i a this reaction are un l i k e l y , although i t i s probably the only chemo-denitrification process occurring i n acid forest s o i l s since n i t r i t e does not decompose above pH 6 (Nommik and Thorin, 1972). Reaction of nitrous a c i d with a-amino acids i n the Van Slyke reaction: RNH2 + HN02 > ROH + N 2 + H 2 0 This reaction probably does not res u l t i n much loss of nitrogen. It occurs at pH 5 or less - conditions where n i t r i t e formation either by b i o l o g i c a l oxidation of ammonia or by enzymatic reduction of n i t r a t e are not favourable. Therefore, even when n i t r a t e i s applied as f e r t i l i z e r , l i t t l e N 2 w i l l be formed (Bremner, 1965). In a reaction s i m i l a r to the Van Slyke reaction, ammonia may react with nitrous acid and form nitrogen: NH3 + HNO2 > N 2 + 2 H 2 0 However, Bremner (1965) considers losses due to this reaction to be n e g l i g i b l e . Reaction of n i t r a t e or nitrous acid with reducing material of the s o i l organic matter, which i s probably of microbial o r i g i n . The phenolic components of the organic matter are believed to be important i n f i x a t i o n of n i t r i t e and reduction to N 2 o and N 2. 11 2.3 Organic Matter Decomposition The conditions which determine organic matter decomposition were studied extensively by Jansson (1958). The fundamental point i n his theory of "continuous i n t e r n a l cycle" of nitrogen i n the s o i l , i s that mineralization and immobilization are going on continuously and simultaneously. The d r i v i n g force behind t h i s cycle i s the heterotro-phic s o i l population, whereas the energy sources required to keep this cycle going are the organic compounds returned to the s o i l i n the form of l i t t e r . When nitrogen i s added to the forest f l o o r , i t may be immobilized by the heterotroph microbial population of the s o i l . This immobiliza-t i o n i s b i o l o g i c a l i n that the microorganisms assimilate the mineral nitrogen for t h e i r own biomass synthesis (Alexander, 1961). Mi n e r a l i z a t i o n , which i s the reverse of immobilization, occurs concurrently. In th i s process, the nitrogen incorporated into the organic structures i s converted into ammonium by other microorganisms. Agronomic research has indicated that nitrogen additions increase the breakdown rate of organic matter (Brown and Dickey, 1970; Black and Reitz 1972). Under forest conditions, Salonius (1972) and Roberge (1976) observed increased r e s p i r a t i o n In black spruce forest f l o o r samples shortly a f t e r urea was added. With a more favourable pH environment and r e a d i l y available nitrogen for c e l l synthesis, more organic matter i s metabolized. That there i s a c o r r e l a t i o n between the C/N r a t i o of the organic matter and nitrogen turnover rate i s understandable. However, what 12 point nitrogen mineralization equals nitrogen immobilization i s not c l e a r . Harmsen and van Shreven (1955) reported values for equilibrium at C/N r a t i o s = 20 to 25 and a nitrogen content of 1.5 to 2.0%. Bremner and Shaw (1957) showed that the c r i t i c a l C/N r a t i o of immobili-zation could exceed 30. Since immobilization i s promoted when the C/N r a t i o i s high, s o i l s with a low C/N r a t i o w i l l have more net m i n e r a l i -zation occurring. Additions of nitrogen f e r t i l i z e r s can reduce the C/N r a t i o . Roberge and Knowles (1966) reported a decrease i n the C/N r a t i o from 58/1 to 42/1 i n the L horizon of an acid black spruce humus. Popovic (1977) also reported a decreased C/N r a t i o i n the humus ranging from 43/1 to 29-34/1 following f e r t i l i z a t i o n . Although a p p l i c a t i o n of nitrogen has been found to stimulate r e s p i r a t i o n and to decrease the C/N r a t i o , increased rates of decom-p o s i t i o n have not always been reported. Z o t t l (1960), Fessenden et a l . (1971), Salonius, (1972) and Roberge (1976) a l l reported decreases i n r e s p i r a t i o n following f e r t i l i z a t i o n with ammonium s a l t s . This would suggest that nitrogen was not l i m i t i n g decomposition i n i t i a l l y . The a v a i l a b i l i t y of carbon may also l i m i t decomposition. Studying the int e r a c t i o n s between mineralization and immobilization, Knowles (1969) reported that additions of nitrogen to a black spruce humus did not immobilize the available nitrogen. By adding sugar as a r e a d i l y a v a i l a b l e carbon source and an ample supply of other minerals i n combination with an increase i n pH, immobilization of the nitrogen was increased. Subsequently, Salonius (1972) showed that microbial a c t i v i t y was limited more by shortages of decomposable organic 13 substrate and the high a c i d i c conditions than by nitrogen. Foster (1979) also found microbial r e s p i r a t i o n stimulated more by an increase i n s o i l pH and a v a i l a b l e carbon than by additions of nitrogen. It seems apparent then, that a v a i l a b i l i t y of nitrogen from organic matter, or from any nitrogen additions applied to the boreal forest f l o o r , i s not dependent on a single l i m i t i n g f a c t o r . Instead, a number of factors such as a c i d i t y and r e a d i l y available energy sources are important when evaluating immobilization-mineralization i n t e r a c t i o n . 2.4 Nitrogen Additions and Their E f f e c t on Nitrogen Cycling Rate Species growing under d i f f e r e n t s i t e conditions can have marked differences i n t h e i r use and c y c l i n g of nitrogen. Conifer species, e s p e c i a l l y , can respond to nitrogen a v a i l a b i l i t y by modifying various p h y s i o l o g i c a l processes ( M i l l e r et a l . , 1979). For example, trees can respond to decreased nitrogen a v a i l a b i l i t y by greater i n t e r n a l nitrogen c y c l i n g . This r e s u l t s i n lower nitrogen concentrations i n the needle l i t t e r . Unfortunately, decreased l i t t e r q u a l i t y with high C/N r a t i o s decompose slowly with a corresponding slow nitrogen mineralization rate. In turn, t h i s lowers the available nitrogen even further and can aid i n the formation of a mor humus (Figure 1). When the pool of available nitrogen i s enlarged through the use of nitrogen f e r t i l i z e r , the added nitrogen can d i r e c t l y a f f e c t the nitrogen transformations within the humus by increasing the rate of nitrogen c y c l i n g . Many studies have shown that additions of nitrogen often result i n increased nitrogen concentrations i n needles (Morrison, 1974; van den Driessche, 1974), as well as i n l i t t e r and throughfall (Mahendrappa Low N a v a i l a b i l i t y i ow N uptake High polyphenol, organic acid production Low N% i n l i t t e r f a l l , stable polyphenol-protein complexes Low decomposition, mineralization rates I Low n i t r i f i c a t i o n (MOR HUMUS FORMATION) High N a v a i l a b i l i t y i High N uptake \ Low polyphenol, organic acid production i High N% i n l i t t e r f a l l , reduced or unstable polyphenol-protein complexes High decompostion, mi n e r a l i z a t i o n rates High n i t r i f i c a t i o n MULL HUMUS FORMATION Figure 1. Influence of the interactions of several processes on nitrogen a v a i l a b i l i t y i n low and high nitrogen s i t e s (Gosz, 1981). 15 and Ogden, 1973). Berg and Staaf (1980) suggest that Increased l i t t e r q u a l i t y can r e s u l t i n a higher decomposition rate and a greater amount of nitrogen released per unit weight of organic matter. A lower per-centage of nitrogen i s immobilized leaving a larger amount of nitrogen available for tree growth (Gosz, 1981). Increased nitrogen concentration can also a f f e c t the l i t t e r q u a l i t y by reducing the amount of polyphenols and tannins present i n the conifer needles. Davies e_t a l . (1964) found that tree seedlings produced higher amounts of polyphenolic substances when nitrogen was i n low supply. Since these protein-complexing substances are notoriously slow to decompose, any reduction i n t h e i r concentration within the needles w i l l u ltimately increase the decomposition rate of the needle l i t t e r (Gosz, 1981). Increased nitrogen supply also a f f e c t s the i n t e r n a l r e d i s t r i b u -t i o n of t h i s nutrient and a f f e c t s leaf persistence on the tree. Under conditions of low nitrogen supply, the tree has a high i n t e r n a l r e d i s -t r i b u t i o n of nutrients before leaf f a l l (Lamb, 1975; 1976; Turner, 1977) thus r e s u l t i n g i n lower nitrogen concentrations i n needle l i t t e r . Furthermore, when the tree increases i t s leaf persistence on poorer s i t e s (Gosz, 1981), the t o t a l amount of nitrogen u t i l i z e d within the tree w i l l be greater. By increasing nitrogen a v a i l a b i l i t y , the with-drawal of nitrogen from the needle w i l l be low, r e s u l t i n g i n higher nitrogen concentration i n the needle l i t t e r . Decreased leaf p e r s i s -tence, of course, translates into greater l i t t e r f a l l . With more high q u a l i t y energy material reaching the forest f l o o r , higher decomposition and mineralization rates w i l l occur. 16 Naturally, other factors than those encompassed by s i t e q u a l i t y have an e f f e c t on the nitrogen cycle by influencing the decomposition rates. The physical component plays as important a role as s i t e q u a l i t y . Such factors as temperature, moisture, aeration, oxygen and carbon dioxide supply a l l influence the decomposition processes by a f f e c t i n g decomposer organisms. However, t h e i r discussion l i e s beyond the scope of t h i s i n v e s t i g a t i o n and they are presented here i n order to complete the scheme of organic matter decomposition. 2.5 The Straw and Optimum Tree N u t r i t i o n Experiments Within the boreal f o r e s t , the c y c l i n g of nutrients, e s p e c i a l l y nitrogen i s slow and the b i o l o g i c a l competition for nitrogen i s intense. The ground vegetation may be an important competitor with trees for the l i m i t e d N a v a i l a b l e . Huikari and P a a r l a h t i (1967) smothered the ericaceous vegetation i n a Finnish pine swamp with straw and noticed an almost immediate response i n tree growth. They a t t r i -buted t h i s response i n part, to the decreased competition and the "green f e r t i l i z a t i o n " by the decomposing vegetation. In order to test the hypothesis that smothering the ericaceous vegetation i n a jack pine stand w i l l increase N uptake by the trees, the straw experiment was i n i t i a t e d (Weetman and Algar, 1974). Plants i n t e r a c t with f e r t i l i z e r additions by a s s i m i l a t i n g nutrients through t h e i r roots, and, i n return, add energy material and organic nitrogen back to the s o i l . In a g r i c u l t u r a l as well as forestry pra c t i c e , the aim i s generally to increase available nitrogen, r a i s e 17 nutrient concentration to optimal l e v e l s i n the plant and so increase y i e l d . Ebermayer (1876) made the f i r s t attempt to estimate the amount of nutrients required for optimal tree growth. Since then, there have been many studies to determine the optimum nutrient requirements of forest trees (Rennie, 1955; Swan, 1960, 1970; Ingestad, 1962; 1976, 1979; Tamm, 1968, 1974). Most early research centred on the use of tree seedlings i n pot cultures, to which additions of various amounts of nutrients were applied. But making predictions of forest tree response based on seedling growth has been d i f f i c u l t . The value of these t r i a l s i n predicting growth response i s dependent on a c o r r e l a t i o n between pot t r i a l s and f i e l d response. However, n u t r i t i o n a l requirements of seedlings and adult trees may not be s i m i l a r . Mead and P r i t c h e t t (1971) reported a generally low c o r r e l a t i o n between f e r t i l i z e r response of Pinus e l l i o t t i i n the f i e l d and those grown i n pots. Added to these n u t r i t i o n a l differences are other factors which can influence tree/ seedling response. A r t i f i c i a l conditions i n the greenhouse are more conducive to plant growth and could also increase/decrease c e r t a i n chemical and physical processes i n the s o i l and so a l t e r nutrient a v a i l a b i l i t y . Even when pot t r i a l s are located under f i e l d conditions, marginal s o i l nutrient d e f i c i e n c i e s may be overlooked because s u f f i c i e n t stress on the nutrient supplying powers of the s o i l has not occurred (Richards, 1968). S t i l l , pot t r i a l s can provide an important diagnos-t i c tool i n assaying s o i l f e r t i l i t y because they can provide informa-t i o n on the nutrient supplying p o t e n t i a l of the s o i l . 