SCOPE 21 -The Major Biogeochemical Cycles and Their Interactions 

ABSTRACT

The key processes involved in the cycling of carbon, nitrogen, sulphur, and phosphorus are examined within the framework of the relationships among grassland ecosystem properties and driving variables. Interactions among nutrients take place in above- and below-ground primary production, in decomposition processes, and in nutrient cycling. Control of these processes through the external driving variables climate, parent materials, vegetation relief, and man's activities determine the soil properties and plant composition. Changes in ecosystem properties (C, N, S, and P quantities and flows) are discussed.

8.1 INTRODUCTION

In this paper we propose to examine the biology of chemical cycling of carbon, nitrogen, sulphur, and phosphorus in grassland ecosystems (rangeland and cropland). Grasslands in some form are found in every continent throughout the world (Coupland, 1979; Breymeyer and Van Dyne, 1980), usually in the areas with 2575 cm of annual precipitation. In general, grassland soils are fertile, yielding high production of food and fibre by virtue of their accumulation of stores of nutrients in organic forms. Significant losses of C, N, S, and P have occurred under intensive cultivation of grassland soils.

Key transformation processes of these elements are examined within a framework of an ecosystem's properties and driving variables. Our conception of the relationships among (1) an ecosystem's driving variables, (2) the important ecosystem processes controlling organic and nutrient turnover, and (3) ecosystem structure are depicted in Figure 8.1. Driving variables are externally imposed variables little affected by the state of the ecosystem. Important ecosystem processes include above- and below-ground primary production, decomposition, and nutrient cycling. Heat and water flow play key roles through their effects on the rates of biological and chemical processes. Our general hypothesis is that the effects of driving variables (controlling factors) are realized through their influence on ecosystem processes.

*Publication No. R 287 of the Saskatchewan Institute of Pedology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.

Examination of changes in elemental concentrations and forms in organic matter of soils developed along environmental gradients have given insight into possible interrelationships between C, N, S, and P. Cole and Heil (1981) summarized changes in C, N, and P that documented close linkages between organic C and N in mature soils and the P content in the original parent material. They hypothesized that given sufficient time, the supply of N comes into balance with the supply of P (assuming S is not limiting) and that microbial growth processes are the principal arenas for the adjustment of N supply to P supply.

Progress in understanding the internal cycling of nutrients in the soilplant system has been fostered by studies designed to monitor the flows of C, N, S, P, or other nutrients (Clark et al., 1980). Knowledge of C flows has only limited value in postulating nutrient flow inasmuch as each nutrient has specific flows and storage compartments that must be considered individually; flows determined for C are not directly transferable to N, nor are those for N the same as those for S or P. McGill and Cole (1981) proposed that the mechanisms stabilizing organic C, N, S, and P are not necessarily common to all four elements and that the mechanisms and pathways of mobilization are specific to the organic material containing the various elements. They proposed a dichotomous system (Figure 8.2) in which N and part of the soil-S may be stabilized as a result of direct association with C (N- and C-bonded S) and may be mineralized as a result of C oxidation (classical biological mineralization) to provide energy. However, S and P, which exist as esters (C-O-S and C-O-P), may be stabilized by the action of the ester with soil components, and may be mineralized by enzymes in response to the need for a specific element. The latter process was termed biochemical mineralization, because it operates largely outside the cell and is controlled by the need for the element released. This concept accounts for the variability found in soil organic matter composition and sets the stage for predicting the relationship between N, S, and P cycling rates in soils. Because the behaviour of S is intermediate between that of N and P, a study of S transformations has proven helpful in elucidating differences found in soil organic matter quality (Saggar et al., 1981a). This review will summarize information on C, N, and P, discuss S in more detail, and examine modes of interaction. In this review no consideration has been given to the redistribution of C, N, S, and P that occurs through the grazing of plant materials by large herbivores and subsequent distribution in soil. This is a deliberate omission because: a) the subject is well researched and has been the subject of other reviews (Batzli, 1978; Till, 1980; Floate, 1981); and b) in terms of net flow of nutrients within grassland ecosystems, the below-ground processes dominate.

