Input control of organic matter dynamics

GEODEI~A ELSEVIER

Geoderma 79 (1997) 25-47

Input control of organic matter dynamics Mary C. Scholes a,*, David Powlson b, Guanglong Tian c a Botany Department, University of the Witwatersrand, P. Bag 3, Wits, 2050, South Africa b Soil Science Department, Rothamsted Experimental Station, Rothamsted, UK c International Institute of Tropical Agriculture, Ibadan, Nigeria Revised 27 November 1996; accepted 9 April 1997

Abstract

The amount and quality of inputs into soil organic matter will be altered by both climate and landuse change. The increase in growth of plants caused by increasing CO 2 concentration implies not only potential increases in yields but also increases in plant residues. Simulation models using doubled CO 2 levels predict global net primary productivity (NPP) to increase by 16.3%, over half of which will occur in the tropics. For tropical ecosystems increases in NPP will be dominated by the effects of elevated CO 2, with water and nitrogen availability and temperature playing a less significant role. Phosphorus limitation may determine whether the potential for increased plant growth will be realized. The distribution of C3 and C4 species in the tropics could be affected by landuse change and estimates of yield increases will be dependent on their proportions. The allocation of photosynthate to the root will increase under elevated CO2, resulting in increased fine root dry weight and root length. Root sink strength and the turnover of roots and associated symbionts are critical knowledge gaps. Carbon : nitrogen ratios in tissues will increase resulting in decreased decomposition rates. The concentration of secondary compounds will be affected more by nitrogen limitations than a direct CO z effect. Changes in lignin, tannin and polyphenol levels are more important in the decomposability of tropical litters than changes in the C : N ratios. Decomposition models will have to be altered to take into account changes in plant composition. The role of models in predicting the effects of management practice on long-term fertility is addressed. © 1997 Elsevier Science B.V. Keywords: elevated carbon dioxide; decomposition; organic residues; secondary chemicals

1. Introduction A number of reviews have addressed aspects of the carbon cycle in relation to global change, including both climate change and landuse change scenarios

*

Corresponding author.

0016-7061/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 1 6 - 7 0 6 1 ( 9 7 ) 0 0 0 3 7 - 2

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M.C. Schole~ et a l . / Geoderma 79 (1997) 2 5 - 4 7

(Cole et al., 1993; Ojima et al., 1993; Zepp and Sonntag, 1995). Soil carbon pools and fluxes are influenced by several factors including carbon inputs (amount and quality of residues) and climatic (rainfall and temperature) and edaphic (texture, structure and pH) conditions. Most of the papers point to complex interactions between plant, soil and atmosphere which involve a host of physical and biological processes. The mechanisms responsible for these processes are generally well understood, but due to the range of possible interactions, conclusions have been general. In addition, most studies of soil responses to climate change have focused on temperate agricultural conditions. Few papers have sketched likely changes in the rates and types of carbon which may enter tropical soils. In this paper we have attempted to merge data and ideas obtained from plant ecophysiological experiments of the effects of climate change on plant growth with data obtained on soil carbon pools and fluxes. It is as important to understand how the quality and quantity of the inputs will change as it is to know how the soil organic pools will respond. The emphasis is tropical. The paper is divided into three sections (a) factors controlling the inputs (b) the influence of inputs on soil organic matter pools in tropical regions and (c) the role of soil organic matter turnover models.

2. Factors controlling carbon inputs under elevated CO 2

The latest global circulation models predict a 1.5-2°C increase in temperature (under 2 × CO 2 concentration), for the humid and sub-humid tropics, with the minimum temperatures being affected more than the maximum temperatures (Houghton et al., 1990). Rainfall in the Intertropical Convergence Zone is likely to increase by approximately 10%. Drier tropical areas will experience only very small changes in total rainfall (Houghton et al., 1990). These estimates are being hotly debated and the only factor which seems certain is that the carbon dioxide (CO 2) concentration in the atmosphere is increasing and will continue to increase into the next century (Conway et al., 1988). Such change will have important impacts on soil organic matter (SOM) dynamics in terrestrial ecosystems through the effects of elevated CO 2 on the quantity a n d / o r quality of products of photosynthesis (van Breemen and van Dam, 1993). 2.1. Net primary productici(v responses to changes in CO 2 and climate

Net primary productivity (NPP) is controlled primarily by the assimilation of CO 2 through the leaves and the assimilation of nutrients by the roots. Elevated CO2 levels increase the potential for plant growth but whether this potential is realized is a function of nutrient and water availability. Data obtained from a number of field experiments showed mixed responses of crop and natural