18 Conventional f e r t i l i z a t i o n experiments have also not given the required information. In these experiments, a single a p p l i c a t i o n of f e r t i l i z e r or applications repeated with long i n t e r v a l s are applied to forest stands. Closed forest stands have mechanisms to recycle nutrients. A d d i t i o n a l l y , the size of the nutrient pools and the rate of nutrient transformations i s impossible to determine with any degree of accuracy. Therefore, l i t t l e i n s i g h t into the n u t r i t i o n a l r e l a t i o n s between forest stand, y i e l d response and s o i l conditions can be made because of the highly variable nutrient status of the trees. Optimum n u t r i t i o n experiments are a newer type of f a c t o r i a l experiments (Tamm, 1968, 1974). Here, the nutrient uptake i s controlled by annual leaf a n a l y s i s . Annual or semi annual nutrient applications are adjusted i n order to keep f o l i a g e l e v e l s of the main nutrients (N, P, K) as constant as possible over an extended period of time. The objective i s to allow the forest stand to e s t a b l i s h a form of 'steady state ' with respect to i n t e r n a l concentration of nutrients and growth. Generally, optimum n u t r i t i o n experiments maintain three d i f f e r e n t constant elevated nutrient regimes. This allows d i r e c t c o r r e l a t i o n s between stand growth response and nutrient l e v e l s . These experiments also permit a comparison of f i e l d and pot t r i a l s . In addition, f i e l d t e s t ing of ' d e f i c i e n t ' , ' s a t i s f a c t o r y ' and 'optimum' f o l i a r nutrient l e v e l s i s possible. The ultimate aim of f e r t i l i z a t i o n research i s to enable the forest manager to predict stand response by using simulation models of forest ecosystems. The use of f e r t i l i z e r s manipulates s i t e conditions by the addition of an external influence. How th i s can a f f e c t nutrient 19 c y c l i n g , nutrient a v a i l a b i l i t y and ultimately, primary productivity, i s s t i l l l i t t l e known. Only by understanding the various regulatory mechanisms forest systems have at t h e i r disposal and how to a l t e r or maximize th e i r function, w i l l the models be able to assess and r e f l e c t true conditions. 2.6 D e f i n i t i o n s L i b e r a l use of the terms forest f l o o r and humus layers are employed. The forest f l o o r refers to a l l organic materials resting on but not mixed with the mineral s o i l surface. Humus, or humus layer, i s defined as the whole of the dead organic matter present i n or on the s o i l and undergoing continuous decay, transfer and synthesis. Although current North American l i t e r a t u r e characterizes the humus layer as a l l decomposing organic remains beneath the l i t t e r layer as well as the A l s o i l horiz on ( P r i t c h e t t , 1979), within the confines of this thesis, forest f l o o r and humus layers are used interchangeably. Likewise, the l i t t e r layer, c o n s i s t i n g of unaltered dead organic matter, i s considered part of the humus. 20 3.0 MATERIALS AND METHODS 3.1 Site Description The research plots are located i n Mont Tremblant Pare, Quebec. Situated i n the southern extremity of the boreal f o r e s t (Forest Section B-7, Rowe 1972), t h i s even-aged jack pine stand was 45 years old at the time the plots were established i n 1970. C l a s s i f i e d as a Site Class I (Plonski, 1974), the stand had a density of 2125 trees/ ha, a dominant height of 14.3 m, a mean diameter of 14.7 cm (DBH) and contained a t o t a l volume of 161 m-Vha (Weetman and Algar, 1974). The forest f l o o r consisted of a t y p i c a l thin raw humus layer commonly found under boreal pine stands (Figure 2). This mor was 5.5 cm i n depth, weighed 43,000 kg/ha oven dry and covered a deep, sandy Ortho Humo-Ferric Podzol (Canada Department of A g r i c u l t u r e , 1970). Ground vegetation as shown i n Figure 3 consisted of an upper stratum of Kalmia a n g u s t i f o l i a (L.) and Vaccinium angustifolium A i t . with a lower moss layer of Pleurozium schreberi (B.S.G.) and Cladonia  r a n g i f e r i n a (L.) Web. 3.2 Treatments The optimum n u t r i t i o n experiment was set up as a completely randomized 2 x 4 f a c t o r i a l with four levels of nitrogen and two l e v e l s of phosphorus and potassium (Table 1). Each treatment was r e p l i c a t e d twice and the f e r t i l i z e r s applied to 0.02 ha c i r c u l a r p l o t s . The f e r t i l i z e r was applied early i n June with an hand cyclone seeder, and over the 10 year period there were s i x applications. The f i r s t S o i l p r o f i l e showing the humo-ferric podzol formed on deep outwash sand and covered with a t h i n mor humus layer. igure 3. Vegetation consists of jack pine with a sparse understory of black spruce. The shrub layer i s composed of Kalmia and Vaccinium. TABLE 1. Design and treatments applied to the forest f l o o r of an immature jack pine stand. Optimum N u t r i t i o n Experiment N Level PK Regime + PK - PK N 0 = 0 kg N/ha 2 plots 2 plots Ni = 56 kg N/ha 2 plots 2 plots N£ - 112 kg N/ha 2 plots 2 plots N3 = 224 kg N/ha 2 plots 2 plots P = 56 kg P/ha as t r i p l e superphosphate. K = 56 kg K/ha as muriate of potash. Straw Experiment Control 2 plots Straw treated 2 plo t s 24 nitrogen addition was as urea; a l l subsequent additions were i n the form of ammonium n i t r a t e . Since the aim was to keep nitrogen concen-trations within a narrow targeted range, the decision to apply f e r t i -l i z e r was based on f o l i a r nutrient concentrations from the previous year. Table 2 l i s t s the t o t a l amount of nitrogen applied each year i n an attempt to maintain f o l i a r N concentrations at 1.4% N = N]_; 1.8% N = N 2 and 2.2% N = N 3 (Weetman and Algar, 1974). The straw treatment was established on 0.25 ha square p l o t s . These plots received approximately 3.6 metric tons of straw which was applied to a depth of 60 cm on top of the snow i n March, 1970 (Figure 4). In the following years, these plots received no further treatments. 3.3 Sample Preparation and Laboratory Methods For the laboratory and greenhouse studies, 5 humus samples, 30 x 30 cm, were col l e c t e d randomly from each plot i n l a t e August, 1979. Placed i n polyethylene bags and shipped to the laboratory, they were a i r dried and stored at 1-2°C u n t i l processed. L i t t e r was co l l e c t e d at the same time from seven p l a s t i c trays 25.5 cm x 52.5 cm which had been located randomly on each of the treated s i t e s early i n June, 1979. The l i t t e r was transported i n paper bags and stored at 1-2°C. Humus samples destined f o r chemical analysis were a i r - d r i e d , sieved through a 6 mm sieve and the r e s u l t i n g organic mixtures of L, F and H horizons ground i n a Wiley M i l l to pass through a 20 mesh screen. Humus samples were then oven-dried at 70°C for 48 hourse. Analysis was completed on 10 humus samples per p l o t . Humus used i n the incubation 25 TABLE 2. Amount and time of f e r t i l i z e r applied to a jack, pine stand during the 10-year period. F e r t i l i z e r applied/year (kg/ha) Total (kg/ha) Treatment 1970 1971 1973 1975 1977 1979 N P K No - - - - - - - - -P 56 56 56 56 56 56 336 K 56 56 56 56 56 56 336 Nl 56 56 56 56 56 56 336 P 56 56 56 56 56 56 336 K 56 56 56 56 56 56 336 N 2 112 112 112 112 112 112 672 P 56 56 56 56 56 56 336 K 56 56 56 56 56 56 336 N 3 224 224 224 224 224 224 1344 P 56 56 56 56 56 56 336 K 56 56 56 56 56 56 336 Figure 4 . Straw a p p l i c a t i o n on top of snow i n early March, 1970. 27 study was not ground af t e r sieving but was bulked for each pl o t . A vegetation survey was undertaken at the time the samples were c o l l e c t e d i n the f i e l d . For each plot, 5 subplots of 1 m^  diameter were randomly l a i d out and the percent presence, abundance and s o c i a b i l i t y estimated using the Braun-Blanquet (1932) method of anal y s i s . Total depth of the L, F and H layers was measured i n the f i e l d on the o r i g i n a l large humus squares dug out of the forest f l o o r . Measurements were taken at opposite corners and the average for each pair calculated. This gives two values for each sample block and 10 values for each p l o t . In order to determine humus weight, 100 cm^ subsamples were cut from each of the larger humus squares, giving a t o t a l of 10 samples per p l o t . Green mosses were removed and roots larger than 1 cm i n diameter and not i n a recognizable state of decomposition were removed. Samples were oven-dried at 105°C f o r 48 hours before weighing. L i t t e r weights for each tray were determined after oven drying at 70°C f o r 48 hours. Humus samples were redried p r i o r to nutrient a n a l y s i s . For t o t a l nitrogen and phosphorus determinations, a modified Kjeldahl digestion (Twine and Williams, 1971) was used. For each sample, 0.2 g of humus was weighed and digested for 12 hours i n 5 mL of digestion s o l u t i o n (100 g potassium sulphate and 1 g selenium i n 1 L concen-trated H2SO4) at about 300°C, the samples were d i l u t e d to 100 mL with d i s t i l l e d water and analysed on the Technicon Autoanalyser. 28 Nitrogen was determined using an ammonia cartridge of a Technicon Auto-Analyser which makes use of the Berthelot r e a c t i o n (reaction of ammonia with sodium phenate and sodium hypochlorite to y i e l d a blue indophenol complex which i s quantified i n a colorimeter). Phosphorus was determined using an ortho-phosphate cartridge of a Technicon Auto-Analyser and involves formation of a reduced phospho-molybdate complex, which i s quantified i n a colorimeter. Results are given as percent of the o r i g i n a l dry weight. To determine potassium, magnesium and calcium concentrations, 1.0 g of humus sample was ashed at 400°C for three hours. The ash was then dissolved i n 7.5 mL of 20% HCL by heating gently on a hotplate. D i s t i l l e d water was added to make 100 mL and the sample analysed on a Varian Techtron Atomic Absorption Spectrophotometer using an a i r -acetylene flame. Following the addition of La203, calcium concen-trations were analysed with a nitrous oxide-acetylene flame. Results are expressed as percent of the o r i g i n a l sample dry weight. The pH of 10 humus samples from each plot was measured using one gram of a i r dried humus, f i n e r than 2 mm. The humus was weighed into 50 mL beakers and 8.0 mL of d i s t i l l e d water added. The pH was measured to the nearest 0.1 u n i t . To lessen the s a l t e f f e c t , 0.5 mL of 0.165 M CaCl2 was added to each sample beaker and the pH remeasured. This method gives an approximate 1:8 d i l u t i o n i n water and 0.01 M CaCl2-Exchangeable cations and t o t a l exchange capacity of the humus was analysed using the sodium chloride method. For exchangeable cations, 2.0 g of sample was weighed into tubes and 30 mL of 1.0 N 29 NaCl solution added. The tubes were then shaken for one hour and the contents transferred to Buchner vacuum suction funnels f i t t e d with Whatman No. 5 f i l t e r paper. The humus was washed with 70 mL NaCl, the leachate transferred to 250 mL volumetric flasks and dilute d to volume with d i s t i l l e d water. The extract was mixed well and analysed on the Atomic Absorption Spectrophotometer for K, Ca and Mg. To determine cation exchange capacity ( C E . C ) , the funnels con-taini n g the saturated humus sample were replaced onto the f i l t e r i n g f l a s k s , washed with 100 mL of isopropanol and the leachate discarded. The humus was then washed with 100 mL 1.0 1J KC1 and the leachate transferred to 250 mL volumetric f l a s k s . The flasks were dilut e d to volume with d i s t i l l e d water, mixed w e l l and analysed for Na on the Atomic Absorption Spectrophotometer. S o i l organic matter was determined t i t r i m e t r i c a l l y using the Walkley-Black method. A i r dried s o i l was ground with an agate mortar and pestle to pass through a 35 mesh screen. Because of the very high organic matter content, 0.1 g of humus was weighed into 250 mL Erlenmeyer f l a s k s . Exactly 15 mLs of 1 N K2Cr207 was added and the flasks swirled to disperse the humus i n the solution. Then 30 mLs of concentration H2SO4 was added and the flasks immediately swirled for 1 minute and l e f t digesting for a t o t a l of 30 minutes. The sol u t i o n was d i l u t e d with 150 mL of d i s t i l l e d water and several drops of Ferroin i n d i c a t o r added. The solu t i o n was then t i t r a t e d with 0.5 N F e S 0 4 . Results were calculated and reported as % C. To examine the pattern of release of NH4+-N and N03~-N during decomposition, incubation studies were carried out. A i r dried humus, 30 f i n e r than 6 mm, was bulked f o r each treatment and 5.0 g equivalent dry weight of humus weighed into 4 oz p l a s t i c jars with screw caps. The samples were moistened with 15 mL d i s t i l l e d H2O to approximate 65% of the water holding capacity of the humus and t h i s moisture content was maintained throughout the incubation study by p e r i o d i c a l l y adding d i s t i l l e d water to restore the samples to t h e i r o r i g i n a l weights. The caps were placed loosely on top and the jars placed i n a thermostatically c o n t r o l l e d incubator and maintained at 20°C under aerobic conditions. At regular i n t e r v a l s (0, 2, 4 and 8 weeks) three jars were removed. The available nitrogen was extracted by shaking the contents of the jar with 50 mL KCL f o r one hour. The supernatant was f i l t e r e d through a Whatman No. 42 f i l t e r paper and analysed for NH4+ and N03~-N (Ke eney and Nelson (1982) on a Technicon Autoanalyser. The bioassay experiments were ca r r i e d out i n the greenhouse at the University of B r i t i s h Columbia. Ten int a c t humus samples (L,F,H) from each treated plot were trimmed to f i t 20 cm diameter pots. The pots were f i l l e d with enough sand so that the surface of the humus layer was kept about 3 cm below the top of the pot. In the green-house, the pots were placed i n a completely random design under phosphorescent grow l i g h t s and the pots l o c a t i o n changed p e r i o d i c a l l y . The pots were sown i n October 1979 with jack pine seed co l l e c t e d from a tree adjacent to the experimental p l o t s . Enough seed was sown to ensure 12-15 germinants i n each pot (Figure 5). The grow l i g h t s were set for one month at 12 hours, one month at 14 hours, two months at 16 hours and then back to one month at each of 14 and 12 hours. 