Figure 8.1 Relationships among driving variables, processes and ecosystem structure

Figure 8.2 Schematic illustration of interrelationship of C, N, S, and P cycling within soil-plant systems (from McGill and Cole 1981). Reproduced by permission of Elsevier Scientific Publishing Co.

8.2 DYNAMICS OF CARBON AND NITROGEN IN GRASSLAND ECOSYSTEMS 

The relationship among plant processes and microbial processes and the effects of these processes on plant production, mircobial secondary production, N production, and N cycling have recently been explored through simulation modelling (McGill et al., 1981). The model (Figure 8.3a, 8.3b) emphasizes the biological and biochemical soil-plant subsystem, including plants, microbes, organic residues, and mineral N. Temperature and moisture are driving variables. The mechanistic process treatments include nitrification, root uptake of inorganic N, mineralization, immobilization, and humification. Treatments of plant growth, phenology, plant response to N supply and N redistribution during growth and senescence are sufficiently mechanistic so that the primary producer submodel can provide substrates to the decomposition portion of the model. The treatments of denitrification and leaching are simplistic, and biological nitrogen fixation is included with a general atmospheric input function. Ammonia volatilization is not included, nor is non-exchangeable ammonia fixation by clays.

This model contains several concepts that were novel. A floating C:N ratio of microbes was used to link C and N cycling and control the decomposition of residues with a high C:N ratio. The model treated bacteria and fungi separately and was the first to attempt to represent two competing groups of decomposers responding somewhat differently to environment and substrate. Plant litter was separated into structural and metabolic components, thereby inferring unequal accessibility of C and N and decomposability of C and N containing molecules. This model assumed that the death of microbes was due to abiotic factors but did imply the presence of micro- and mesofauna. Mineralization and immobilization of N were concurrent and independent processes controlled by the C:N ratio of mircrobes. Organic matter stabilization was achieved by both humification and adsorption. Microbial decomposition of this stabilized organic matter left a residue that was more resistant to decomposition.

The concepts embodied in McGill et al. (1981) and in other recent reviews (Jones and Woodmansee, 1979; Clark et al., 1980) have been instrumental in directing research approaches to the study of P and S.

8.3 DYNAMICS OF CARBON AND PHOSPHORUS IN GRASSLAND ECOSYSTEMS 

The immobilization, mineralization, and redistribution of P is best discussed with reference to P cycling in soils (Cole et al., 1977; Chauhan et al., 1979, 1981) as represented by Figure 8.4. The main source of P in soils is from primary P minerals (mainly hydroxyapatite in North American grassland soils). This material is sparingly soluble and releases small amounts of phosphate to solution, where it enters in equilibrium with labile inorganic P (P_i) (i.e. surface-adsorbed or precipitated as soluble calcium phosphates, such as octocalcium phosphate, that are exchangeable to ³²P and extractable by anion-exchange resins). Secondary P_i compounds of reduced solubility and occluded forms of P_i may also be present, especially in more weathered soils.

Figure 8.3 Compartment design for (a) C and (b) N flows. The asterisk indicates state variables replicated by depth (02, 26, 614, 1430 cm) (from McGill et al., 1981). Reproduced by permission of Swedish Natural Science Research Council

Figure 8.4 Compartments and flows of P in the P cycle (Chauhan et al., 1981). Reproduced by permission of Agricultural Institute of Canada