M.C. Scholes et al. / Geoderma 79 (1997) 25-47

27

species to elevated CO 2 (Dahlman, 1993). The majority of experiments demonstrate a positive gain, with most crop responses ranging from 30 to 50% increase in yield, and some woody species showing a 100 to 300% increase in yield. In all cases the magnitude of the responses varied greatly (Dahlman, 1993). A process-based terrestrial ecosystem simulation model (TEM) predicted that for doubled CO 2 with no climate change global NPP will increase by 16.3% (Melillo et al., 1993). Over half of this increase will occur in the tropics with most of the production attributable to tropical evergreen forests. The TEM generally estimates that nitrogen limitation of NPP is much weaker in tropical forests than in temperate and boreal forests. Thus the potential to incorporate substantial amounts of CO 2 into tropical production is great. This potential may be limited by phosphorus availability since phosphorus limitations are widespread in the tropics (Sanchez, 1976). Phosphorus limitations and species compositional changes need to be included in future TEM predictions. Changes in climate with no change in CO 2 concentrations are predicted to have little effect on global estimates of NPP. For tropical and dry temperate ecosystems, increases in NPP are dominated by the effects of elevated CO 2. But for the northern and moist temperature ecosystems, NPP increases reflect primarily the effects of elevated temperature in enhancing nitrogen availability (Melillo et al., 1993). Tropical vegetation boundaries are expected to experience only minor spatial distribution shifts (Neilson, 1993).

2.2. Controls on plant productiuity and biomass partitioning 2.2.1. Photosynthesis and photorespiration Plant primary production is controlled by a number of physiological and morphological factors. These factors include the rate of photosynthesis, photorespiration, respiration and nutrient and water acquisition which are in turn influenced by environmental factors such as irradiance and temperature. Plants are divided into three categories depending of their photosynthetic pathways, C3, C4 and crassulacean acid metabolism. For C3 plants, representing more than 95% of all plant species in the world (Houghton et al., 1990), elevated CO 2 levels will lead to enhanced ribulose bisphosphate carboxylase oxygenase (rubisco) and decreased diffusion limitations and photorespiration rates. Large quantities of nitrogen are cycled through amino acids during the process of photorespiration. This is one way in which nitrogen can be reallocated to growing tissues. If photorespiratory rates are reduced, increased turnover of proteins may be required under elevated CO 2 conditions to supply amino acids to growing tissues. The primary carboxylation step in C4 plants is carried out by phosphoenol pyruvate carboxylase which responds to elevated CO 2 in a less marked way. The distribution of C4 grasses is higher in the tropics than in any other biome in the world and this must be taken into account when predicting NPP changes for

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M.C. Scholes et al. / Geoderma 79 (1997) 2 5 - 4 7

the tropics. There is evidence to suggest that the predicted increases in NPP will not be realized due to the down-regulation (acclimation) of the photosynthetic mechanisms. Increases in photosynthetic rates occur after short-term exposures to elevated CO 2 but rates often decrease after long-term exposure. Stomatal conductance changes may be responsible but likely causes appear to be (1) chloroplast damage due to excessive carbohydrate accumulation, (2) limited sink strength for utilization of photosynthate and (3) changes in rubisco activity (Thomas and Strain, 1991).

2.2.2. Water effects Elevated carbon dioxide tends to ameliorate the effects of water stress and water-use efficiency and has been shown to increase under elevated CO 2 due to decreased stomatal conductance and transpiration rates (Lawlor and Mitchell, 1991; Kimball et al., 1993; Tyree and Alexander, 1993). In dry areas water availability limits productivity more than nitrogen availability. Research findings show that increases in water-use efficiency with elevated CO 2 are generally greatest in water-stressed systems. This may have implications for increased NPP for the sub-humid tropics but will be less important in the humid tropics. 2.2.3. Nutrient effects Carbon assimilation rates must be matched by nitrogen assimilation rates if marked responses to elevated CO 2 are to be realized. There is much debate in the literature on the availability of nitrogen under elevated CO 2 conditions. Some findings provide evidence of a feedback mechanism limiting plant response due to microbial immobilization (Diaz et al., 1993) whereas others provide evidence of increased mineralization rates due to enhanced temperatures (Zak et al., 1993). It has been suggested that tropical soils are more likely to be limiting in phosphorus than nitrogen due to the high phosphorus fixation capacity of many tropical soils (Sanchez, 1976). Phosphorus availability will therefore play an important feedback role to increased NPP. It is possible that net photosynthetic assimilation will increase in an elevated CO 2 atmosphere even if soil nitrogen availability limits plant growth (Zak et al., 1993). Studies have also shown that even if plants grown at elevated CO 2 show a 40% loss of their active rubisco, net photosynthetic rates will still be enhanced at elevated CO 2 when temperatures exceed 22.5°C (Long, 1991). It would seem that except in cases of extreme nutrient deficiency or in cold environments, plant growth is likely to be stimulated by elevated CO 2 in most field situations. 2.2.4. Biomass partitioning and root functioning There are currently two hypotheses which are put forward to explain the biochemical and physiological control of biomass partitioning between plant roots and shoots (Davidson, 1969; Huber, 1983; Johnson, 1985; Robinson, 1986; Chu et al., 1992). Huber (1983) proposed that partitioning of carbon between