31 Figure 5. Jack pine seedlings growing on treated humus i n the greenhouse. 32 This gave a t o t a l of s i x active growing months. The s o i l i n the pots was kept moist. In early May, 1980, the seedlings were harvested. They were c a r e f u l l y removed from the humus i n order to preserve as much of the root system as possible. Roots were washed clean and shoot and root weights determined a f t e r oven drying for 48 hours at 70°C. Nutrient analysis was completed on the fol i a g e and followed the procedure as outlined above for the humus. Results are given as percent of the orginal dry weight. 3.4 S t a t i s t i c a l Analysis In order to compute the analysis of variance for the in d i v i d u a l parameters, the optimum n u t r i t i o n experiment was treated as a com-pl e t e l y randomized block design. The factor + PK was analysed as the block e f f e c t , the factor N as treatment l e v e l and the sample observa-tions were nested within the r e p l i c a t i o n s . The analysis was based on the following l i n e a r model: i j l k i j I J ( i j ) l ( i j l ) k Where: Y ^ j i k = measurement of the k t n observation i n the l t n r e p l i c a t i o n i n the j t l a treatment i n the i t h block, V = mean of a l l observations, a = e f f e c t of the i t h block (-PK, +PK), Y = e f f e c t of the j t n treatment (NQ, NJ_, N 2, N 3 ) , 33 w = e f f e c t of the i n t e r a c t i o n between the i t l 1 block, and j t h treatment (N x PK), \ i j ) l = e ^ ^ e c t °f t n e l t n r e p l i c a t i o n i n the i n t e r a c t i o n between the i c ^ block and j t n treatment, £ ( i j l ) k = experimental error. Sources of v a r i a t i o n , degrees of freedom and expected mean squares for the analysis of variance are given i n Table 3. The i n t e r -action between the v a r i a b i l i t y within treatments and the v a r i a b i l i t y within observations was tested for s i g n i f i c a n c e . If found s i g n i f i -cant, no further data analysis was performed. If the i n t e r a c t i o n was not s i g n i f i c a n t , the two sources of v a r i a t i o n were pooled to create a new, l a r g e r , source of v a r i a t i o n . The analysis of variance was performed using t h i s experimental erro r . Duncan's Multiple Range Test (Steel and T o r r i e , 1980) was used to compare treatment means with P = 0.05. Because some of the measured c h a r a c t e r i s t i c s are commonly expressed as a percentage, an arc sine transformation ( G i l b e r t , 1973) was used to normalize the data d i s t r i b u t i o n . 34 TABLE 3. Analysis of variance table. Source of Degrees of Sum of Mean Expected mean v a r i a t i o n freedom squares squares squares PK b-1 SSj MSj ° 2e + K l ° 2 r + K 3 a 2 p K N t - 1 SS I ] ; MS-J--]- + K l d 2 r + K 2 o 2 N PK x N ( b - l ) ( t - l ) S S I I T MS T T T rj2 . v n 2 e + K l r -4- v o"2 i - K.4 P K * N V a r i a b i l i t y within treatments b t ( r - l ) S S I V MS I V 2 2 CT + K i a e 1 r V a r i a b i l i t y wi thin observations btr(m-l) SS V MSy Where: b = no. of blocks (PK) = 2 t = no. of treatments (N) = 4 r = no. of r e p l i c a t i o n s (R) = 2 m = no. of observations = 1 0 °2e = r e s i d u a l variance o2r = variance between r e p l i c a t i o n s ° 2PK*N = variance due to i n t e r a c t i o n between PK and N ° 2 N = variance due to N O 2 P K = variance due to PK K l ~ K 4 = c o e f f i c i e n t s of the variance components. 4.0 RESULTS AND DISCUSSION 4(a) SOME PHYSICAL-CHEMICAL CHANGES IN THE GROUND VEGETATION, LITTER  AND HUMUS Following 10 years of sustained nitrogen additions or a single heavy straw a p p l i c a t i o n , both v i s u a l physical and chemical changes were evident i n the organic matter and ground vegetation. This chapter w i l l deal with some of these changes revealed by f i e l d and laboratory analyses. 4.1 Ground Vegetation The moss, herb and shrub layers were examined to ascertain the eff e c t s of the various treatments. As a secondary component of the forest ecosystem, t h i s l e s s e r vegetation i s affected by the chemical and physical c h a r a c t e r i s t i c s of the forest f l o o r and s o i l . A summary of the ground vegetation survey i s given i n Table 4. The c o n t r o l plots exhibited the t y p i c a l Kalmia-Vaccinium association as seen i n Figure 3 . These shrubs cover much of the forest f l o o r whereas the moss and lich e n layer appear i n somewhat smaller patches. As the nitrogen le v e l s are increased (Figure 6), both the abundance and s o c i a b i l i t y of the shrub layer i s decreased u n t i l at the N3 l e v e l Kalmia covers under 5% of the plots and occurs s i n g l y rather than covering the ground. Vaccinium covers le s s than 25% but s t i l l grows i n clumps. Large nitrogen additions also e f f e c t i v e l y reduces the moss layer and eliminates Cladonia r a n g i f e r i n a e n t i r e l y . Instead, TABLE 4. Ground vegetation analysis of the optimum n u t r i t i o n and straw experiments. •—-JJegetation T r ea tmeivtrs^ -----^ ___^  Kalmia a n g u s t i f o l i a Vaccinium angustifolium Pleurozium schreberi Polytrichum spp. Other species * + o * + 0 * + o * + o -PK N 0 100 3.3 100 5.4 100 3.3 2.2 Cladonia r a n g i f e r i n a Nl 100 3.2 100 2.2 100 4.4 10 + 1.1 + Cladonia r a n g i f e r i n a Hypnum c r i s t a - c r i s t e n s i s N 2 90 2.2 90 2.3 80 2.3 N 3 90 1.1 100 2.2 10 + 10 + + 1.2 Sambucus canadensis Carex and Calamagrostis spp. +PK No 100 2.2 90 2.2 80 3.4 10 + 1.1 Cladonia r a n g i f e r i n a Nl 90 3.3 90 3.3 100 2.3 + 2.1 + + Solidago macrophylla Carex spp. Sambucus canadensis Hieracium pratense N 2 100 2.2 10 + 50 1.2 10 + 2.2 + 2.1 Carex and Calamagrostis spp. Viburnum cassinoides Solidago macrophylla N 3 20 1.1 30 2.2 10 + 40 1.2 1.1 + 1.2 Carex spp. and V i o l a spp. Sambucus canadensis Aster spp. Straw treatment 100 2.1 100 2.1 100 2.2 100 1.2 * Frequence of occurrence (%). + Degree of abundance. Scale: + = rare; 1 = <5%; 2 = 5-25%; 3 = 25-50%; 4 = 50-75%; 5 = >75%. o Degree of s o c i a b i l i t y . Scale: 1 = isolated; 2 = troups; 3 = patches; 4 = colonies; 5 = complete coverage. Figure 6. Reduction of the Kalmia-Vaccinium shrub layer i n a jack pine stand following N 2 treatments. 38 other species such as Carex trisperma and Sambucus cover the forest f l o o r (Figure 7). On plots receiving additions of phosphorus and potassium as well as nitrogen, r e s u l t s mirror those found on plots not receiving these ad d i t i o n a l n u t r i e n t s . On plots receiving only P and K, the shrub layer was much reduced when compared to the controls. Additions of nitrogen raised the percent presence somewhat at the PK l e v e l , but as nitrogen additions increase, there i s a marked drop i n frequency and abundance. Pleurozium i s reduced u n t i l at the N3PK l e v e l i t occurs only sparsely on 10% of the sample p l o t s . However, the occurrence of exotic species such as Solidago macrophylla, Viburnum cassinoides, V i o l a , Aster, Sambucus and Carex species i s more frequent than on plots not receiving P and K additions. As seen i n Figure 8 the straw addition was very e f f e c t i v e i n smothering the ground vegetation. Now, 10 years l a t e r , the o r i g i n a l ground vegetation seems ready to re-dominate the s i t e . The moss and shrub layer a l l exhibited 100% frequency i n the sample p l o t s , but the abundance r a t i n g i s s t i l l low. It i s highly l i k e l y that many more years w i l l be required to eliminate the e f f e c t s of the straw. It i s known that the e x i s t i n g ground vegetation i s a very active competitor for added nutrients. Their roots are much closer to the surface than tree roots, enabling them to absorb greater quantities of nutrients than trees per unit weight. However, plant species have varying c a p a b i l i t i e s i n reacting to changes i n the nutrient status. In this case, the Kalmia and Vaccinium species were not p a r t i c u l a r l y e f f e c t i v e i n u t i l i z i n g the available nitrogen since they decreased i n Figure 7. Appearance of exotic species i n a jack pine stand following N3 treatments. 40 Figure 8. A straw a p p l i c a t i o n e f f e c t i v e l y reduced the Kalmia -Vaccinium shrub layer for almost 10 years i n a jack pine stand. 41 both frequency and abundance with increasing nitrogen additions. Damman (1971) studying a Kalmia heath i n Newfoundland, found t o t a l s o i l nitrogen to be very high, but i t was not i n an available form to the plant. Kalmia appears capable of dominating a s i t e and maintain-ing i t s e l f i n d e f i n i t e l y on substrates with low decomposition and mineralization rates and a corresponding immobilization of nutrients. The increased nutrient a v a i l a b i l i t y , e s p e c i a l l y of nitrogen, was obviously detrimental to the ericaceous vegetation growing on plots receiving high nitrogen additions. Other investigators have reported s i m i l a r r e s u l t s . Following f e r t i l i z a t i o n with nitrogen, phosphorus and lime, V i r o (1965) noted that on a Scots pine s i t e i n Finland, over 50% of the Vaccinium had died. He at t r i b u t e d t h i s phenomenon to f r o s t . Because of the extra nutrients a v a i l a b l e , growth continued l a t e r than normal. This resulted i n the c e l l u l a r tissue being unable to l i g n i f y before winter. Thus, although Vaccinium i s hardy i n i t s resistance to climate, on poor s o i l s i t seems s e n s i t i v e to variations i n f e r t i l i t y (Schroeter, 1923). Whether f r o s t accounts for the dramatic reduction of the ericaeous vegetation i n th i s case i s doubtful. However, since no observations were made on possible f r o s t damage, i t may have been a contrib u t i n g factor. I n t e r e s t i n g l y , there was increased tree mortality on higher nitrogen plots which possibly may have been caused by f r o s t . In a si m i l a r experiment i n Sweden, Tamm (1974) noted climate i n j u r i e s such as "winter drought" increased markedly with nitrogen f e r t i l i z a t i o n . 42 Other researchers have noted that f e r t i l i z e r a p p l i c a t i o n can cause a "burning" e f f e c t on lesser vegetation - including mosses and l i c h e n s . Roberge et a l - (1968) noted the toxic e f f e c t s to feather mosses following f e r t i l i z a t i o n with 450 kg urea-N/ha, with the moss f l o r a turning brown and dying. Studying the e f f e c t of N, P and K f e r t i l i z a t i o n on the ground vegetation i n Norway spruce stands i n Finland, Malkonen et a l . (1980) noted that the moss coverage declined immediately following f e r t i l i z a t i o n . Even af t e r eight years, the moss had not completely recovered. S i m i l a r l y i n this study, Pleurozium  schreberi decreased with additions of more nitrogen but there i s no d e f i n i t e trend evident with Polytrichum, as i t s occurrence i s sporadic i n most cases. Cladonia however, disappeared at the N 2 l e v e l . These high nitrogen amounts were most l i k e l y t o x i c , not only to the moss and l i c h e n layer but to the ericaceous vegetation as w e l l . Although the e x i s t i n g vegetation did not respond favourably to the changed environment, other species d i d . For example, grasses and herbs became more abundant with higher nitrogen additions. Malkonen et a l . (1980) also noted that grasses and herbs benefited from n i t r o -gen f e r t i l i z a t i o n . Jeglum (1971) investigated the r e l a t i o n s h i p be-tween the vegetational community and the pH of the upper s o i l horizon. He found the abundance of Carex and Calamagrostis sp. greater at a higher pH range of 4.0-4.9. These grasses f i r s t appear on the N^PK p l o t s which were found to have a pH of about 3.8 (see Figure 13). They increased i n abundance on the N 2PK (pH 4.0) but declined somewhat at the N3PK treatment l e v e l . This treatment recorded a drop i n the hydrogen ion concentration to a pH of 3.8. On plots not receiving PK 43 additions, the grasses did not appear u n t i l the N3 treatment l e v e l , which had an average pH of 3 . 9 . A l l other plots had much lower pH values• 4.2 Humus Weight and L i t t e r f a l l 4.2.1 Results 4.2.1.1 Humus mass and depth Estimates of average t o t a l humus weight (kg/ha) f o r each trea t -ment as well as the average depth (cm) are presented i n Figures 9 and 10. A d d i t i o n a l l y , photographs taken i n the f i e l d show the differences between the (Figure 11) and N3 (Figure 12) treated humus layers. The r e s u l t s for both variables can be summarized as follows: 1. The addition of nitrogen s i g n i f i c a n t l y affected both the weight and depth of the humus. 