P in solution may be taken up by plant roots or by bacteria and fungi. Bacteria and fungi differ in the way they immobilize P, which alters subsequent redistribution of organic P (P_o) in soil (Hedley and Stewart, 1982). Mineralization rates of P_o depend on the availability of solution P to the microbial population because the C:P ratio of the bacteria and fungal biomass influences mineralization rates (Chauhan et al., 1981). Both the availability of P from solution and the amount of energy available for microbial growth influence the composition of the bacterial and fungal cells and also their subsequent redistribution in soil. Chauhan et al. (1981) found that in soils that had an adequate supply of resin-extractable P_i, incubation with cellulose resulted in a build-up of labile and moderately labile organic-P forms (extractable by NaHCO₃ and NaOH, respectively). When resin-extractable P_i content was low, although the addition of cellulose resulted in approximately the same microbial biomass, the net result was markedly different in that there was no change in the amount of labile P_o, moderately labile P_o, or aggregate-protected P_o forms. The C:P ratio in the soil biomass was large (45:1 to 60:1), and with repeated addition of cellulose the supply of P limited the microbial decomposition of the cellulose.

These studies give us some insight into the possible redistribution of P in soils. Studies carried out in microcosms in which combinations of single bacterial, amoebal, and nematode species were added to sterilized soils in the presence of an energy source (Cole et al., 1978) also have provided insight into the biological transformations of P_o in soils and into the abiotic effects, such as freezing and thawing (Cole et al., 1977).

8.4 DYNAMICS OF CARBON AND SULPHUR IN GRASSLAND ECOSYSTEMS

Detailed descriptions of the S cycle have been reported elsewhere (Postgate, 1968; Peck, 1974). Sulphur, a constituent of amino acids and co-factors, is essential to the structure and function of all living cells. Unlike C and N, S can be utilized to some extent in its most highly oxidized, naturally occurring form, SO₄^2-. In general however, plants and micro-organisms reduce sulphate to S(-II) and use this to form S-containing amino acids and other reduced-S compounds required for their existence. Higher animals utilize these reduced organic compounds, after which they are reformed to the original inorganic compounds, a process that occurs to varying extent in all organisms (Roy and Trudinger, 1970).

1. Sulphur Amino Acids and Related Compounds.

   These include methionine, cysteine (cystine), cellular metabolites such as glutathione, and S amino acid intermediates such as homocysteine and cysteic acid. Other reduced species of biological importance include co-factors such as co-enzyme A, antibiotics, vitamins, and iron sulphur proteins.

2. Sulphonic Acids and Related Compounds.

   The sulphonic acids contain direct C-S linkages and are important as intermediates in the oxidation and reduction of cysteine. The common property of these compounds is that they cannot yield SO₄^2- under conditions where SO₄^2- esters are rapidly hydrolysed.

3. Oxyacids.

   Sulphate, thiosulphate, sulphite, dithionate, and polythionate have been shown to be involved as intermediates in various phases of the reductive and oxydative metabolism of S (Roy and Trudinger, 1970).

4. Ester Sulphates.

   Ester sulphates or sulphonic acid esters and other related sulphoconjugates constitute a very diverse group of compounds widely distributed throughout micro-organisms, plants, and animals. The true S ester has the general formula ROSO₃ and contains a C-O-S bond. Related to the true ester SO₄^2- are compounds in which sulphate is present in other linkage forms (Fitzgerald, 1976, 1978). These compounds fall into two categories: N-O-S linkage, typified by glucosinolates, and N-S linkage, typified by sulphamates. As commonly used, the term `ester sulphates' includes compounds containing these linkages.

5. Sulphatophosphates.

   Adenosine 5'-sulphatophosphate (APS) and adenosine 3'-phosphate 5'-sulphatophosphate (PAPS) have important functions in the activation of SO₄^2- in the formation of ester SO₄²- (Postgate, 1968; Roy and Trudinger, 1970; Schiff and Hodson, 1973; Siegel, 1975).

In general, plants contain approximately as much S as P, with an average content of about 0.25% (by dry weight). A wide variety of S-containing compounds have been found in plants, but only a few are known to be required for normal cell function. The essential compounds include the S amino acids, glutathione, thiamine, vitamin B, biotin, ferredoxin, lipoic acid, co-enzyme A, and the sulpholipid of the chloroplasts. Compounds of non-essential or unknown function in plants include glucosinolates, which are found in the cruciferaes, choline sulphate, and penicillin. A variable portion of the S in plants is in the form of inorganic SO₄^2-.

