Management control of soil organic matter dynamics in tropical land-use systems

GEODE~ ELSEVIER

Geoderma 79 (1997) 49-67

Management control of soil organic matter dynamics in tropical land-use systems Erick C.M. Fernandes a,*, Peter P. Motavalli b, Carlos Castilla c, Linus Mukurumbira d a The Department of Soil, Crop and Atmospheric Science, Cornell University, 622 Bradfield Hall, Ithaca, NY 14850, USA b Agricultural Experiment Station, University of Guam, Mangilao, Guam 96923, USA c The International Council for Research in Agroforestry (ICRAF), c / o EMBRAPA CPAF/RO, C.P. 406, Porto Velho, 78900 RO, Brazil d Soil Productivity Research Laboratory, DRSS, Private Bag 3757, Marondera, Zimbabwe Revised 4 December 1996; accepted 9 April 1997

Abstract

Given the rapid conversion of tropical forests to crop and pasture land, the economic constraints to widespread fertilizer use, and the potentially negative ecological impacts of fertilizer misuse, there is an urgent need to improve the management of organic inputs and soil organic matter (SOM) dynamics in tropical land-use systems. One desirable goal is the ability to be able to manipulate SOM dynamics via management practices so as to promote soil conservation, to ensure the sustainable productivity of agroecosystems, and to increase the capacity of tropical soils to act as a sink for, rather than a source of, atmospheric carbon. Soil organic matter dynamics are affected by management activities such as: (1) manipulation of the soil environment via tillage, mulching, and application of organic or inorganic fertilizers; (2) varying not only the quantity and quality of organic inputs, but also the placement and timing of application; and (3) manipulation of soil fauna. Although simulation models based on ecosystem concepts, such as Century, offer a dynamic conceptual framework to examine the effects of long-term management, the predictable management of SOM dynamics in tropical agroecosystems is constrained by the lack of appropriate methodologies to isolate SOM pools that are responsive to management. © 1997 Elsevier Science B.V.

Keywords: organic inputs; CENTURY; organic fractions; soil macro fauna; agroforestry; mulching

* Corresponding author. Tel.: ecf3 @cornell.edu

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607 255-1712; Fax:

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1. Introduction As a renewable resource, soil organic matter (SOM) is both a source and a sink of plant nutrients (Duxbury et al., 1989); it is an ion exchange material; it promotes the formation of soil aggregates and thereby influences soil physical properties and soil moisture; and it is an energy substrate for soil microbes and macrofauna (Allison, 1973). In temperate and some tropical soils, tillage, irrigation, and chemical fertilizers have been used to complement and even enhance the functions of SOM (Sanchez and Miller, 1986). The capital intensive nature and negative environmental impacts of fertilizer and pesticide abuse, however, dictate an urgent need for improved management of SOM for sustaining tropical land-use systems. Besides the impacts of SOM on soil productivity, the accelerated decomposition of SOM due to agriculture and the resulting loss of carbon to the atmosphere and its contribution to the greenhouse effect is a serious global problem. For example, in the early 1980s, land-use changes were estimated to have resulted in the transfer of between 1 and 2 × l015 g C yr -1 from terrestrial ecosystems to the atmosphere. Between 15% and 17% of this C came from the decomposition of SOM (Houghton et al., 1991; Houghton and Hackler, 1994). The level of SOM in any agroecosystem at a given time is determined by the interaction among the factors which determine its formation and those which promote its breakdown (Nye and Greenland, 1964). The prevailing hypothesis predicts a decline in SOM content following the conversion of natural forests to plantations, agriculture or pasture (Lundgren, 1978; Detwiler, 1986; Houghton et al., 1991). The reduction in SOM content can be due to increased soil erosion, faster SOM mineralization and oxidation of soil organic carbon (SOC), smaller quantities of organic inputs, a n d / o r more easily decomposed organic inputs in managed systems as compared to natural forests. In some managed systems, however, increases in SOM contents have occurred due to improvements in plant productivity and the consequent increases in additions of above- and below-ground organic inputs to the soil (Sanchez et al., 1982; Lugo and Brown, 1993). One desirable goal, is to be able to manipulate via management practices, those factors that regulate the synthesis and decomposition of SOM, so as to contribute to the sustainable productivity of agroecosystems. It is important to note, however, that SOM is only one of several renewable resources (water, nutrients, seed, labor) which are essential to sustaining productivity at the agroecosystem level.