2. Additions of PK did not s i g n i f i c a n t l y a f f e c t these two variab l e s . 3 . The largest increase i n both weight and depth occurs at the l e v e l . Humus weight increases by more than 65% compared to the control and the depth increases by more than 2 cm. 4. Further additions of nitrogen decreased the weight compared to the Ni l e v e l , but weights s t i l l remained higher than the co n t r o l . 5. Increased nitrogen additions had a si m i l a r e f f e c t on the depth. However, the shrinking of the humus layer was more dramatic than the weight loss with the average for the N3 l e v e l being 2 cm lower than the c o n t r o l . 44 12 - -11 - -10 - -9 - -CO x: 3 - -2 - -1 - -N 0 N-, N 2 N 3 N 0PK N-jPK N 2PK N 3PK TREATMENTS Figure 9. Estimates of the average weight (kg/ha) of the combined L, F, H humus layer s (± standard d e v i a t i o n ) of the optimum n u t r i t i o n experiment and straw treatment p l o t s . 45 12 - -11 - -10 - -E u LU a co 9 - -8 7 - -6 - -5 - -4 - -1 - -N 0 N-| N 2 N 3 N 0PK N-|PK N2PK N3PK Straw TREATMENTS Figure 10. Average forest floor depth (cm) of the optimum nutrition experiment and straw treatment plots (± standard deviation). 4 6 Figure 11. Humus weight and depth increased d r a m a t i c a l l y f o l l o w i n g treatments. Figure 12. With increased nitrogen additions, humus layers decreased i n weight and depth as shown here on N 3 treated humus. 48 6. Results for the humus receiving the PK additions generally r e f l e c t those trends found on the plots receiving nitrogen alone. S t i l l , the PK treatments appeared to reduce some of the dramatic changes as both Figures 9 and 10 show. 7. The addition of the straw s i g n i f i c a n t l y affected the weight of the humus but i t s e f f e c t on the depth was not s i g n i f i c a n t . 4.2.1.2 L i t t e r Estimated three month l i t t e r weights f or each treatment, separated into the components f o l i a g e , bark and t o t a l weights, are presented i n Table 5. To provide a better i n t e r p r e t i v e tool In explaining decomposition rates, the decomposition rate factor (k) (Olsen, 1963) was calculated using the equation (k = L/Xss). Here, L = annual l i t t e r f a l l (3 month l i t t e r data calculated on a yearly basis by multiplying the 3 month l i t t e r data by 4) and X = weight of the forest f l o o r . The k values for each treatment are also given i n Table 5. Although the f i e l d data did not show s t a t i s t i c a l differences between the nitrogen^levels nor the PK additions, the trend shows that f e r t i l i z e r applications increased the t o t a l amount of l i t t e r f a l l . Foliage l i t t e r weights increased s l i g h t l y with larger nitrogen additions and were also higher on PK p l o t s . No trends are di s c e r n i b l e for the bark l i t t e r so that t o t a l l i t t e r weights r e f l e c t the fo l i a g e l i t t e r pattern. L i t t e r weights f or the straw treatment were much higher than the c o n t r o l for each of the l i t t e r components. TABLE 5. Estimated three month l i t t e r weights (kg/ha) i n the treated jack pine stand and th e i r corresponding above ground decomposition rate factor k, calculated on an annual basis. 49 Treatment Foliage L i t t e r components (kg/ha) Bark Total k x 10 N 0 Nl N 2 N 3 151.5 182.5 248.5 228.8 101.9 92.8 77.5 104.9 253.5 275.1 326.0 333.6 .237 .155 .228 .263 N 0PK N]PK N 2PK N3PK 170.4 267.2 205.5 291.3 55.5 107.6 200.9 78.0 223.9 374.7 406.5 368.9 .164 .233 .313 .268 Straw 241.1 133.6 374.5 .258 50 4.2.2 Discussion The r e s u l t s show that the lowest l e v e l of nitrogen addition resulted i n the largest accumulation of organic matter. Since the l i t t e r weights f o r were only s l i g h t l y higher than the controls, the majority of the organic matter buildup must be due to a slower rate of decomposition. In f a c t , the calculated k value was found to be the lowest of a l l treatments. The periodic small doses of nitrogen seemed not to have stimulated the microorganisms into increased mi n e r a l i z a t i o n rates as other investigators have reported (Mahendrappa, 1978). Addition of 1,344 kg N/ha did increase the decomposition over the 10 year period. The k value was calculated at 0.0263 and the t o t a l humus depth on the N3 plots had shrunk considerably i n sp i t e of the increased l i t t e r input. Total humus weight was s t i l l somewhat higher than the controls however, suggesting that the most of the ce l l u l o s e and hemicellulose had decomposed, leaving more decay r e s i s t a n t phenols, waxes and other materials behind. Investigating the decay curves for i n d i v i d u a l substrate fract i o n s and t h e i r r e f l e c t i o n on the t o t a l weight l o s s , Minderman (1968) found annual loss rates varying from 10%, 25% and 50% for phenols, waxes and l i g n i n r e s p e c t i v e l y . Indeed, the humus on these high nitrogen plots f e l t 'greasy' to the touch, which may have been caused by the r e l a t i v e l y high concentrations of these waxes and possibly the phenol components. Addition of PK and nitrogen increased decomposition rates even more, with humus on the N2PK plots having the highest decomposition 51 rate factor of a l l treatments. Average humus weight was s t i l l higher than the c o n t r o l , but this was l i k e l y due to the larger amounts of l i t t e r which f e l l on the N2PK plots (Table 5). The influence of the PK additions to increasing the organic matter decomposition i s probably due more to the addition of phospho-rus than potassium. Phosphorus was found to increase decomposition rates when coupled with nitrogen additions ( K e l l y and Henderson, 1978; Mahendrappa, 1978). The ap p l i c a t i o n of PK alone however, decreased the k factor (k = 0.0164) considerably suggesting that nitrogen was a l i m i t i n g factor i n c o n t r o l l i n g the decomposition rate. Kelley and Henderson (1978) reported s i m i l a r r e s u l t s . In their l i t t e r decomposi-tio n study, additions of P alone decreased the microflora populations and s i g n i f i c a n t l y retarded decomposition. The straw a p p l i c a t i o n consisted of 3.7 metric tons of straw applied on top of the forest f l o o r . Almost 10 years l a t e r , t h i s straw i s s t i l l v i s i b l e as a d i s t i n c t layer within the humus formation. Total humus weight has increased by more than 15,000 kg/ha over the controls and i s partly the r e s u l t of over 1,500 kg/ha l i t t e r f a l l i n g on the forest f l o o r . S t i l l , decomposition rates are high, i l l u s t r a t i n g that nitrogen i s not l i m i t i n g microorganism a c t i v i t y i n spite of the high C/N r a t i o normally a t t r i b u t e d to straw. Indications that smothering the ground vegetation was so successful that s u b s t a n t i a l l y more nitrogen was avai l a b l e for tree growth. The increased l i t t e r f a l l and straw i s now supporting an active microbial population as r e f l e c t e d i n the high decomposition rate f a c t o r . 52 4.3 Humus pH 4.3.1 Results The r e s u l t s of the pH determinations recorded i n H2O and CaCl2 solutions are presented i n Figure 13. Although a s i g n i f i c a n t i n t e r a c t i o n between N and PK was found to occur at the N 3 , N3PK l e v e l , Figure 13 does show the following trends: 1. Additions of PK s i g n i f i c a n t l y raised the pH on a l l but the N3 PK plot which recorded a s l i g h t l y lower value than N3. 2. Increased nitrogen a p p l i c a t i o n also increased the pH i n most instances. However, t h i s increase was not found to be s i g n i f i c a n t . 3. The straw treated plots had s i g n i f i c a n t l y higher hydrogen ion concentrations compared to the control p l o t s . 4.3.2 Discussion The pH has important influences on the nitrogen transformation processes which occur i n the organic layers. Therefore, i t was one of the f i r s t humus c h a r a c t e r i s t i c s measured. Generally, the pH of the co n t r o l plots was low, as can be expected from a humus formed under a jack pine stand. The pH was s i g n i f i c a n t l y raised however, when PK was added. Much of t h i s increase was due to the calcium content of the phosphorus f e r t i l i z e r . Triplesuperphosphate was used as the P source and i n th i s form i t can contain 12-16% calcium (Jones, 1979). In fac t , the calcium content of the humus was found to be s i g n i f i c a n t l y higher on those plots which N 0 N-i N 2 N 3 N 0PK N-|PK N2PK N3PK Straw TREATMENTS Average pH of the humus (± standard d e v i a t i o n ) measured i n 1 : 8 d i l u t i o n d i s t i l l e d H 2 0 and 0 . 0 1 M C a C l r 54 received PK additions. The cause for the pH decline at the N3PK treatment l e v e l r e l a t i v e to N2PK, i s uncertain. A measurement error cannot be discounted but other variables investigated seem to i n d i c a t e that the pH drop may be r e a l . E f f e c t of the nitrogen additions on the pH can be seen on those plots not r e c e i v i n g PK additions. This increase i s i n contrast to most in v e s t i g a t o r s , who have found no increase i n humus pH values following f e r t i l i z a t i o n with ammonium n i t r a t e . In f a c t , s l i g h t decreases are generally reported when using this inorganic nitrogen form (Overrein, 1967; Weetman et a l . 1972). Because nitrogen s a l t s are r e a d i l y ionized i n the s o i l s o l u t i o n , the NO3 - i s moved e a s i l y out of the s o i l system. At the same time, increased leaching of bases occurs as the C a + + , Mg"*-*" and K + move down with the n i t r a t e . Results of the optimum n u t r i t i o n experiment show a s l i g h t decline at the N\ l e v e l . But, as larger doses of ammonium n i t r a t e are applied, the pH increases. Possibly, the organic exchange s i t e s have become so saturated with the ammonium ion, that they accept 0H~ groups and form ammonium hydroxide. The i n t e r a c t i o n between the N and PK l e v e l s was mentioned above. This i n t e r a c t i o n c h i e f l y occurred at the N3PK l e v e l . Whereas the pH increases as more nitrogen i s added, the addition of PK adversely affected the highest nitrogen l e v e l by decreasing the pH somewhat. This i n t e r a c t i o n however, was found s i g n i f i c a n t for the pH recorded i n H2O only. The pH values measured i n CaCl2 (+ 0.225) were less variable than those recorded i n H2O (+ 0.244) and this may explain why the i n t e r a c t i o n was not s i g n i f i c a n t i n CaCl2» 55 The pH of the straw treated p l o t s was s u b s t a n t i a l l y higher than the controls. The straw came from a l o c a l farm and i t seems reason-able to assume that the o r i g i n a l pH of the straw was higher than the acid humus on which i t was placed. During the past 10 years, however, t h i s straw layer has undergone considerable decay. Since the products of decomposition are predominantly a c i d , a continuous decrease i n the pH of th i s substrate has l i k e l y occurred. The dispersion of organic matter has been related to increases i n the pH (Hubert and Gonzales, 1970; Ogner, 1972; Salonius, 1972). With the s o l u b i l i z a t i o n of organic matter, the a v a i l a b i l i t y of carbon i s increased. Salonius (1972) and Foster (1979) found microbial a c t i -v i t y not limited i n i t i a l l y by the nitrogen but rather by unavailable organic substrate. With the increase i n humus pH following heavy f e r t i l i z e r a p p l i c a t i o n , heretofore unavailable carbon sources may have become available for microbial use, r e s u l t i n g i n increased decomposi-tio n rates as discussed above. The pH of the humus also has very important influences on the nitrogen transformation processes which occur i n the humus. For example, ammonia v o l a t i l i z a t i o n losses are greater when the pH i s increased (Bernier et a l . , 1969; Weetman and Algar, 1974). At higher pH values, the phenolic hydroxyl and carboxyl groups of organic matter tend to di s s o c i a t e and become more r e a c t i v e . Ammonium ions may then be subjected to non-biological immobilization by reacting with the l i g n i n or other organic constituents (No'mmik, 1970; A l l i s o n , 1973). The highly acid conditions i n raw humus can explain the low l e v e l of n i t r i f i c a t i o n i n boreal f o r e s t s . Roberge and Knowles (1966) detected 56 n i t r i f i c a t i o n only when the pH rose above 5.6 after repeated f e r t i l i -z a t ion with urea. Some n i t r i f i c a t i o n i s always occurring however, and a r i s e i n the pH should r e s u l t i n an increase i n n i t r i f i c a t i o n . 4.4 Nitrogen 4.4.1 Results 4.4.1.1 Total Nitrogen Concentrations of t o t a l nitrogen i n the treated humus are given i n Figure 14 and Table 6. The analysis again showed a weak i n t e r -action between PK additions and nitrogen l e v e l s . Using the estimated humus weight and the nitrogen concentration, the t o t a l amount of nitrogen i n kg/ha for each treatment was calculated and the r e s u l t s presented i n Table 6. Inspection of the data i l l u s t r a t e s the following points: Total nitrogen (%) 1. Average t o t a l nitrogen on the con t r o l plots was 1.15% N, with no s i g n i f i c a n t increase u n t i l the N 2 l e v e l (1.31% N). 