M.C. Scholes et al./ Geoderma 79 (1997) 25-47

29

starch and sucrose at the leaf level during the day alone governs carbon allocation patterns between shoots and roots. An alternative hypothesis is that the partitioning is not solely dependent on carbon acquisition and allocation, but rather is controlled by the relative rates of carbon fixation and nitrogen uptake or the carbon and nitrogen substrate concentration levels in the plants (Davidson, 1969; Johnson, 1985; Robinson, 1986). Neither hypothesis is strongly supported with data. Partitioning of photosynthate to root and shoot dry matter may be affected by temperature regime, with proportionally more retained by shoots at higher temperatures (Sena-Gomes and Kozlowski, 1987). The proportion of carbohydrate that is assimilated and transferred to the roots will be critical in understanding soil organic matter dynamics. Eighty-seven percent of the articles reviewed by Rogers et al. (1994) report that root dry weight increases under elevated atmospheric CO 2, regardless of climatic conditions. This increase is almost entirely attributed to fine root production. The most pronounced effects were increases in root length (110%) and root dry weight (100-200%) (Idso and Kimball, 1992; Rogers et al., 1992; Stulen and den Hertog, 1993). A number of studies show variable results especially with respect to root : shoot ratios, rooting volume and root length (Del Castillo et al., 1989; Chaudhuri et al., 1990). This variability may be due to the experimental conditions of growth. Increases in root allocation is strongly dependent on sink strength which is in turn partially controlled by the volume of soil the plants are growing in (Thomas and Strain, 1991). In one of the few studies conducted with C4 grasses from the tall-grass prairie under elevated CO 2 it was found that CO 2 and water treatments had no significant effect on root dry weight in the 0 - 4 0 cm depth and there was no change in root: shoot ratios (Mo et al., 1992). The authors point out that there are very few studies on plant communities to predict effects of elevated CO 2 in the field. This is especially true for the tropics where large areas are C4 grassland used for rangelands. The increase in growth of plants caused by increasing CO 2 concentration implies not only potential increases in yield but also increases in plant residues (Kimball, 1985). Plant ontogeny, the duration of productive plant tissue and litter fall will all be important factors in quantifying the amount of tissue that enters the soil organic matter pool. In an experiment conducted by Korner and Arnone (1992) 15 tropical plant species were grown in controlled environment chambers. The results showed no significant changes in stand biomass, leaf area index, nitrogen or water consumption between ambient and elevated CO 2 treatments. Steady state leaf area index was accompanied by the onset of leaf litter production, which was greater under elevated CO 2. Major responses under elevated CO 2 included massive starch accumulation in the tops of the canopies, increased fine-root biomass, and a doubling of CO 2 evolution from the soil. Increased root biomass can arise for a number of reasons: (a) production can increase while mortality remains constant, (b) production can remain constant while mortality decreases and (c) both processes can increase. Each possibility

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M.C. Scholes et al. / Geoderma 79 (1997) 2 5 - 4 7

provides different amounts of carbon input to the soil system with different amounts of available energy (Zak et al., 1993). Increased root growth has the potential to increase root respiration. Root respiration is significantly greater than leaf respiration on a dry mass basis (Farrar, 1981).

2.2.5. Soil biological acti~'itv Elevated CO 2 could increase carbon exudation resulting in elevated microbial activity in the rhizosphere, increased mycorrhizal infection and increased N-fixation (Diaz et al., 1993; Pregitzer, 1993; Zak et al., 1993). Microbial carbon was 1.5 times greater in the rhizosphere of plants grown under elevated CO 2 compared with plants grown under ambient CO 2. Respiration was also shown to increase but the cause of this was difficult to determine (Zak et al., 1993). Many studies have suggested that plant response to elevated CO 2 may be limited by nutrient acquisition but few have discussed their findings in relation to microbial biomass dynamics in the rhizosphere. Korner and Arnone (1992) and Diaz et al. (1993) both provide evidence that under elevated CO 2 carbon exudation and microbial activity in the rhizosphere were enhanced. In the former case this led to increased leaching and in the latter case it led to increased immobilization and nutrient limitation for the plants. More studies of this type are required. Increases in mycorrhizal colonization have been measured under elevated CO2; this has important implications for plants growing in phosphorus-limited environments. Becard and Piche (1989) showed that hyphal growth was greatly stimulated by a synergistic interaction between volatile and exuded factors produced by roots. Mycorrhizal respiration rates are higher than root respiration rates and if colonization were to be increased this would compound the respiratory costs of the root system (Pregitzer, 1993). A positive feedback loop between nitrogen fixation and photosynthesis was observed in nodulated plants growing at elevated CO 2. This feedback may be an important way in which the potential carbon drain of nitrogen fixation on the host plant could be compensated, especially in infertile habitats (Arnone and Gordon, 1990). 2.2.6. Litter quality and decomposition The most important chemical characteristics influencing litter decomposition, nutrient release and soil organic matter dynamics are C : N ratio, lignin and polyphenols (Swift et al., 1979; Anderson and Flanagan, 1989; Tian et al., 1992a; Tian et al., 1992b). Elevated CO: tends to decrease N content, leading to an increased C : N ratio (Mooney et al., 1991). Melillo (1983) predicted that litter produced at high CO 2 would have a high C : N ratio due to increased carbon assimilation relative to nitrogen assimilation. This effect may be counteracted in leguminous tissue if nodulation efficacy increases under elevated CO 2 (Norby, 1987). van Breemen and van Dam (1993) reported results from growth chamber studies where abscised chestnut leaves contained less nitrogen when