Generally, the S amino acids contain approximately 90% of the S found in plants (Blair, 1979). However, in some species, a considerable portion of the total S content of the plant may be contained in secondary plant products (Anderson, 1975). The total C: S ratios of plant material are shown in Table 8.1 (Stotzky and Norman, 1961).

The largest portion of S in plants is in protein either as cysteine, cystine, or methionine. Plant proteins generally contain 1% S and 17% N (Dijkshoorn et al., 1960; Pumphrey and Moore, 1965). A review of published data up to 1967 indicates that the average N:S ratio in proteins was 13.7 for gramineous plants and 17.5 for legumes (Dijkshoorn and van Wijk, 1967). For three field crops (wheat, corn, and beans) an N:S ratio of about 1215:1 was required for protein synthesis (Stewart and Porter, 1969).

For any plant species the composition of a given protein, which is controlled by genetics, is constant. Therefore, environmental factors such as N and S supply, age of plant, etc., should have no influence on the N:S ratio in plant proteins. However, the total N:total S ratio can be greatly affected by environmental factors. When S is adequate, non-protein S (mainly SO₄^2-) will accumulate in the plant and the total N:total S ratio will be less than the N:S ratio in protein. When S is deficient, protein formation is suppressed and non-protein N accumulates. The resulting bulk plant N:S ratio is greater than the N:S ratio of the proteins.

Table 8.1 Total S content and C:S ratio of plant material^a

In most plants inorganic SO₄^2- is the main component of the HI-reducible S fraction. Plants absorb S as SO₄^2-, which is reduced and incorporated into amino acids as a precursor for protein synthesis. Any excess SO₄^2- that is not metabolized accumulates unchanged in the tissue. When S is in short supply, most of this SO₄^2- is incorporated into protein and little remains free in the tissue. Westermann (1975) found that increases in total S above 0.14% in alfalfa were the result of accumulation of SO₄^2-. In plants of the Cruciferae family, ester SO₄^2- of glucosinolates could make up a significant portion of the HI-reducible fraction. The glucosinolates are a class of organic anions (see section 8.4), with more than 60 individual compounds identified in plants (Kjaer, 1976).

A considerable range in the concentration of inorganic sulphate and HI-reducible fraction in plants has been reported. Dijkshoorn and van Wijk (1967) indicated the absence of SO₄^2- in rape, radish, and clover growing in S-deficient soil with an abundant supply of other nutrients; all available SO₄^2- was apparently used. The percentage of total S that was HI-reducible was recently reported as being as high as 60% in rapeseed of S-sufficient plants sampled at the rosette growth stage and as low as 8% of the total S concentration in S-deficient rape plants (Maynard and Stewart, 1980). In wheat, HI-reducible S concentrations decreased from 250 to 60 ppm in the tops as the wheat matured ((Freney et al., 1978). However, when expressed as a percentage of the total S, the critical value indicating S sufficiency remained constant at 10%. Critical values of 10% and 2628% have been reported for green panic grass (Smith and Dolby, 1977), and rape plants (Maynard and Stewart, 1980), respectively.

There is little published data on other ester SO₄^2- in plants. Nissen and Benson (1961) first reported the identification of choline SO₄^2- in roots and leaves of barley, corn, and sunflowers. Sulphated polysaccharides have not been found in higher plants but are present in cell wall constituents of algae (Schiff and Hodson, 1973) and may constitute up to 70% of the dry matter of some red seaweed (McCandless and Craigie, 1979). These algal polysaccharides include carrageenan in red algae, agar and fucodin in brown algae (Bidwell, 1972). From the information available, Schiff and Hodson (1973) suggest that the occurrence of sulphated polysaccharides probably represents the adaptation of particular groups of organisms to special needs.