2. Nitrogen concentrations of the N3 treated humus was down (1.24% N) i n spite of the large nitrogen additions these plots received (1,344 kg N/ha). 3. The additions of P and K as a whole had no s i g n i f i c a n t influence on the nitrogen concentration of the humus. However, means for the PK treatments were higher (1.25% N) than for treatments not rece i v i n g PK (1.21% N). 4. The lowest nitrogen concentration was measured on plots receiving PK only (1.11% N). 1.6 1.5 J -1.4 1.3 UJ o z o o 1.2 + LU O 1.1 o CC r -1.0 0.9 -I-N 0 N-j N 2 N 3 N0PK N-|PK N2PK N3PK Straw TREATMENTS Figure 14. Average nitrogen concentration (± standard deviation) of the treated humus. TABLE 6: Average nitrogen concentration, t o t a l nitrogen and available nitrogen (NH4+-N + N03~-N) i n the treated humus (kg/ha)*. Treatments N-concen- Total-N Available-N (kg/ha) t r a t i o n (kg/ha) weeks (%) 0 2 4 8 N 0 1.15ab 491 5.1 16.2 17.9 19.5 Ni 1.16ab 814 12.1 31.5 33.8 36.5 N 2 1.31ef 749 30.0 47.9 52.9 48.5 N3 1.24de 628 27.2 37.2 48 51.9 NoPK 1.11a 605 11.7 18.3 19.8 19.8** N]PK 1.20bc 772 11.8 26.1 29.3 32.5 N 2PK 1.31ef 680 16.0 34.5 28.2 30.0 N3PK 1.38f 760 27.6 40.2 42.2 39.6 * Values i n the same v e r t i c a l column followed by the same l e t t e r are not s i g n i f i c a n t l y d i f f e r e n t (P <0.05). **0ne plot only. N 0 1.15a 441 5.1 16.2 17.9 19.5 Straw 1.38b 800 13.4 45.8 45.9 46.50 * Values i n the same v e r t i c a l column followed by the same l e t t e r are not s i g n i f i c a n t l y d i f f e r e n t (P = <0.05). 59 5. The highest % N was recorded on the N3PK (1.38% N) which had received the largest nitrogen additions and the PK treatments. 6. The straw plots also recorded high concentrations (1.38% N) although these plots had received no nitrogen additions. Total nitrogen (kg/ha) 1. A l l treated plots recorded higher t o t a l nitrogen amounts compared to the c o n t r o l . 2. The l e v e l contained the largest amount of nitrogen with su b s t a n t i a l declines at the N 2 and N3 l e v e l . 3. Additions of PK alone increased the nitrogen by more than 100 kg N/ha. 4. Subsequent nitrogen and PK additions raised t o t a l nitrogen, but the r e s u l t s were not uniform. 5. The straw plots recorded very high amounts, second only to the N^ l e v e l . 4.4.1.2 Available Nitrogen The available NH.4+-N and N03~-N concentrations of the humus determined during the incubation study are l i s t e d i n Table 7. In order to better evaluate the magnitude of the processes involved, some of the data were graphed and are presented i n Figures 15 to 18. The a v a i l a b l e nitrogen content for each treatment was calculated by using estimated humus weight and the available nitrogen concentration. The r e s u l t s are l i s t e d i n Table 6. 60 TABLE 7. Available nitrogen (ppm) i n the humus layer following eight weeks of incubation at 20°C and 60% WHC*. NH 4 +-N N03~-N Time (weeks) Treatment 0 2 4 8 0 N 0 105a 364ab 401b 441a 14ab 14a 17a 16a Nl 160b 433c 453c 505b 12a 15a 17a 15a N 2 509g 81 le 864f 790c 19c 27b 26b 58b N 3 500g 653d 808e 807c 36c 80d 113d 218d NoPK 20 Od 319a 317a 576b 14ab 17a 12a 22b N]PK 176c 390bc 397b 486b 17ac 16a 15a 21.5a N2PK 280e 425c 470c 520b 29d 40c 41c 58b N3PK 374f 606d 59 7d 581b 128f 122e 136e 136c *Values i n the same v e r t i c a l column followed by the same l e t t e r are not s i g n i f i c a n t l y d i f f e r e n t (P <0.05). NQ 105a 364a 401a 441a 14a 14a 17a 16a Straw 218b 433b 734b 784b 15a 16a 18a 19a *Values i n the same v e r t i c a l column followed by the same l e t t e r are not s i g n i f i c a n t l y d i f f e r e n t (P <0.05). 61 NHJ-N N03-N • • • • • • • • *M • • • • • • • • • «l 900 -f-N 0 10 20 30 40 50 1 60 TIME (days) Figure 15. Total available nitrogen (NH^ -N and N03"-N) in the humus of the control, and N2 treated plots following 0, 2, 4 and 8 weeks incubation at 20°C and a water content of 60% WHC. 62 1000 300 +-• •••••••••••••••«•••••••••••••••••••••••••••••••• - - ) • • • • • • • • • • • • • • • • • • * • • • • • • • • • • • • • • • • • • • • • • • • • • • € • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • A • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • a • •••••••••••••••••••••••••••••••••••••••••••••••« • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • A v e • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • A * * * • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • a • •••••••••••••••••••••••••••••••••••••••••••••••« • • •••••••••••••••••••••••••••••••••••••••••••••A* • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • a • . • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • a • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • A • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • a • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • a • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • a * • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • a • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • a * * • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • a 100 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • A • • • • • • • • • • • • • • • • • • • • • • • • • • • a • ••••••••••• 10 20 30 40 50 60 TIME Cdays) Figure 16. Total available nitrogen (NH4 +-N and N03~-N) in the humus of the N3 treated plots following 0, 2, 4 and 8 weeks incubation at 20°C and a water content of 60% WHC. 63 900 NHJ-N .»••••• mm • « TIME (days) Figure 17. Total available nitrogen (NH4 +-N and N03~-N) in the humus of the N2PK and N3PK treated plots following 0, 2, 4 and 8 weeks incubation at 20°C and a water content of 60% WHC. 64 900 +-^ 7 0 0 £ a a Z 111 a o cc 1-500 LU - J QQ < I 300 100 -h: Straw 10 20 30 40 50 60 TIMECdays) Figure 18. Total available nitrogen (Nrl^ + -N and NO^'-N) in the humus of the straw treated plots following 0, 2, 4 and 8 weeks incubation at 20°C and a water content of 60% WHC. 65 Under the conditions reported here, an increase i n the concen-t r a t i o n of NH^+-N i s the res u l t of mine r a l i z a t i o n of the orga n i c a l l y bound nitrogen and i s brought about by the microorganisms present i n the humus. The increase i n NO3 -N concentration i s the res u l t of the actions of n i t r i f y i n g b acteria. Total available nitrogen r i s e s when the sum of NH4 + -N and NO3 -N increases. Immobilization of NH4+-N by the microflora leads to a decrease i n NH4+-N concentration. N i t r i f i c a t i o n can also decrease NH.4+-N by o x i d i z i n g ammonium to n i t r a t e . Following the interactions between min e r a l i z a t i o n and immobili-zation, the rate at which nitrogen became available showed the following trends: 1. Additions of nitrogen s i g n i f i c a n t l y increased the KC1 extract-able NH4+-N over the controls i n time zero and this trend continued over the eight week period. 2. The rate of NH4+-N production was most rapid during the f i r s t two weeks of incubation f o r a l l treatments except N3. At the N3 l e v e l , NH4+-N production continued f a i r l y r a p i d l y u n t i l the fourth week. 3. Following t h i s two week period, NH4+-N increased only s l i g h t l y during the remaining weeks for the control and N^ humus. For the N 2, N3 and N3PK treated humus, NH4+-N decreased i n amount. 4. In plots which received PK along with N additions, the produc-t i o n of NH4+-N was somewhat lower, although the differences were not always s i g n i f i c a n t . 66 5- Generally, NC>3~-N l e v e l s were low on the con t r o l , PK and Ni p l o t s , with s i g n i f i c a n t n i t r i f i c a t i o n not recorded u n t i l the N 2 treatment. On the N3 humus, some nitrate-N was present before incubation began, but by the the eighth week, nitrate-N production was greatly increased. 6. Humus which received nitrogen and the addit i o n a l PK treatments showed some N03~-N production at the N2PK l e v e l . Nitrate-N was much higher at the beginning of the incubation period for N3PK, and continued at approximately the same rate throughout the experiment. 7. On the straw p l o t s , NH4+-N l e v e l s were s i g n i f i c a n t l y higher than those measured for the c o n t r o l . In f a c t , ammonium-N leve l s were higher than those recorded f o r the N3PK l e v e l and almost as high as those recorded on N3 humus. Nitrate-N remained low. 4.4.2 Discussion The optimum n u t r i t i o n experiment provided an excellent opportu-n i t y to study what e f f e c t increases i n nitrogen supply had i n changing the nitrogen cycle on a low nitrogen s i t e . Generally, the nitrogen additions did raise the nitrogen concentrations of the humus as shown i n Figure 14. In some cases however, the increases seemed much smaller than would be expected. For instance, at the N^ l e v e l , 336 kg N/ha was applied over the 10 year period (Table 2), yet the nitrogen concentration was raised only s l i g h t l y , from No = 1.15% N to Ni = 1.16% N. Further additions of 1,000 kg N/ha at the N3 67 l e v e l raised the concentration only 0.09 units - down i n fact from N 2 = 1.31% N to N3 = 1.24% N. When t o t a l nitrogen (kg N/ha) i s calculated f o r the treated humus (Table 6) a d i f f e r e n t picture emerges. Nitrogen content for the control was 491 kg N/ha and for Ni 814 kg N/ha - an increase of 323 kg N/ha. Since 336 kg N/ha was applied, i t appears that l i t t l e was l o s t from the s i t e . At the N 2 l e v e l , t o t a l nitrogen was calculated at 749 kg N/ha or 258 kg N more than the con t r o l . Since N 2 received 672 kg N/ha, 414 kg N was either partly taken up by the vegetation, leached from the organic layers or v o l a t i l i z e d into the atmosphere. Although these processes were not a c t i v e l y investigated In t h i s study, some inferences can be made. Some of the nitrogen at the N2 l e v e l was probably leached out as N03~-N due to the nature of ammonium n i t r a t e . In contrast, very l i t t l e of the NH.4+-N was l i k e l y leached because i t i s r e a d i l y held by the CEC of the humus. Nitrate-N remained the same for one month of the incubation period (Figure 15), increasing somewhat at eight weeks, so that the p o s s i b i l i t y of N03~-N loss through n i t r i f i c a t i o n does e x i s t . Some loss of nitrogen through v o l a t i l i z a t i o n can not be discounted since an increase i n humus pH occurred and amounts of available ammonium-N were large. The remaining nitrogen was most l i k e l y taken up by the ground vegetation and trees. Nitrogen concentrations i n the current year f o l i a g e f o r trees growing on the N2 treated plots were a c t u a l l y kept at approximately 1.65% N (Weetman and Forunier, 1984). In a study of a P_. banksiana stand f e r t i l i z e d with 300 kg urea-N/ha, 68 Morrison and Foster (1977) were able to raise the nitrogen concentra-t i o n to 1.26 % N. They calculated that 23% of the applied nitrogen was d i s t r i b u t e d within the trees a f t e r three years which was equal to 70 kg N/ha. It would be f a i r to assume therefore, that substantial amounts of nitrogen were taken up by the trees on the N 2 plots over the 10 year period considering the nitrogen concentration of the jack pine needles was much higher (Weetman and Fournier, 1984). Remarkably, most of the applied nitrogen on the Ni treated humus was immobilized within the humus layers. Low decomposition rates resulted i n a large build-up of organic matter. This immobiliz-ed nitrogen represents a nutrient gain by the humus, which acts here as a large storage re s e r v o i r, r e l e a s i n g only small amounts of nitrogen for tree use. As nitrogen additions are increased, transformation processes are greatly altered. Total nitrogen content, although higher than the con t r o l , decreases compared to the Ni l e v e l . Available nitrogen increases s u b s t a n t i a l l y , e s p e c i a l l y on the N 2 and N3 treated p l o t s . N i t r i f i c a t i o n , f i r s t beginning at the N 2 l e v e l , becomes a major fac t o r with the N3 treatment. The increased p o t e n t i a l for leaching los s of NO3 -N probably accounts f o r the majority of nitrogen l o s t from the forest f l o o r . After 10 years, only 628 kg N/ha remained of the 1,344 kg N/ha which had been applied, i n d i c a t i n g that over 714 kg N/ha was l o s t or taken up by the vegetation. With increased n i t r i t e and n i t r a t e production i n spite of the low pH, the po t e n t i a l for chemodenitrification e x i s t s . N i t r i t e i s re a d i l y transformed to nitrous acid which can be further oxidized to 69 n i t r i c oxide and nitrogen dioxide. Most of the NO2 so produced w i l l react further with H2O to form n i t r i c acid and nitrous a c i d , but some of the NO and NO2 i s l i k e l y to escape. The s i g n i f i c a n c e of t h i s reaction i n s o i l has been i n dispute for some time, but Nelson (1982) considers the chemical decomposition of nitrous acid an impor-tant avenue of nitrogen loss i n a c i d i c s o i l s . Conditions favouring b i o l o g i c a l d e n i t r i f i c a t i o n are l i k e l y to occur only infrequently, and should not r e s u l t i n major losses of nitrogen. Ammonia v o l a t i l i z a t i o n probably does not account for large nitrogen losses e i t h e r . However, v o l a t i l i z a t i o n can occur even at low pH (N3 = pH 4.9) when organic matter of high nitrogen content decom-poses (Freney et a l . , 1981). Ap p l i c a t i o n of PK alone affected the