M.C. Scholes et al. / Geoderma 79 (1997) 25-47

31

Table 1 C : N ratio and fraction of sweet chestnut leaf litter decomposed after one year as influenced by CO 2 level during litter production and interaction with soil fauna CO 2 level (vppm)

C : N ratio Decomposed fraction (without fauna) Decomposed fraction (with protozoa, nematodes and insects)

330

700

34 0.30 0.40

80 0.15 0.50

From van Breemen and van Dam (1993).

grown at higher CO 2. Decomposition trials using these leaves indicated that decomposition of high-CO 2 litter was slower than that of l o w - C O 2 material without the addition of a range of decomposers. With a change in the composition and the activity of the microflora rapid decomposition of the high-CO 2 material took place (Table 1). Several studies have shown that sugar and starch in the plant increased with elevated CO 2, either due to enhanced photosynthesis or reduced photorespiration (Ho, 1978; Acock, 1990). This may be the main cause for an increased C : N ratio. The decomposability of the tissue would be no different to material grown under ambient conditions as sugar and starch concentrations are not the determinants of decomposition rate (Tian et al., 1992b). Polyphenol levels may increase in plants grown under elevated CO 2 due to soil acidification. Plants growing on acid soils, contain more polyphenols than those on neutral soils (McVey et al., 1978; Muller et al., 1978). Lack of nitrogen may increase polyphenol and lignin concentrations in the plant (van Breemen and van Dam, 1993). If such changes in the carbon quality do occur at higher CO 2 levels, then decomposition may be slower.

3. The influence of inputs on soil organic matter pools and fluxes

3.1. The concept of SOM equilibrium and balance of inputs and outputs Soil organic matter levels are controlled by the balance between inputs of above- and belowground litter and outputs from mineralization. In a steady-state situation, these inputs are equal to the NPP of a system which is largely controlled by climate with some influence of soil fertility status, soil texture, vegetation, fire and herbivory. The decomposition rate is also a function of climate, the form of the response function is different to that for primary production (Scholes and Scholes, 1995). Organic matter contents are concentrated in the surface horizons of most soils and are responsive to ambient vegetation (Cole et al., 1993). The total amount of carbon in an ecosystem is

M.C. Scholes et al. / Geoderma 79 (1997) 2 5 - 4 7

32

constrained, within minimum and maximum limits by the supply of other essential nutrients in the system. Whether climate change will lead to an increase or decrease in soil C in the tropics will depend on the balance between climate change effects on productivity (hence inputs to the soil) and effects on decomposition rates. Several long-term field experiments in temperate areas have showed a linear relationship between C inputs and soil C levels which supports the concept of equilibrium levels (Cole et al., 1993). Inputs to the soil are strongly controlled by management practices that may dampen the effects of climate change. In contrast, decomposition rates are generally less manageable and more variable. In the tropics where decomposition is rapid, the steady-state SOM levels per unit of C input are lower. Equilibrium values for the tropics as a result of changes in temperature and precipitation are estimated in Cole et al. (1993). Examples for two tropical sites, Warra in Australia and Hat Yai in Thailand are given in Fig. 1. The site in Australia is strongly water-limited, such that an initial increase in temperature increases evapotranspiration, reducing the water availability and the decomposition rate, leading to initial increases in soil carbon levels. However, further warming leads to a decrease in equilibrium soil carbon. In Thailand, temperatures for decomposition are already near optimum; thus with a 5°C increase in temperature, decomposition rate decreases (Cole et al., 1993). A number of modelling exercises predict the impact of global change on soil properties. These studies concentrate mainly on agroecosystems and grasslands in temperate regions where detailed data of inputs and the various carbon pools are available (Buol et al., 1990; Cole et al., 1993). There are models available for the tropics where the changes in total soil carbon pools has been predicted Warra

Hat Yai

NPP= I I0g C.m :

1

NPP= 750gC.m-2

6

41

.4

3.9

.2

I0 37 0 I

5

10 I

5

"q'~/'~Co_4 "~10

,X~V >

Fig. 1. Potential soil C levels (0-20 cm) for two sites calculated from steady-state analysis of Century, based on climate, soil texture and C input data. Values represent C levels for 0-5°C increases in mean annual temperature (MAT) and -10% to + 10% change in mean annual precipitation (MAP), (Cole et al., 1993).