Studies on the decomposition of plant material indicate that net mineralization of S was mainly related to the content of S in the materrial added to the soil (Freney, 1967). No net mineralization of S occurred from plant material if it contained less than 0.13% S. Stewart et al. (1966) found that it was necessary for the S content of wheat straw to exceed 0.15% to obtain the maximum rate of decomposition. Barrow (1960, as cited in Freney, 1967) observed only a broad relationship between net S mineralization from organic materials and the C:S ratio of the material during a 12-week incubation; increased S concentration of plant material did not affect decomposition rates. Atmospheric deposition of SO₄^2- has been shown to inhibit decomposition, probably due to the accumulation of toxic products of SO₄^2- oxidation such as H+ (Dodd and Lauenroth, 1981).

8.4.2 Sulphur in Soils

In soils most of the S occurs in organic rather than inorganic form (Biederbeck, 1978; Kowalenko, 1978). Soil organic-S is generally divided into two major fractions called hydriodic acid-reducible (HI-reducible S) and C-bonded S. The carbon-bonded S fraction, operationally defined as soil sulphur not reduced by a hydriodic acid mixture, contains cysteine and methionine and other sulphonic acids but not ester SO₄^2- HI-reducible S is more precisely defined as that fraction of organic S that is reduced to H₂S by a mixture of hydriodic, formic, and hypophosphorous acids. This mixture produces H₂S only from compounds containing C-O-S, N-S, and NOS linkages, but does not rupture the C-S bond.

In many ways the cycling of S in soils (Figure 8.5) is analogous to that of P. The main difference is that the atmosphere is an important source of S, primarily in the form of sulphate. In addition to this, the weathering of minerals may provide solution SO₄^2-. Solution SO₄^2- adsorb onto the surfaces of clay minerals, although the strength of such adsorption is much weaker than of SO₄^3- and would be of some consequence only in weathered soils of low pH (<5). In extremely weathered soils, SO₄^2- is occluded by iron and aluminium compounds. Various organic sulphur compounds are also volatilized from water-saturated agricultural and tidal marsh soils (Farwell et al., 1979).

Figure 8.5 Compartments and flows of S in the S cycle

Both bacteria and fungi accumulate SO₄^2- and at lower concentrations most of the SO₄^2- is converted to amino acid (C-S bonds). At higher soil solution concentrations, bacteria do not accumulate excess SO₄^2-, whereas fungi have the ability to store SO₄^2- as organic SO₄^2- compounds. Fitzgerald (1976), in a review of the literature, suggests that choline sulphate is the most probable form of organic SO₄^2- that accumulates. Saggar et al. (1981b) developed a method for the measurement of microbial-S and found differences in the accumulation of C-bonded S and HI-reducible S between bacteria and fungi (Table 8.2). On the death or decrease of microbial biomass, organic-S in two forms, bonded S (C-S) and ester sulphate (C-O-S), would be released to interact with the soil mineral colloids and form soil organic S. Because there are two types of organic S in the soil, the rates of mineralization will depend on two different processes, viz., biological and biochemical mineralization.

The hydrolysis of ester SO₄^2- probably occurs by the splitting of the C-O-S bond by sulphatase enzymes (sulphohydrolases). There is a large number of sulphohydrolases, each apparently characterized by a high specificity towards the organic part of the molecule and an absolute specificity for the S moiety (Houghton and Rose, 1976). Harada (1964) showed that at least three sulphohydrolasestwo arysulphatases and a choline sulphatasewere formed by Pseudomonas aeruginosa.

Table 8.2 Sulphur concentrations of bacteria and fungi grown at three sulphur concentrations (from Saggar et al., 1981b). Reproduced by permission of Pergamon Press Ltd.