nitrogen transformations as seen i n Figures 14 and 17 and Tables 6 and 7. Decreased nitrogen concentrations as well as a reduction i n the decomposition rate, very l i k e l y led to the b u i l d up of organic matter (Figure 9). However, the t o t a l nitrogen content of the humus as well as the supply of available nitrogen increased somewhat when compared to the controls. Additions of phosphorus have been shown to improve the a b i l i t y of plants to recover mineral nitrogen from s o i l s by approximately 20% (Black, 1968). Increased nitrogen uptake from the mineral s o i l may have con-tributed to increased nitrogen i n the l i t t e r returning to the forest f l o o r s . When nitrogen was added with the PK, the opposite r e s u l t s were achieved; nitrogen concentration and decomposition rates increased but available nitrogen decreased. Furthermore, n i t r i f i c a -t i o n was somewhat reduced during the incubation experiment when 70 compared to humus treated with nitrogen alone. Viro (1963) reported s i m i l a r r e s u l t s when phosphorus was added to incubating humus samples. This interplay of nitrogen and phosphorus i l l u s t r a t e s the i n t e r -connection between these two elements. Unfortunately, the linkage between nitrogen and phosphorus i s not s u f f i c i e n t l y understood. For example, Walker et a l . (1959) reported that nitrogen f e r t i l i t y can be improved by a p p l i c a t i o n of phosphorus, and i n this i n v e s t i g a t i o n available nitrogen did increase on the PK treated p l o t s . Additions of nitrogen and PK decreased nitrogen a v a i l a b i l i t y however, exemplifying that the important effects of phosphorus on the mine r a l i z a t i o n processes probably involve the r e l a t i v e requirements of these elements by the decomposer organisms (Cole and H e i l , 1981). Nitrogen r e l a t i o n s i n the straw treated humus were very i n t e r -esting and contrary to expectations. Concentrations of t o t a l nitrogen was very high (1.38% N) and the release of NH4+-N during the incu-bation period was likewise high. This translated into t o t a l and ava i l a b l e nitrogen contents equal to or greater than those found on plots r e c e i v i n g 672 - 1,344 kg N/ha! Weetman and Algar (1974) have speculated on the absence of nitrogen immobilization on a substrate with such a high C/N (>100) r a t i o . They a t t r i b u t e d t h i s to a l l or part of the following: reduced competition from the ericaceous vegetation for the limited nitrogen supply, increased nitrogen a v a i l a b i l i t y due to the decomposing ground vegetation or possible n i t r o g e n - f i x a t i o n on the straw layer. A g r i c u l t u r a l research has shown that a p p l i c a t i o n of substrates with high C/N r a t i o s need not reduce decomposition of organic residues. 71 Studying the dynamics of carbon and nitrogen i n a simulation model, M c G i l l et a l . (1981), noted that high C/N r a t i o of 100 had no e f f e c t on plant processes although i t stimulated microbial growth. In t h i s i n v e s t i g a t i o n , decomposition rates were found to be high on the straw treated humus. Ferguson (1967) reported that a p p l i c a t i o n of straw had not depressed cereal y i e l d , but had i n f a c t , increased production. S i m i l a r l y , tree volume production was s u b s t a n t i a l l y increased on the straw plots (Weetman and Fournier, 1984). Part of the answer may possibly l i e i n the addition of more e a s i l y decomposable carbohydrates as an energy source for micro-organisms. Coupled with the reduction i n nitrogen demand plus the nitrogen released by the decomposing ground vegetation, increased nitrogen was available for tree growth. L i t t e r q u a l i t y was possibly better, with enough nitrogen returning to the forest f l o o r to enable a t h r i v i n g microbial population to e x i s t on the straw l a y e r . A d d i t i o n a l l y , the higher pH of the straw as well as i t s better moisture holding capacity, possibly contributed to the creation of more desirable environmental conditions for microbial growth. Although mycorrhizae were not investigated i n t h i s study, an increase i n the organic matter (straw) may have led to an increase i n mycorrhizal development. Mikola (1969) noted that s o i l moisture was very important for mycorrhizal formation because moist s o i l s prevent mycelia from drying; hence s u r v i v a l i s greater. Moist s o i l s also of f e r better substrates for sporophore production and spore germina-t i o n and a corresponding increase i n mycorrhizal formation on tree roots. The straw-treated humus may have retained moisture longer than 72 the thin humus layer of the control plots and so promoted mycorrhizal growth and increased nutrient procurement for the trees. 4 .5 Carbon and Decomposition 4.5.1 Results The r e s u l t s for the carbon (% C) and the C/N r a t i o are given i n Table 8. With reference to this data, the conclusions are: 1. % C increases with low nitrogen additions at the l e v e l . Larger applications have no further e f f e c t . 2. The C/N r a t i o increases at the N^ l e v e l and decreases again to c o n t r o l l e v e l s at the N 2, N3 treatment l e v e l s . 3. Additions of PK alone increases the % C, and C/N r a t i o to t h e i r highest l e v e l s . Additions of nitrogen reduce the % C somewhat and decrease C/N r a t i o to control l e v e l s . 4. % C f o r the straw treatment r e f l e c t those values found at the control l e v e l . The C/N r a t i o however, i s the lowest at C/N = 32. 4.5.2 Discussion In general, there i s a c o r r e l a t i o n between the C/N r a t i o of organic matter and nitrogen m i n e r a l i z a t i o n during decomposition, with low r a t i o s favouring nitrogen release. However, available carbon may also l i m i t microorganism a c t i v i t y , so that the C/N r a t i o i s not as sen s i t i v e to the nitrogen mineralization rate as could be expected. Evaluating the r e s u l t s for the optimum n u t r i t i o n and straw experiment, the r a t i o was generally found inadequate i n explaining transformation processes. TABLE 8. Percent carbon, and the C/N r a t i o of the treated humus. Treatment % C C/N N 0 43.9 38 Ni 47.2 41 N 2 47.7 36 N3 47.4 38 NnPK 53.1 48 NiPK 51.1 43 N 2PK 49.4 38 N3PK 49.2 36 Straw 44.6 32 74 Compared to the controls, the C/N r a t i o of the treated humus was higher. This value corresponds well to the lower decomposi-t i o n rate factor (k = 0.0155) calculated f o r when compared to the con t r o l (k = 0.0237). Indeed, most of the nitrogen added was found immobilized within the organic layers as the high carbon to nitrogen r a t i o would i n d i c a t e . However, heavier nitrogen applications did not decrease the C/N r a t i o s i g n i f i c a n t l y below the co n t r o l , although decomposition factors and e s p e c i a l l y available nitrogen increased greatly. Addition of PK alone increased the C/N r a t i o considerably. With increased nitrogen applications however, C/N ra t i o s r e f l e c t those recorded from the humus receiving nitrogen alone. Similar increases due to phosphorus and potassium were reported by V i r o (1963). The straw treated humus had the lowest C/N r a t i o of a l l t r e a t -ments. E a r l i e r discussions have already indicated the high m i n e r a l i -zation rates t h i s humus exhibited. The high C/N r a t i o s t r a d i t i o n a l l y associated with straw had decreased to C/N = 32 i n d i c a t i n g that much of the carbohydrates i n the straw had rapidly decomposed over the 10 year period. Other researchers have speculated on the d i f f e r i n g q uantities of nitrogen mineralized from organic matter with s i m i l a r C/N r a t i o s . In his incubation experiments, Benoit (1974) reported a net immobiliza-t i o n of NH4 + i n black spruce humus, but with jack pine humus, constant rates of nitrogen mineralization was recorded. According to Z o t t l (1960), no v a l i d conclusions can be made about the nitrogen 75 supply from the C/N r a t i o , when material d i s s i m i l a r i n decomposability i s compared. From the l i t e r a t u r e , i t appears that the organic chemical com-position of l i t t e r plays an important role i n nitrogen release. For example, l i g n i n may be more of a determinant i n decomposition rates than the C/N r a t i o (Swift et a l . , 1979). Percentages of l i g n i n have been shown to increase with increased nitrogen f e r t i l i z e r applications (Berg and Straaf, 1980). Nilsson (1973) investigated the suppressing e f f e c t of l i g n i n on c e l l u l o s e decomposition and his observations support the concept that the l i g n i n acts as a b a r r i e r enclosing the carbohydrates. This b a r r i e r must f i r s t be degraded before the c e l l u l o s e and the hemicellulose components can be decomposed. However, l i g n i n content w i l l a f f e c t the decomposition rate only i f i t i s a dominant chemical f r a c t i o n towards the l a t e r stages of decomposition. Berg and Straaf (1980) present some suggestions regarding the i n t e r a c t i o n of carbon and nutrients (nitrogen): 1. At l e a s t three groups of carbon compounds a f f e c t the rate of decomposition - the soluble compounds with a rapid turnover rate, the somewhat slower c e l l u l o s e s and hemicelluloses and the slowest turnover compounds containing l i g n i n or l i g n i f i e d carbohydrates. 2. Elements such as nitrogen and phosphorus influence decomposition rates of needle l i t t e r when soluble compounds and parts of the u n l i g n i f i e d carbohydrates were already degraded. 76 3. As the concentration of l i g n i n increased, the influence of plant nutrients decreased. 4. A high l i g n i n l e v e l i n i t i a l l y can negate the stimulating e f f e c t of plant nutrients on decomposition rates. _j_ | | | | 4.6 Cation Exchange Capacity (CEC) and the Cations K , Ca and Mg The CEC of the humus determined at f i e l d pH with the sodium chloride method i s given i n Table 9. Since the CEC here i s a measure of the r e v e r s i b l y adsorbed cations retained by the organic matter, t h i s value w i l l be r e l a t i v e l y high due to the active organic f r a c t i o n s . Increases i n CEC can also be expected when the pH r i s e s (Thompson and Troeh, 1978). Table 9 shows that the CEC f o r the control i s 42 meq/ 100 g, with a s l i g h t increase to 48.3 meq/100 g at the N3 l e v e l as more nitrogen i s added. This increase i s probably d i r e c t l y a t t r i b u t e d to a r i s e i n the pH following the saturation of the exchange s i t e s with NH4+ ions. Camire (1981) also noted a higher CEC i n jack pine humus following increases i n the pH a f t e r higher rates of urea f e r t i l i z a t i o n . Addition of PK s i g n i f i c a n t l y raises the CEC. Again, the higher pH recorded on these plots probably accounts for most of the increase (Figure 13). The addition of straw increased the CEC of the humus. Increased decomposing organic matter may have led to higher oxygen-containing functional groups with a larger adsorption capacity. The impact of f e r t i l i z a t i o n on the exchangeable cations K +, Ca"1-1" and Mg"*-1" i s given i n Table 9. Concentrations of these cations 77 TABLE 9. Exchangeable bases K , Ca , Mg (me/100 g), CEC (me/100 g) and Base Saturation (%) of the treated humus. K + Ca"1-1" Mg4"*" CEC BS Treatment (me/100 g humus) % No 3.09 5.09 Ni 1.60 5.61 N2 1-49 6.70 N3 1.41 6.66 NQPK 2.86 12.68 N]_PK 2.90 13.15 N 2PK 2.01 15.54 N3PK 3.02 11.90 Straw 2.90 6.97 0.76 42.95 21 0.65 43.21 18 0.67 46.35 12 0.67 48.70 18 0.68 45.80 35 0.65 45.50 37 0.66 52.45 35 0.69 51.95 30 1.42 48.12 24 78 i n the humus i s given i n Table 10. Results of these two tables can be summarized as follows: 1. The control plots had the highest exchangeable K + values. 2. Additions of PK increased the exchangeable KT and Ca"1-*" but had no e f f e c t on Mg"1-*". 3. Increasing nitrogen additions reduced K + s i g n i f i c a n t l y and i I increased C a ^ somewhat on plots not receiving PK additions. 4. Potassium concentration decreased s i g n i f i c a n t l y with increased nitrogen additions but a p p l i c a t i o n of PK kept concentrations at around 0.08% K. 5. Concentrations of calcium also increased s i g n i f i c a n t l y with PK additions but nitrogen additions had no e f f e c t . 6. Magnesium remains constant at 0.03% Mg regardless of treatment. Following increased a p p l i c a t i o n of ammonium n i t r a t e , large amounts of NH4T-N are introduced into the organic layers. Under moist conditions, the N H 4 + ions can e a s i l y displace KT ions from the exchange complex because these two ions have the same valency and sim i l a r s i z e . With more K+ ions i n solut i o n , the pot e n t i a l for increased leaching e x i s t s , e s p e c i a l l y considering the mobile nature of N03~-N added i n the form of ammonium n i t r a t e and as a re s u l t of n i t r i f i c a t i o n . Indeed, exchangeable K+ ions decreased with increased nitrogen additions as did the concentration of K i n the humus. Whereas other researchers have also noted decreases i n Ca and Mg concentrations following nitrogen f e r t i l i z a t i o n (Beaton et a l . , 1969; Otchere-Boateng and B a l l a r d , 1978), there was no decrease i n these TABLE 10. Average concentrations (%) of phosphorus, potassium, magnesium and calcium i n the treated humus*. Treatment P K Mg Ca N 0 0.12a 0.09e 0.03a 0.22a Ni 0.11a 0.06b 0.03a 0.23a N 2 0.11a 0.05b 0.03a 0.30a N3 0.11a 0.04a 0.03a 0.24a NoPK 0.15b 0.08d 0.03a 0.45b N]PK 0.16bc 0.08d 0.03a 0.65b N 2PK 0.19cd 0.07c 0.03a 0.51b N3PK 0.17d 0.08d 0.03a 0.44b *Values i n the same v e r t i c a l column followed by the same l e t t e r are not s i g i f i c a n t l y d i f f e r e n t (P <0.05). NQ 0.12a 0.09a 0.03a 0.22a Straw 0.14b 0.09a 0.06b 0.30b *Values i n the same v e r t i c a l column followed by the same l e t t e r are not s i g n i f i c a n t l y d i f f e r e n t (P <0.05). 