M.C. Scholes et al. / Geoderma 79 (1997) 25-47

33

for various landuses (Greenland et al., 1992). A study conducted for seven grassland and savanna sites in the tropics, using two different general circulation models and double CO 2 climates, showed that these ecosystems lost the least amount of soil carbon per unit area ranging from no change to 300 g C m - 2 . The Eurasian grasslands lost the greatest amount of soil C (approximately 1200 g C m -2) and in the other temperate grasslands losses ranged from 0-1000 g C m - 2 (Ojima et al., 1993). However, little information is available which links changed levels and types of inputs with specific soil carbon pools. The time is ripe for studies of this nature on tropical systems. An experiment carried out at the Rodale Institute Research Centre's long-term farming systems trial in Pennsylvania provides critical information on changes in different SOM pools with management. The purpose of this trial was to test if 10 years of organic or conventional management practices generated differences in biologically active soil organic matter pools. Soil CO 2 evolution, available inorganic pools and N mineralization rates, water dispersible organic carbon and particulate organic matter were measured. Results showed that soils receiving organic treatments accumulated biologically active C. Accumulated organic matter in the manure-amended soil was the most labile whereas the cover-cropped soil accumulated the most organic matter overall. The measurement of particulate organic matter was the best index of biologically active SOM because it was sensitive to both changes in quality (i.e. biological lability) and quantity of the soil organic matter (Wander et al., 1994). Studies of this nature and completeness are urgently required under tropical conditions. 3.1.1. Is microbial biomass a good indicator of change? Small, gradual changes in SOM are difficult to detect over short periods of time because of the high background and natural variability of soils. It is possible to detect changes in microbial biomass long before changes in the total organic C (Sparling, 1992; Sparling et al., 1994). The changes in the index defined by the microbial C:organic C ratio reflects inputs to the soils and the efficiency of conversion to microbial C. Monitoring of microbial C may be a useful indicator of SOM changes (Sparling, 1992; Sparling et al., 1994). It seems too simplistic to expect that the measurement of soil microbial biomass alone (or any other single soil property) can provide a universally applicable index of sustainability. However, biomass measurements have been shown to provide a sensitive indication of changes in total SOM content caused by changes in management or landuse in the tropics (Ayanaba et al., 1976), subtropics (Saffigna et al., 1989) and temperate environments (Powlson et al., 1987). 3.1.2. Changes in SOM pools as a result of input amounts There are many data sets which show a rapid decline in soil organic matter with the cultivation of previously untilled soils (Houghton, 1991). Data also

34

M.C. Scholes et al. / Geoderma 79 (1997) 2 5 - 4 7

show that soil organic carbon can be lower, the same as, or greater than mature tropical forests and that soil organic carbon can increase rapidly after the abandonment of agricultural fields (Brown et al., 1993). The type of landuse following the clearing is critical in determining the subsequent effects on SOM. It is estimated that a 40% reduction in soil C content takes place if cropping follows clearing but only a 20% reduction if pasture follows clearing. Large root biomass in pastures and a lack of tilling account for these differences. Recent evidence shows that the fractional loss of soil carbon is not positively correlated to the amount of carbon initially present in uncultivated soils (Davidson and Ackerman, 1993). The proportion of microbial C to organic C is consistently higher under pasture than native forest and arable cropping. Microbial C recovers more rapidly than organic C on returning to pasture following cropping (Sparling, 1992; Sparling et al., 1994). Soil microbial biomass was measured over one year in three adjacent systems in the central Amazon region: (1) virgin rainforest, (2) slashed and burned forest and (3) recently established pasture. The microbial biomass-C decreased upon slashing and burning to 64% of its original value (1287 ~g g-1) in the forest (0-5 cm layer) and increased after establishment of pasture to 1290 ~g g - ~, but remained unchanged in the 5-20 cm layer (Luizao et al., 1992). The introduction of deep-rooted pasture grasses in South American savannas has led to a marked increase in soil carbon in the profile especially at depth (40-100 cm) (Fisher et al., 1994). The use of ~ ~3C is a powerful and fairly widespread tool in determining the changes in pool sizes and turnover following clearing. Accurate estimates of bulk density, knowledge of previous landuse and sampling depth are critical in the calculation of these values (Veldkamp, 1994). Feller (1993) concluded from a number of tropical studies that plant residues play a major role in the 'active' fraction as seen from their turnover rates. Desjardins et al. (1994) showed that in two soils from the Eastern Amazon, one under native forest and another after ten years under pasture, had very similar total C contents, 31 vs. 29 t ha-J and 16 vs. 15 t ha J, in the 0-20 and 20-40 cm layers, respectively. 6 ~3C data showed that 46-49% of the total C in the 0-20 cm layer of the soil under pasture was derived from the pasture species. Particle size fractionation showed that the changes were restricted to the upper soil layers (0-10 cm). Distribution of the stable and labile C pools are shown in Fig. 2. The replacement of forest carbon by pasture carbon was greater in the sand-sized fraction (55-65% of the total C) than in the clay-sized fraction (34-45% of the total C). The passive pool ( > 5000 yr) of forest SOM was smaller than the labile pool and represented about 17 and 26% of total C in the 0-10 and 0-20 cm layers respectively. Similar results were obtained from a study in a West African savanna, where an area previously dominated by C4 grassland has, as a result of the exclusion of fire, been converted to a mixed savanna dominated by C3 woody plants.