The sulphohydrolases of micro-organisms also play an important role in the mineralization of soil ester SO₄^2- outside the cell. Recent studies lend support to the theory that several sulphohydrolases are located on microbial cell surfaces (Fitzgerald and Scott, 1974; Fitzgerald and George, 1977; Fitzgerald, 1978; Maca and Fitzgerald, 1979).

Arylsulphatase activity has been reported in soil (Tabatabai and Bremner, 1970; Cooper, 1972; Speir, 1977; Lee and Speir, 1979). However, little correlation between sulphatase activity and S mineralization has been found. Lee and Speir (1979) did find that sulphatase activity correlated with plant uptake of S from organic sources but indicated that the evidence was inconclusive. They also indicated that, because sulphatase is only one of many enzymes involved in organic S mineralization, a direct relationship between sulphatase activity and organic-S mineralization is improbable. Speir et al. (1980) found differences in arylsulphatase activity between cropped and fallow soils, with the latter soils showing a decrease in activity over 21 weeks. These authors postulated that temperature-dependent denaturation of sulphatase occurred in both soils but that roots replaced lost sulphatase in cropped soils.

8.5 INTERRELATIONSHIPS OF CARBON, NITROGEN, SULPHUR, AND PHOSPHORUS

Major interactions of C, N, S, and P are observed in the effects of these elements on the plant and animal productivity of ecosystems. Changes in species composition and the balance between grasses and legumes and the nutrient quality of the plants have been observed (Chapin, 1980). N, P, and S all affect C accumulation by plants. Another category of interactions occurs where P uptake by plants is influenced by available N in the soil and N content of plant material (Williams, 1948 Grunes, 1959; Cole et al., 1963). Similarly, S supply has a major effect on N accumulation and, conversely, N can affect the utilization of S within plants (Rendig and McComb, 1959). Although the ratio of N to S in protein remains constant under various environmental conditions, the total N to S ratio can be greatly affected by environmental factors.

The second major category of interaction for these elements relates to the decomposition of plant material and formation of soil organic matter. In general, nutrient-poor plant residues require additional supplies of N, P, and S for microbial decomposition because decomposition processes are very sensitive to imbalances in the supplies of these elements. Another major example of interactions mediated by microbes is the effect of P supply on N fixation in plants, algae, and heterotrophic organisms (Cole and Heil, 1981).

In cultivated systems, residue management affects the distribution of C residues in the upper layers of the soil. This affects N, S, and P cycling and may increase the amounts of these elements in organic forms. Management practices also alter moisture and temperature regimes in the soil, and these effects may be compounded. Management practices that accelerate losses of C from ecosystems result in increased net mineralization of organic forms of N, P, and S. Inorganic forms of N and S are subject to leaching and gaseous loss, whereas P may revert to less available mineral forms.

Various approaches have been adopted to study the C, N, S, and P relationships in soils. These include comparisons of the amounts of organic matter N, P, and S in soils from environmental gradients. Bettany et al. (1973) found that the amounts of total S, organic N, and organic C, and HI-reducible S obtained from the A_P layers of 54 soils collected from five major soil zones vary considerably with location within any one zone. However, in the soil organic matter the mean C:S and N:S ratios and the percentage of total S that is HI-reducible showed similar and consistent trends. The C.N:S ratios ranged from 58:6.4:1 in the arid Chernozenic Brown soils to 129:10.6:1 in the more moist Luvisolic (Gray-Wooded) soils. The HI-reducible S was approximately 50% of the total S the Chernozenic soils, in contrast to 36% in the Luvisolic soil. Separation of representative soils from each zone by use of a combined chemical and physical technique showed greater differences in the C: N: S ratios of conventional humic acid (HA-A) fractions than in those of the total soil organic matter. The HA-A fraction was thought to be representative of the end-product of the humification processes and to represent the most resistant fraction (Bettany et al., 1979). However, chemical separation can be criticized due to the severity of the procedure and the possibility of mixing and re-precipitating organic matter.