80 cations noted here. Possibly, the increased CEC following increases i n pH associated with the larger nitrogen additions, enabled more of these divalent cations to be adsorbed onto the exchange s i t e s . This reduced the presence of Ca"1-*" and Mg + + ions i n solu t i o n and prevented t h e i r loss through leaching. Humus which had received the PK treatment showed higher potas-sium and calcium and concentrations. The increase i n calcium was due to the 12-16% Ca content of the superphosphate f e r t i l i z e r . Potassium concentrations did decrease somewhat with larger nitrogen doses but re s u l t s were not s i g n i f i c a n t . There was no change recorded for the Mg"*-** ion and concentrations remained at 0.03%. 4.7 Phosphorus The addition of PK s i g n i f i c a n t l y raised phosphorus l e v e l s i n the treated humus as shown i n Table 11. However, considering that these plots recived 336 kg P/ha over 10 years, very l i t t l e phosphorus remained i n the organic layers. Since increased l e v e l s of phosphorus i n the jack pine f o l i a g e , which occurred on PK treated plots (Weetman, unpublished data), cannot account for the bulk of this l o s s , leaching i s assumed to have been large. Generally, o r g a n i c a l l y bound phosphorus i s considered very immobile and the immobility of phosphorus i n s o i l s has been reported by many researchers. Studying the loss of phosphorus from f e r t i l i z e d peat i n containers under f i e l d conditions, Malcolm et a l . (1977) found that up to 60% of the added phosphorus was leached from the system. They suggest two c h a r a c t e r i s t i c s which l a r g e l y determine the rate of 81 TABLE 11. Average t o t a l phosphorus (kg/ha) i n the treated humus. Treatment P (kg/ha) N 0 51.3 Nx 77.9 N 2 62.9 N3 55.8 NQPK 81.7 N]PK 102.9 N 2PK 98.6 N3PK 93.7 Straw 81.2 82 phosphorus leaching: the s o l u b i l i t y of the phosphate f e r t i l i z e r and the phosphorus adsorption capacity of the organic matter. Superphos-phate i s f a i r l y soluble at the lower pH range and low phosphorus adsorption capacity of the humus must also be suspected. Fox and Kamprath (1971) demonstrated that i n acid (pH <4.6) s o i l s containing no inorganic c o l l o i d s , phosphorus adsorption was low, r e s u l t i n g i n high phosphorus m o b i l i t y . 4(b) SEEDLING GROWTH AND NUTRITION In order to determine whether the changes i n the humus brought about by repeated f e r t i l i z a t i o n and the straw a p p l i c a t i o n would af f e c t seedling growth and n u t r i t i o n , a bioassay experiment was performed. The re s u l t s of the dry matter production and nutrient uptake of jack pine seedlings grown on the treated humus are discussed. 4.8 Seedling Growth 4.8.1 Results A f t e r growing i n the greenhouse f o r s i x months, seedling dry matter production f o r each treatment was determined. Total seedling weights and the component shoot and root weights are presented i n Figure 19. In Table 12, the root/shoot r a t i o s f o r each treatment are l i s t e d . The r e s u l t s for a l l variables are summarized below: 1. The addition of nitrogen did not s i g n i f i c a n t l y a f f e c t seedling shoot weights. However, shoot biomass did increase at N^ 83 9 - -O) O o UJ Q LU UJ CO 8 - -7 - -I-I g LU >• 5 QC O 6 - -4 - -2 - -1 - -N 0 N-, N 2 N 3 N0PK N-jPK N2PK N3PK TREATMENTS Straw Figure 19. Average dry matter production (grams) of seedlings grown on treated jack pine humus for six months. TABLE 12. Root/shoot ra t i o s of seedlings grown on treated jack pine humus*. Treatment R/S (+ S.D.) NQ 0.56 + 0.13a Ni 0.65 + 0.07bcd N 2 0.69 + O.llc d N3 0.69 + 0.08d NoPK 0.60 + 0.09abc NiPK 0.58 + 0.06ab N 2PK 0.66 + O.llcd N3PK 0.78 + 0.07d *Values i n the same v e r t i c a l column followed by the same l e t t e r are not s i g n i f i c a n t l y d i f f e r e n t (P <0.05). No 0.56 + 0.13b Straw 0.36 + 0.07a *Values i n the same v e r t i c a l column followed by the same l e t t e r are not s i g n i f i c a n t l y d i f f e r e n t (P <0.05). 85 l e v e l , only to decline as more nitrogen was added, f a l l i n g below the control at N3. 2. Root biomass responded s i g n i f i c a n t l y to nitrogen additions with greatest root growth again at the l e v e l , but declining at higher nitrogen l e v e l s . Root weights were s t i l l marginally higher at N3 compared to the controls. 3. The addition of PK increased root and shoot biomass for a l l but the N3 PK l e v e l , however, the differences were generally not s i g n i f i c a n t . 4. For the straw treatment, root and shoot weights were s i g n i f i -cantly d i f f e r e n t compared to controls. 5. There were no s i g n i f i c a n t differences i n the root/shoot r a t i o s for treatments receiving nitrogen additions and PK applications, although r a t i o s were generally greater at higher nitrogen l e v e l s . In contrast, the root/shoot r a t i o s for the straw experiment were s i g n i f i c a n t l y lower than the controls. 4.8.2 Discussion Using dry matter production as a measure of growth, the r e s u l t s show that f o r the optimum n u t r i t i o n experiment, seedlings grown on humus treated with low levels of nitrogen additions attained the highest biomass production. With the addition of phosphorus and potassium, dry matter production was increased a further 11% over the group which did not receive these nutrients. Additional large n i t r o -gen additions decreased seedling oven dry weights on both -PK and +PK plots to below c o n t r o l values (N3-PK = 0.473 g; N3+PK = 0.470 g). 86 Both a g r i c u l t u r a l and forest research has found a decrease i n the root/shoot r a t i o with increasing amounts of nitrogen and phos-phorus a p p l i c a t i o n (Rohrig, 1958; Paavilainen and Norlamo, 1975). This i s attributed to the stimulating influence these nutrients, e s p e c i a l l y nitrogen, have i n increasing shoot growth. Thus, although the root biomass also increased, the r a t i o of roots to shoots decreases. Results recorded i n this study did not agree with these findings. In f a c t , the rate of growth was greater for the roots than the shoots r e s u l t i n g i n an increase i n the root/shoot r a t i o . The depression i n y i e l d which occurred on humus treated with high l e v e l s of nitrogen may have been caused by p h y s i o l o g i c a l d i s -orders within the seedlings. When levels of nitrogen n u t r i t i o n are increased, the l e v e l s of amino acid concentration also increases since they are not being used because of a shortage of other plant nutrients (Mengel et al^., 1979). With very high l e v e l s of available Ntfy"*", amino acid synthesis cannot keep pace and Nlty"1" i s stored within the plant c e l l . Research with a g r i c u l t u r a l plants has shown that NH4 + can be toxic to plants and reduce y i e l d s (Jungk, 1967; Maynard and Barker, 1969). The most i n t e r e s t i n g aspect of the bioassay experiment i s that the growth r e s u l t s generally show the same trends as those found for the mature jack pine stand. Figure 20 g r a p h i c a l l y presents a compari-son of the t o t a l periodic volume growth response of the stand (Weetman and Fournier, 1984) and seedling dry matter production, both expressed as a percent of the c o n t r o l set at 100%. Greatest stand volume 160 - - SEEDLING 150 140 130 O 1 2 o 110 100 90 80 • • • . • • • • • • • 9 9 9* > 9 • • • • • < > • • • • • • f > • • • • • • < > • • • • • • < > • • • • • • < STAND 4 # • • • • • • • • • • 9 9 9* 9 9 9 V . V • • • • • • • • • • • • • 9 9 9* 9 9 9-9 9 9 9 • • • •    9 9 9 t 9 9 9 • 9 9 9 9 9 9 • 9 9 9 9 9 9 9 9 9 ' • • • » • • • • • • • • • • • • • •   • • • • • • • • > • • • • • • V . V » • • • • • • 9 9 9 9 9 9 9 9 9 9* r 9 9 9 9 9 9 W 9 W '1 •999 • • • « • • • i • • • • • • • • • < <999 • • • • • • • > 9 9 9 9 9 9 • • • • • • • • • • • > • • •* 9 9 9* ' • • • i • • • •  • ' 9 9 9 • • •_. 19 9 9 9 9 9 > • • • • •  • • • « > 9 9 9 • • • • • • • • • • « • • • • • • • • • • • • • • ' • • • • • • • • • • • • • • • 9 9 9 9 9 9 9 ' 9 9 9 • • • • ' 9 9 9 • • • • 9 9 9 9 9 9 9* 9 9 9 9 9 9 9* i • • • ' 9 9 9 » • • * .9 9 9' 9 9 9 9 9 9 9* • • • • > 9 9 JL-*. ' ^9 9 9* ft • • • • • • < 9 9 9 9 9 9 9 * .9 9 9* 9 9 9 9 9 9 9 t \ 9 9 9 9 9 9* ' 9 9 9 9 9 9 t > 9 9 9 ' 9 9 9 9 9 9* ' 9 9 9 • • • 1 • • • • 9 9 9 < <999 .9 9 9 i 9 9 9 9 9 9 9* ft • • • • • • i ' 9 9 9 . 9 9 9* ft • • • 9 9 9 i 9 9 9 9 1777.r • • • i 9 9 9 9 « NO control Figure 20. Ni N 2 N 0PK N-jPK N 2PK N 3PK Straw TREATMENTS Periodic total volume increment (1970-1979) in m3/ha of the jack pine stand and total seedling dry weight (6 months) in grams expressed as percent of the control NQ. 88 increment and seedling biomass was attained with low l e v e l s of nitrogen plus the addition of PK. At the N3 and N3PK l e v e l s , negative growth (compared to c o n t r o l = 100%) or extremely low growth response i s also recorded f o r both seedling and stand response. However, some anomalies did occur. Whereas the addition of PK alone increased seedling weight, t h i s treatment reduced stand response considerably. At the N 2 and N2PK l e v e l s of nutrient additions, seedling response was generally low, whereas stand response was s t i l l high. The straw treatment again provided an i n t e r e s t i n g comparison to the optimum n u t r i t i o n experiment. Seedling biomass production was highest on the straw treated humus with gains of 77% over the c o n t r o l . Stand response was 46% greater than the control and equal to the response recorded at the Nj^-PK (44%) and N2+PK (45%) l e v e l s . Taking into account the l i m i t a t i o n s inherent i n bioassay experi-ments, plus the fact that seedlings were grown on the top organic layers and not the whole s o i l p r o f i l e , the bioassay was sensi t i v e enough to r e f l e c t general the trends observed at the stand l e v e l . 4.9 Seedling N u t r i t i o n 4.9.1 Results In order to evaluate the n u t r i t i o n a l status of the seedlings grown on the treated humus, analysis of f o l i a r macronutrient concen-trations were conducted. The re s u l t s are presented i n Figure 21, Table 13 and summarized below: 89 2.3 -f-2.2 "I 2.0 -h 1.9 1.8 1.7 UJ O 1.6 O O 1.5 -fr-LU o g 1.4 r — 1.3 ! N 0 N! N 2 N 3 N0PK N-,PK N2PK N3PK Straw TREATMENTS Figure 21. Average fo l ia r nitrogen concentrations (± standard deviation) of the seedings grown for six months on the treated humus. TABLE 13. Average f o l i a r concentrations of nitrogen, phosphorus, potassium, magnesium and calcium of jack pine seedlings grown on treated humus*. Treatment N P K Mg Ca N 0 1.47a 0.17a 0.36bc 0.11a 0.38a Ni 1.39a 0.18a 0.31ab 0.13c 0.45b N 2 1.75bc 0.21b 0.31a 0.14d 0.51c N3 1.78c 0.21b 0.30a 0.14d 0.51c NoPK 1.45a 0.21b 0.45d 0.12b 0.41ab NiPK 1.51ab 0.20b 0.38c 0.12b 0.44b N 2PK 1.38a 0.24c 0.32ab 0.12b 0.52c N3PK 1.52ab 0.25c 0.30a 0.14d 0.61d *Values i n the same v e r t i c a l column followed by the same l e t t e r are not s i g n i f i c a n t l y d i f f e r e n t (P <0.05). N 0 1.47a 0.17a 0.36a 0.11a 0.38a Straw 2.16b 0.16a 0.41b 0.11a 0.32a *Values i n the same v e r t i c a l column followed by the same l e t t e r are not s i g n i f i c a n t l y d i f f e r e n t (P <0.05). 91 Nitrogen 1. S i g n i f i c a n t differences were observed i n the nitrogen concentra-tions were observed only i n those seedlings growing on the N 2 and N3 treated humus. 2. Nitrogen concentrations were highest however, on seedlings grown on the N 2 and N3 treated humus with nitrogen l e v e l s at 1.75% N and 1.78% N res p e c t i v e l y . Control l e v e l s were recorded at 1.46% N. 3. Lowest nitrogen l e v e l s occurred i n seedlings growing on the Ni, and N 2PK treated humus at 1.39% N and 1.38% N re s p e c t i v e l y . 4. Additions of PK reduced f o l i a r nitrogen concentrations by about 7%. Likewise, as Figure 21 shows, nitrogen concentration v a r i a -b i l i t y between the nitrogen l e v e l s was reduced on PK treated humus. 5. Straw treated humus produced the highest nitrogen l e v e l s i n seedlings (2.16%N) and re s u l t s were s i g n i f i c a n t l y d i f f e r e n t from the control (1.46% N). Phosphorus 1. Addition of nitrogen s i g n i f i c a n t l y increased phosphorus concen-tr a t i o n s . 2. Additions of PK further increased phosphorus l e v e l s i n seedlings by more than 20% over those growing i n humus which did not receive a PK treatment. 3. Phosphorus concentration was s i g n i f i c a n t l y lower i n seedlings growing on the straw treated humus. 92 Potassium 1. In contrast to phosphorus, additions of nitrogen decreased f o l i a r potassium le v e l s s i g n i f i c a n t l y . 