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M.C. Scholes et al. / Geoderma 79 (1997) 25-47

Carbon T ha-1 (lOcm)-1 0.

0

5

10

15

20

25

5o

~~--~ 75 ~

[] Forest labile C pool [] Forest stable C pool

Fig. 2. Distribution of stable and labile C pools derived from forest, and of C derived from Pennisetum, in the soil after 10 years of pasture (Desjardins et al., 1994).

Although the total C content did not show significant changes, 52-70% of the original C4 carbon was turned over when vegetation cover was changed. Turnover of the coarse fraction ( > 250 p~m) was much greater (97%) than the mineralization (50%) of the clay associated fractions ( < 20 Ixm) (Martin et al., 1990). These two studies are important in that in one case a C3 system is tending towards a C4 system and the reverse in the other case. In both cases the total C pool sizes stay approximately the same and the coarse fraction is the one most severely affected. This has interesting implications and may give some clues as to the relative importance of physical protection versus chemical protection on carbon stabilization. In another study, using 6t3c, conducted in a semi-arid savanna in South Africa, there was a 51% turnover of clay-associated carbon in less than three decades when the savanna was cleared and cropped with maize. This is an extremely rapid turnover relative to northern hemisphere studies. The savanna trees and grasses contribute to the organic pool in approximate proportion to their primary production. Reductions of between 9-37% in soil organic matter occurred on sandy soils in Southern Africa when the natural vegetation was cleared and continuous maize cropping took place; declines were not as great on vertisols (Woomer, 1993). Data from a tropical forest in India showed a decline of 40-46% and 52-58% of soil organic C and microbial biomass C, respectively when the forest was converted into savanna and ploughed land (Basu and Behera, 1993). The implications of these studies are that changes in inputs may be more important on sandier soils because of the lower stability of these less protected soil carbon pools.

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M.C. Scholes et al. / Geoderma 79 (1997) 2 5 - 4 7

60 5O

40f\ 2O 10

...... •.................... ~ . . . . . . . . . .

0 0'"'"

-

I

I

I

I

I

5

10

15 Time (y)

20

25

SOC,

SOCp

30

TOC ,It

Fig. 3. Soil organic C changes with time in the top 0 to 0.1 m following forest clearing and conversion to pasture in Costa Rica. Markers are observations, lines are fitted curves. (SOCf: soil organic carbon, forest; SOCp: soil organic carbon, pasture; TOC: total organic carbon), (Veldkamp, 1994).

Bonde et al. (1992) showed that there was a rapid decline in soil C in the initial 12 years of cropping after clearing a tropical forest. D e c o m p o s i t i o n in the sand and silt fractions was fastest. The following 38 year cropping period resulted in a m u c h slower decline in forest-derived carbon. Data f r o m a study conducted in Costa Rica are presented in Fig. 3. Soil organic carbon was sampled f r o m a deforestation sequence on an Andic Humitropept. Total soil carbon rapidly decreased after clearing mainly due to litter and root decomposition. The amount of carbon in the forest soils stabilized after five years at 12 M g h a - 1 for the 0 - 1 0 c m layer. O f the soil organic carbon originally present (26.9 M g h a - J for the 0 - 1 0 c m layer) about 45% consists o f passive soil organic carbon and the rest of d e c o m p o s a b l e organic carbon. Pasture derived carbon

Table 2 Average inputs of grass (C4) carbon and losses of rainforest (C3) carbon from the > 1.6 Mg m -3 soil fraction after 35 and 83 years Depth (cm) 0.0-7.5 7.5-15.0 60.0- 80.0

Inputs of C4 (mg C g- z yr-

1)

Losses of C3 (mg C g- 1 yr- J)

35 yr

83 yr

35 yr

83 yr

0.769 0.263 0.026

0.733 0.449 0.033

0.906 0.417 0.014

0.543 0.337 0.021

From Skjemstad et al. (1990).

M.C. Scholes et al./ Geoderma 79 (1997) 25-47

37

starts accumulating after three years and after 30 years the total organic carbon stabilizes at about 24 Mg ha-1 which is about one-half the original value (Veldkamp, 1994). In Northern N.S.W., Australia, the change in vegetation cover from a C3 lowland subtropical rainforest to C4 pastures was used to follow input rates and turnover of organic matter over an 83 year period (Fig. 4, Table 2). The area between the curves represents the soil organic matter derived from the grass communities. Table 2 estimates average yearly inputs and losses. Loss of C3 carbon was most rapid in the first 35 years suggesting disappearance of the more labile fraction; inputs from the C4 source roughly balanced losses; subsequent loss of C3-derived carbon from more recalcitrant pools was slower, but sustained input rates of C4-derived C led to an overall increase in C content. With time this rate decreased leaving only the more recalcitrant protected organic matter pools rich in old carbon. The stabilization of SOM in microaggregates was found to increase the turnover times in the high density ( > 1.6 Mg m -3) fraction from 60 to 75, 75 to 108 and 276 to 348 years in the 0-7.5, 7.5-15 and 60-80 cm depths respectively. Charcoal was a major constituent and reduced the value of the light ( < 1.6 Mg m -3) fraction in determining the natural abundance of the input carbon at any time (Skjemstad et al., 1990). Mineralogy also affects the stabilization of carbon. Soils having a high level of amorphous components and Al-organic matter complexes show less carbon loss on cultivation. Much higher losses were found on an Oxic Humitropept in Costa Rica than on an Andic Humitropept (Veldkamp, 1994).