Comparison of the amounts and forms of C, N, and S in soil organic matter fractions after 65 years of cultivation reveal large differences in organic matter contents and relatively lower net losses of sulphur (38%), compared with carbon (44%) and nitrogen (49%) losses (Bettany et al., 1980). Smaller C:N:S ratios in the cultivated soil indicated that S is more resistant to mineralization during cultivation processes than C and N.

In a similar study the effect of long term cultivation on the concentration and total amounts of carbon, nitrogen, organic and inorganic phosphorus was a loss of 35% in the carbon concentration and losses of between 18 and 34% in nitrogen concentration, depending on the inclusion of legume crops in rotation. Phosphorus concentrations were reduced by 12%, and in clay and silt soils all P losses were from the organic matter. In a similar period of cultivation of a lighter-textured sandy loam, there were greater reductions of C, N, and P (46, 46, and 29%, respectively), and P was lost from both organic and inorganic fractions (Tiessen et al., 1982).

It has not yet been demonstrated that humus can be chemically removed from soils without alteration or that any isolated fraction is either biologically or chemically homogeneous. One method that attempts to do this is the separation of soil organic matter on the basis of particle size fractions. Various techniques involve dispersion in water, either by continued agitation or ultrasonic energy, followed by sieving, sedimentation, and centrifugation. Results from studies of this sort (Turchenek and Oades, 1979; Young and Spycher, 1979; Anderson et al., 1981) indicate that most organic matter is associated with the <20-µm size fractions and that the C:N and C:S ratios decrease significantly with decreasing particle size. Similar studies with P show that the more labile fractions of inorganic and organic P are associated with the clay size particles.

8.6 CYCLING STUDIES

By using radioisotope and stable isotope tracers and ¹⁴C dating, the fractions that are significant in cycling organic-bound nutrients may be identified (McGill et al., 1975; Paul and Van Veen, 1978; Chauhan et al., 1979, 1981). Additional information on the rates of P and S immobilization and redistribution has been gained from incubations of soil that have had cellulose added (Cole et al., 1978; Coleman et al., 1978; Chauhan et al., 1979, 1981; Hedley et al., 1982). In a study of S transformations during an incubation of two grassland soils (Saggar et al., 1981 a), changes in biomass S, C, and N were evaluated and the transformations of ³⁵S-labelled sulphate among organic matter fractions were followed. During the 64 days of incubation (Figure 8.6) there was a net immobilization of S with and without the addition of cellulose or sulphate. In contrast, a net mineralization of N occurred. Cellulose decomposition rates responded to the supply of S available for synthesis of new microbial cells. Fluctuations in the amounts of biomass S during incubation followed biomass C and N changes, and C:S and C:N ratios of the biomass ranged from 47 to 121 and 4.9 to 4.7, respectively. Microbially incorporated S was concentrated within the biomass or partially transformed into soil organic matter.

Fractionation of the soil after incubation by a 0.1 M NaOH0.l M Na₄P₂O₇ extraction technique revealed a significant increase in the C:N ratio in the conventional humic acid (HA-A) and fulvic acid (FA-A) and humin (<2 µm) fractions. Biomass C accounted for 2064% of the observed increases in these fractions, suggesting that differences were due partly to transformed microbial products and partly to microbial cell organic constituents released on lysis of cells or trophic effects during incubation. In contrast to C and N, the contents of total S and HI-reducible S increased in the FA-A fraction only and  accounted for 45 to 76% of the immobilized labelled S. These results indicated that inorganic SO₄^2- was immobilized and organic S was accumulated simultaneously in the cellulose treatment of both soils. Incorporation of S mainly in the FA-A fraction obtained during this short incubation further substantiates the idea that organic S compounds synthesized during short periods are relatively labile in nature, but over a prolonged period these compounds are transformed into relatively resistant fractions, such as HA-A, HA-B, and humins. In contrast, rates of S mineralization were highly correlated with rates of N mineralization when SO₄^2- -S was removed periodically by leaching during the incubation period (Tabatabai and Al-Khafaji,1980).