2. Additions of PK could not of f s e t t h i s decline although potassium l e v e l s were s i g n i f i c a n t l y higher i n seedlings grown on PK treated humus than i n those which grew on humus not receiving t h i s treatment. 3. The potassium concentration of straw treated seedlings were s i g n i f i c a n t l y higher than the c o n t r o l . Magnesium and Calcium 1. Additions of nitrogen s i g n i f i c a n t l y increased f o l i a r magnesium and calcium l e v e l s . 2. Additions of PK had no e f f e c t on Mg or Ca l e v e l s i n seedling f o l i a g e . 3. Straw treated humus also had no e f f e c t on f o l i a r Mg and Ca l e v e l s . 4.9.2 Discussion The c r i t i c a l l e v e l of f o l i a r nutrient concentration i s generally defined at that point where y i e l d attains 90% of the possible maximum (U l r i c h , 1952; Richards and Bevege, 1972). According to Swan (1970), f o l i a r nutrient concentrations for jack pine are i n the c r i t i c a l to adequate range at N = 1.3%; P = 0.16%; K = 0.33%; Ca = 0.10% and Mg = 0.07%. Results f o r the bioassay experiment showed that seedling nitrogen, phosphorus and magnesium n u t r i t i o n were s u f f i c i e n t for most treatment l e v e l s - Potassium le v e l s were generally within the c r i t i c a l 93 l i m i t s and could be considered adequate i n only a few cases. Calcium concentrations however, were very high and more than adequate for a l l treatment l e v e l s . Magnesium lev e l s are also considered adequate for a l l treatments. Generally, conditions i n the greenhouse were such that, i r r e s -pective of treatments, the humus seemed capable of rele a s i n g s u f f i -c ient nitrogen f o r adequate seedling n u t r i t i o n . Humus which had received l i t t l e or no nitrogen additions and whose nitrogen concentra-t i o n was low (Section 4.4.1, Figure 14), did not necessarily induce low nitrogen l e v e l s i n seedling f o l i a g e . For example, at the NQPK l e v e l , humus concentration was 1.11% N, yet seedlings growing on t h i s treatment l e v e l had higher nitrogen concentrations than seedlings growing on humus which had received the equivalent of 672 kg N/ha. Data on seedling dry matter production and stand basal area increment (Figure 20) indicated a substantial response to moderate applications of nitrogen. Seedling response was highest on the N^PK treated humus. Sustained high growth response of the jack pine stand was also achieved with repeated l i g h t doses of nitrogen with additions of PK as supplied with the N^PK treatment regime. Seedling nitrogen concentrations generally fluctuated between +1.4 - +1.5% N over the treatment l e v e l s NQ, NI and humus receiving PK additions (Figure 2 1 ) . Optimum f o l i a g e % N for the stand was 1.4% N (Weetman and Fournier, 1984). Nitrogen f e r t i l i z a t i o n can a f f e c t the concentrations of other nutrients i n the f o l i a g e . Some researchers have found decreased f o l i a r P, K, Mg and Ca lev e l s following nitrogen additions (Krauss, 94 1969; Timmer, 1979) which have been a t t r i b u t e d to either antagonistic or d i l u t i o n e f f e c t s . In this study, nitrogen additions raised P, Mg and e s p e c i a l l y Ca lev e l s s i g n i f i c a n t l y . Nitrogen a p p l i c a t i o n promotes plant uptake of phosphorus i n several a g r i c u l t u r a l crops (Grunes, 1959). However, i t has also been reported that nitrogen depresses phosphorus uptake by pine seedlings (Fowells and Krauss, 1959). This may be pa r t l y explained by the e f f e c t nitrogen f e r t i l i z a t i o n has i n retarding mycorrhizal development and hence, i n decreasing phosphorus uptake. However, Taber and McFee (1972) found that nitrogen can also increase phosphorus absorption. Increases i n the nitrogen concentrations caused an increase i n r e s p i r a t i o n rates with a corresponding increase i n ATP demand. Thus, nitrogen addition may have increased metabolic a c t i v i t y and created a demand for increased phosphorus. Increased mineralization rates which occurred i n the humus following heavier nitrogen a p p l i c a t i o n , may also have released immobilized phosphorus i n the organic matter. With more phosphorus available for plant uptake, concentrations of t h i s nutrient increased i n the seedling f o l i a g e . Potassium l e v e l s were within the lower end of the c r i t i c a l range for a l l treatments which received high nitrogen additions. The average potassium concentration of nitrogen treated humus (Table 10) was also low and, as discussed e a r l i e r , i s probably due to the mobile nature of K + - e s p e c i a l l y following f e r t i l i z a t i o n . Seedling potassium le v e l s were higher on PK treated humus but a s i m i l a r decline i n f o l i a r potassium occurred as increased amounts of nitrogen were applied. 95 The low growth response of the seedlings and the jack pine stand on plots r e c e i v i n g high N additions, may be due to a deficiency i n boron and other micronutrient l e v e l s . According to B a l l a r d (1983), s o i l s which have high concentrations of a v a i l a b l e N may aggravate or induce deficiency i n boron. These conditions were c e r t a i n l y met on high N p l o t s . Unfortunately, no boron analysis was conducted on the seedling f o l i a g e . Since plant growth i s a factor not only of nutrient i n t e n s i t y but also balance (Shear et a l . 1948), the r a t i o s of one nutrient to another i s important when s t r i v i n g f or maximum y i e l d . Ingestad (1966) expressed as a r a t i o several macronutrients to nitrogen (N = 100). Values for Pinus s y l v e s t r i s , an e c o l o g i c a l l y s i m i l a r tree species, are thought to be i n the following range (Ingestad, 1979): N P K Ca Mg 100 14 43 6 6 Only the r e l a t i v e phosphorus to nitrogen r a t i o for the PK treated seedlings approached or exceeded the above stated value for P_. s y l v e s t r i s . A l l values for potassium were below the recommended values, while those for calcium and magnesium were higher. Simple r a t i o s of the major nutrients N, P, K, Ca and Mg are also used to express nutrient balances for a number of tree species. Interpretations of these element concentration r a t i o s have recently been published by Ballard (1983). Looking f i r s t at the N/P r a t i o s , the values for the optimum n u t r i t i o n experiment were between 5.5 and 8.5. These r a t i o s would indicate no P deficiency i n seedling f o l i a g e . The N/P r a t i o s for the straw experiment were higher at 13.5 and a possible P deficiency may be indicated. 96 The K/Ca r a t i o may also be useful i n i d e n t i f y i n g possible potassium calcium imbalances since these two nutrients are known to be antagonistic (Mengel, 1972). Bjorkman (1953) noted n u t r i t i o n a l imbalances i n Norway spruce when K/Ca was 0.3, but a much healthier green colour when K/Ca was 0.9. Seedlings growing on the various treated humus had lower K/Ca values with increasing nitrogen additions, from Ni = 0.95 to N3 = 0.59 and f o r PK treatments NjPK = 0.86 to N3PK = 0.49. The straw treatment had a K/Ca r a t i o of 1.28 which possibly i l l u s t r a t e s that optimum K/Ca r a t i o s may well l i e higher than 0.5 f o r jack pine. Depression i n y i e l d s which occurred at c e r t a i n nutrient l e v e l s , i n s p i t e of adequate nitrogen n u t r i t i o n , may well have been caused by c e r t a i n imbalances i n macro- and micro-nutrient l e v e l s . It appears, that diagnosis of nutrient deficiency i s f a i r l y straightforward, but determining optimum f o l i a r nutrient concentrations i s not. Much more research i s required i n t h i s area of tree n u t r i t i o n i n order to set up r e l i a b l e nutrient balances f o r our major tree species. 97 5.0 SUMMARY The optimum n u t r i t i o n experiment and the straw treatment provided an excellent opportunity to study the ef f e c t of sustained nitrogen additions and a large carbohydrate a p p l i c a t i o n to the forest f l o o r of a jack pine stand and on c e r t a i n aspects of the nitrogen cycle. The r e s u l t s showed that some nitrogen transformation processes were greatly affected within the organic layers of the forest f l o o r . And yet, the buffering capacity of the humus and i t s resistance to change was remarkable, considering the magnitude of some of the t r e a t -ments. In evaluating the observed treatment impacts, the following conclusions are drawn: 1. The addition of nitrogen d r a s t i c a l l y changed the ground vegeta-t i o n . Mosses, lichens and ericaceous shrubs were greatly reduced at higher nitrogen applications, to be replaced by exotic species such as Sambucus, Aster and V i o l a . The straw treatment e f f e c t i v e l y smothered the ground vegeta-t i o n . Only a f t e r 10 years did the o r i g i n a l ground cover begin to regain control over the area. 2. Repeated low additions of nitrogen e f f e c t i v e l y increased humus weight by 65% and depth by 2 cm. Heavier nitrogen applications reduced both weight and depth. Humus which had received the add i t i o n a l PK treatments, exhibited s i m i l a r trends; however, the changes i n weight and depth were smaller. Weight of the straw treated plots was 35% higher but there was l i t t l e e f f e c t on depth. 98 The lowest decomposition rates observed were for humus receiving low nitrogen additions, i n d i c a t i n g slow nutrient turnover and nitrogen immobilization. Addition of PK alone also resulted i n low rates and th i s was at t r i b u t e d to phosphorus decreasing the decomposer populations. Increased nitrogen raised decomposition rates markedly, i n d i c a t i n g higher nutrient release. Decomposition was also high on the straw treatment, approaching values found on high nitrogen addition plots. In spite of high C/N r a t i o s associated with straw, t h i s treatment seemed to be supporting an active microbial population. The decrease i n pH commonly associated with ammonium n i t r a t e f e r t i l i z e r was observed at low nitrogen l e v e l s . As nitrogen additions increased, pH values rose, possibly due to saturation of exchange s i t e s with NH4+ which i n turn acts as a base i n accepting 0H~ groups remaining a f t e r water reacts with ammonium n i t r a t e . Humus receiving the addit i o n a l PK treatments had con-s i s t e n t l y higher pH values due to the calcium content of the superphosphate f e r t i l i z e r . The pH of the straw treated plots was also higher than the co n t r o l . Quite possibly, the o r i g i n a l pH of the straw was higher. Nitrogen concentration of the humus was raised s i g n i f i c a n t l y only when 672 kg N/ha was applied and decreased with the higher nitrogen a p p l i c a t i o n , probably due to increased leaching losses. Addition of PK alone depressed nitrogen concentrations, but when applied i n addition with nitrogen, concentrations of nitrogen were increased. 99 Straw plots had one of the highest humus nitrogen concentra-tions at 1.38% N. 6. A l l treated plots recorded higher t o t a l nitrogen (kg/ha) than controls, with humus receiving the low l e v e l of nitrogen additions containing the largest amount of nitrogen. High nitrogen plots apparently l o s t much of the applied nitrogen. Indications point to a very t i g h t nutrient c y c l i n g at low l e v e l s of nitrogen input. 7. Results of the aerobic mineralization study confirm that n i t r i -f i c a t i o n was proceeding, i n spite of pH < 4, on high nitrogen p l o t s . As expected, a v a i l a b l e nitrogen increased as more nitrogen was applied, but additions of PK generally depressed av a i l a b l e nitrogen on plots receiving nitrogen. Straw plots showed high amounts of available nitrogen but no n i t r i f i c a t i o n was observed. 8. Large additions of nitrogen had no ef f e c t on the C/N r a t i o , but PK applications increased C/N, e s p e c i a l l y when none or l i t t l e nitrogen was applied. The straw treatment had the lowest C/N r a t i o of a l l treatments. 9. The CEC rose s t e a d i l y as more nitrogen was applied which i s at t r i b u t e d to the increase i n humus pH. Base saturation declined with increased nitrogen l e v e l s - most l i k e l y due to increased leaching loss of N O 3 - . Potassium seemed to have caused most of the decline as magnesium and calcium l e v e l s remained f a i r l y constant. Additions of PK s i g n i f i c a n t l y increased CEC as well as exchangeable bases. 100 10. Although PK applications did increase phosphorus l e v e l s i n the humus, much of the applied phosphorus was leached from the organic horizons. 11. Humus derived from sustained low additions of nitrogen plus a p p l i c a t i o n of PK resulted i n the highest seedling biomass and th i s correlated with stand response. Seedlings grown on the straw treated humus exhibited the greatest response of a l l treatments. 12. Seedling nitrogen concentrations were adequate for a l l t r e a t -ments and r e f l e c t e d the a b i l i t y of the humus, under greenhouse conditions, to release s u f f i c i e n t nitrogen. Additions of nitrogen increased f o l i a r phosphorus, calcium and magnesium le v e l s but decreased potassium concentrations. 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