3.2. Input quality and soil organic matter A number of experiments carried out under tropical conditions show that after clearing, the decline in soil C was less when mulches were returned to the soil.

0-7.

7.5-15 cm

~ 4

4

2

1

2

[ I

0

60-80 cm

1 ~g:3~l I ....

c3 I

I

20

I

I

40

I

I

60

I

I

i

80 0

I

I

I

I

I

40

i

71"7

{42:0)[ /

~3,

20

~

I

60

I

I

80 0

/

C3 I

/

20

I

I

I

40

60

I

80

Time (years) Fig. 4. Changes in relative contribution of C3 and C4 carbon with time in the > 1.6 Mg m -3 fraction from 0-7.5, 7.5-15, and 60-80 cm. Numerals represent the percentage of C3 carbon present. Numerals in parentheses represent C3 carbon expressed as a percentage of the original carbon content (Skjemstad et al., 1990).

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M.C. Scholes et al. / Geoderma 79 (1997) 2 5 - 4 7

Addition of sorghum residue to an Australian vertisol increased the total soil carbon pool in the 0-10 cm layer by 8% even though the respiration was increased by 45%. Residue retention caused larger percentage increases in microbial biomass C than in total organic carbon (Ayanaba et al., 1976; Saffigna et al., 1989). Additions of Siam weed (Chromolaena odorata) to cropping systems on Alfisols in Nigeria's tropical zone increased the soil organic matter levels from 3.7 to 4.5% after eight months as well as increasing the proportions of humic acids over fulvic acids (Obatolu and Agboola, 1993). In an experiment carried out on dry tropical arable land in India microbial biomass levels were higher under a lentil crop than a rice crop. This was attributed to the increased root biomass of lentils providing a direct input of organic matter and nutrients through exudation and mortality. The microbial biomass levels were well correlated with crop biomass and grain yield indicating that the microbial biomass was contributing significantly to crop production by enhancing mineralization and releasing nutrients. There was a synergistic effect with the addition of NPK fertilizers (Goyal et al., 1992; Srivastava and Lal, 1994). The addition of leguminous litter from alley-cropping species led to enhanced total soil C and microbial carbon levels in continuous cropping systems in Costa Rica (Mazzarino et al., 1993). In a Sri Lankan experiment, the addition of residues with a high C : N ratio (rice husks) increased the percentage of C over time. The addition of Leucaena leucocephala over six years contributed substantially to the inorganic nitrogen content of the soil and plants but did not increase the soil organic matter content or the microbial biomass C significantly (van Holm, 1993). How litter quality changes will affect the various soil carbon pools remains uncertain; for the active pool, it seems that input amount is more important that quality (Norby et al., 1986; Couteaux et al., 1992; van Breemen and van Dam, 1993). There are well established relationships between litter decomposition rates and resource quality. Plant materials high in C : N ratio, lignin and polyphenols break down more slowly (Tian et al., 1992a; Tian et al., 1992b). Increases in these levels are expected to decrease the decomposition rates of organic inputs, and to increase the fraction of plant carbon stored. Palm and Sanchez (1990) showed that polyphenol levels in tropical litters appeared to influence rates of decomposition more than percent nitrogen or percent lignin. Constantinides and Fownes (1994) and Oglesby and Fownes (1992) presented information on the decomposition rate of a number of tropical litters and green manures. They showed that nitrogen accumulation was most strongly correlated with initial N concentration in the litter material whereas soluble polyphenols were the controlling factors in green material. There is no clear trend in the change in the stabilized soil organic matter following change in the quality of plant materials under elevated CO 2. In general, organic inputs high in polyphenols led to a more rapid formation of stable forms of soil organic matter than those low in polyphenols (Stott et al.,

M.C. Scholes et al./ Geoderma 79 (1997) 25-47

39

1983; Kelly and Stevenson, 1987). Azhar et al. (1986) found that phenolic compounds bound mineralized nitrogen in the nitro and nitroso-forms in soil humus. This could lead to formation of stabilized carbon at depth due to the presence of leached phenolics. Although the effect of change in C : N ratio and lignin content on the formation of the physically protected SOM fraction is yet to be characterized, it can be expected that higher C : N ratio and lignin plant materials entering the decomposer system under the elevated CO 2 may have a high potential for the development of stabilized SOM pools as they decompose slowly.