Figure 8.6 The effect of sulphate-S and cellulose addition on 10 mM CaCl₂-extractable-S concentrations (µg S g^-1 soil) and nitrate-N concentrations (µg NO₃N g^-1 soil) during 64 days incubation (from Saggar et al., 1981a). Reproduced by permission of Pergamon Press Ltd.

Microbial biomass plays a major role in the redistribution of p into different forms (Stewart et al., 1980). Incubation of the soils with added cellulose in the presence of a large supply of P_i resulted in the accumulation of labile P_o forms in the soil. When soils deficient in phosphorus were incubated under the same treatments, labile P_o forms did not increase. Chauhan et al. (1981) and Hedley et al. (1982) postulated that these changes were due to increased mineralization rates of labile P_o forms when labile P_i was in short supply and to differences in the microbial P contents and forms.

Structured models of microbial populations are becoming more important in ecological research. These models take simultaneous account of several population variables, such as number, mean cell size, total biomass, and nutrient content (Hunt, 1977). Hunt et al. (1977) modelled the growth of bacteria limited by either C, N, or P. Their model distinguished between storage compartments for C and P and a pool of small precursor molecules, as well as the structural and synthetic components. The model was adjusted to fit published data on the chemical composition of bacteria growing in a chemostat limited by either C, N, or P. The model successfully predicted the kinds of waste products produced under various conditions, the transient response to a change in dilution rate, and the response to a change in the make-up of the medium. These predictions were made even though the observations were not used to formulate the model or establish parameter values.

Phosphorus-cycling, nitrogen-cycling, and sulphur-cycling simulation models (Cole et al., 1977; Chapin et al., 1978; Harrison, 1978; Mishra et al., 1979; Coughenour et al., 1980; McGill et al., 1981) have integrated the effects of moisture, soil properties, plant phenology, and microbial decomposition of organic matter on P flows. These models revealed gaps in knowledge of key processessuch as organic P mineralization, influences of phophatases on P mineralization, and influence of the microbial biomass on the redistribution of P_i and P_o in soilsthereby giving direction to future research.

We are currently modelling the interaction of ecological driving variables and plant processes, microbial processes, and soil chemical processes in an attempt to integrate C, N, P, and S interactions in grassland ecosystems. To date we have represented these processes in the flow of C, N, and P in soil microcosms (Cole, Hunt, Stewart, Woods, and Coleman, in preparation). A simulation model of C, N, S, and P flows based on these concepts is described in a companion paper (Hunt et al., chapter 10, this volume).

8.7 REFERENCES

Anderson, J. W. (1975) The function of sulphur in plant growth and metabolism, in McLachlan, K. D. (ed.) Sulphur in Australasian Agriculture, Sydney, Sydney University Press, 87-97.

Barrow, N. J. (1960) The effect of varying the nitrogen, sulphur and phosphorus content of organic matter on its decomposition, Aust. J. Agric. Res., 11, 317-330.

Bettany, J. R., Saggar, S., and Stewart, J. W. B. (1980) Comparison of the amounts and forms of sulphur in soil organic matter fractions after 65 years cultivation, Soil Sci. Soc. Amer. J., 44, 70-75.

Bettany, J. R., Stewart, J. W. B., and Saggar, S. (1979) The nature and forms of sulphur in organic matter fractions of soils selected along an environmental gradient, Soil Sci. Soc. Amer. J., 43, 981-985.

Biederbeck, V. O. (1978) Soil organic sulphur and fertility, in Schnitzer, M., and Khan, S. U. (eds) Soil Organic Matter, New York, Elsevier Scientific, 273-310.

Breymeyer, A. I., and Van Dyne, G. M. (eds) (1980) Grasslands, Systems Analysis and Man, IBP Synthesis Series 19, Cambridge, Cambridge University Press.

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