4. Decomposition models and their role in assessing the impact of global change on soil organic matter Mathematical models of SOM turnover can be used to explore the possible impacts of various changes. Any projections must be regarded as a guide as they will reflect the structure and assumptions built into the model. Several models for SOM turnover are now available and reasonably well validated, though much less so for tropical situations than temperate. Century (Parton et al., 1987) is the most widely used and has been tested in the widest range of situations. The Rothamsted Carbon Model (Jenkinson and Rayner, 1977; Jenkinson et al., 1987; Jenkinson, 1990) has the merit of being simple in structure. Both models require the following data. Soil type (e.g. clay a n d / o r silt content) as a factor controlling decomposition rate and distribution of C between pools. Meteorological data, to give soil temperature and moisture. The degree of detail required (e.g. daily, monthly) varies and may be changed depending on the duration of simulations. Quantity of organic C inputs - - often estimated from actual or expected aboveground production. Decomposability of inputs - - see below. Timing of inputs, e.g. spread evenly through the year or in a large pulse during plant senescence or after harvest. Before using a model to predict changes in SOM following a change in the quantity or composition of input, the model should first be validated for the appropriate soil type and climatic conditions. Ideally this is done by testing its success in simulating past changes as measured in long-term experiments. In practice such data are rarely available for tropical regions, so it will often be necessary to make projections without a robust validation of the model for the particular environment. Some degree of validation can be achieved by making

40

M.C. Scholes et al. / Geoderma 79 (1997) 2 5 - 4 7

contemporary measurements of total soil carbon and some individual pools, such as microbial biomass. If the history of the site is known and reasonable estimates of past input quantity and composition can be made then the model can be run and simulated values compared to the measured. To simulate the impact on SOM of changes in climate or landuse, the following assumptions must be made: Changes in soil moisture and temperature - - presumably defined by the chosen scenario. Changes in quantity of C input. Changes in decomposability of C input. Changes in timing and placement of C input. Soil fauna. Changes in the quantity of C input depend on assumptions regarding ' C O 2 fertilization' among other things, e.g. changes in the vegetation type, management practices and nutrient inputs. How much will NPP for the vegetation type be changed by enhanced atmospheric CO 2 concentration? Will the deposition of organic matter below- and aboveground be altered? Will nutrient availability modify NPP and hence inputs? Thus, there is a need for data from enhanced CO 2 experiments that are done under realistic conditions relevant to tropical soils and environments. Changes in the decomposition rate of C inputs (both litter and belowground) are difficult to predict but have the potential of being an important influence on SOM accumulation and turnover. Lack of knowledge on this point may well be a limitation to the use of models to make reliable prediction of changes in SOM under climate change. However, the evidence and theory suggest that changes in C input rates and plant partitioning are likely to be much more important areas of uncertainty. In the Rothamsted Carbon Model incoming plant material is divided into two fractions: decomposable plant material (DPM) and resistant plant material (RPM). Both decompose to CO 2, biomass and humus but with different rate constants: 0.3 and 10 yr J, for RPM and DPM, respectively. An example of the use of this model is given in Fig. 5A,B where input amounts have changed under different climates. Soil organic C is predicted to fall from 43 to 28.5 t ha-1 in 26 years with continuous inputs of 2.0 t ha 1 instead of ll.0 t ha -j. These levels correspond approximately to humid tropical conditions changing from forest cover to arable crop production. For drier savanna conditions, a fall from 18 to 16 t ha ~ in 24 years is predicted. In Century inputs are partitioned into 'structural' and 'metabolic' components on the basis of lignin:N ratio, structural material having the higher lignin content. Decaying metabolic and non-lignin structural material is transferred to the active SOM pool (which contains the microbial biomass), while the lignin fraction does not pass through the microbial pool, but is assumed to decompose

M.C. S c h o l e s e t a l . / G e o d e r m a 79 (1997) 2 5 - 4 7

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directly into the slow organic carbon pool. The turnover time of the structural pool is 1-5 years with the decay rate being a function of lignin content. The metabolic pool has a turnover time of 0.1 to 1 year. An example of the application of the Century model is given in Fig. 6 where the importance of photosynthetic pathways and management practices on the foliage productivity and soil carbon characteristics of a semihumid temperate grassland are assessed. Photosynthetic pathway, precipitation and temperature were the variables which affected foliage production most, whereas management practices had the greatest influence on soil carbon storage (Seastedt et al., 1994). Whatever model is chosen it will be essential to know the influence of enhanced CO 2 on input composition. At present there is not a clear consensus on this. Another possible complication is that a change in input composition may alter the microbial community structure which, in turn, may alter the decomposition pattern (rate or distribution of metabolites). A change in community structure in the rhizosphere, as a result of changes in the composition of

M.C. Scholes et al. / Geoderma 79 (1997) 25-4 7

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exudates, may influence the c o m p o s i t i o n o f the C reaching the microbial population in the bulk soil. H o w e v e r , it is probably sensible to assume that these more subtle effects are negligible until evidence indicates otherwise.

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43

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