2. Land-use effects on native SOM dynamics The conversion of natural forest to an agroecosystem, results in profound changes in the biological and chemical processes at the plant-soil interface. The

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net result is an initial decline in SOM. The magnitude and duration of the decrease in SOM, however, depends upon the method of forest conversion, the intensity of subsequent land use, the climate, and the soil physical and chemical properties (Lugo et al., 1986). Subsistence cropping and pasture establishment are probably the leading causes of deforestation in the tropics, where shifting cultivation is still considered to be the dominant form of agriculture. The practice involves the clearing of forests, cropping for 1 to 3 years, and then abandoning the land and allowing it to recover during a period of fallow. The length of the fallow varies across the tropics and depends on the soil and vegetation type, climate, and even cultural conditions (Snedaker and Gamble, 1969; Turner et al., 1977). In general, fallow periods on fertile soils tend to be considerably shorter (2 to 15 years) than for acid, infertile soils (10 to 25 years). Estimates of the average loss of SOC in the top 1 m during the cropping cycle in shifting cultivation vary from 15% to 40% (Nye and Greenland, 1964; Sanchez et al., 1982; Detwiler, 1986; Schlesinger, 1986; Davidson and Ackerman, 1993). When the crop lands are abandoned, the fallow vegetation generally results in an increase in soil carbon, although the recovery of SOM to levels approaching that of the forest soil may take as long as 35 years (Detwiler, 1986). The clearing of forests for planted pastures has been reported to result in a lower loss of SOC than when soils are cultivated (Houghton et al., 1991). In some cases, pasture soils had similar SOC contents (Buschbacher, 1984), or more SOC in the top 50 cm than soils of adjacent mature forests (Lugo et al., 1986). There is evidence that aggressively growing pasture grasses can compensate for the loss of carbon in recently deforested soils. Half of the SOC in an 8-year-old pasture in Brazil was shown to have originated from the decay of pasture grass litter (Cerri et al., 1991). The below-ground C contribution of pastures is likely to be closely linked to management and grazing pressure. In a properly grazed pressure, Castilla (1992) observed almost no accumulation of aboveground litter and a low contribution of roots to SOM. The accumulation of SOM under 20-year-old forest plantations was found to be less than that for pastures, even though total C accumulation in biomass was greater in the plantations (Lugo and Brown, 1993). The establishment of forest plantations on degraded soils, the lower litter production, and the lower root turnover in trees relative to grazed pasture grasses, may explain the low SOM accumulation rates. In agroforestry systems involving shade/timber trees and perennial crops (Alpizar et al., 1988) and alley cropping with fast-growing leguminous trees and food crops (Fernandes et al., 1993a), high rates of shoot and root litter production, and additions of prunings, result in rapid recovery of SOM contents to pre-deforestation levels. While the removal of forest vegetation and declining soil productivity are often correlated with declining SOM contents, high SOM contents do not

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necessarily mean high soil productivity (Sanchez and Miller, 1986). The lack of consistent and predictable correlations between soil productivity and SOM contents, has led to the hunt for management-sensitive and agronomically meaningful SOM pools based on different turnover times in the soil and their association with soil particles (Jenkinson and Rayner, 1977; Parton et al., 1987). It is assumed that: - several SOM fractions exist, each of which responds in a specific way to management and land-use practices, - the synthesis and decomposition of these fractions are regulated by physical, chemical and biological factors that can be altered by management.

3. Predictable responses of S O M dynamics to m a n a g e m e n t

The fundamental hypothesis of this paper is that SOM can be predictively controlled by management of the factors that determine the quantity and quality of organic inputs to the soil, the synthesis of SOM from these organic inputs, and the decomposition of SOM (Ingrain and Swift, 1988). Perhaps the greatest scope for managing SOM dynamics lies with the production and use of organic inputs to achieve desirable levels of the different SOM fractions in the soil. Organic inputs are commonly used in many tropical land-use systems throughout the world (Gaur et al., 1984; Garrity and Flinn, 1988; Stromgaard, 1991). Benefits to soil productivity from application of organic inputs include improvements in soil biological (Sangina et al., 1992), chemical (Bell and Bessho, 1993; Lungu et al., 1993), and physical characteristics (Inoue, 1991; Geiger et al., 1992). A conceptual model for the role of organic inputs in soil organic matter dynamics is provided by the Century SOM model which proposes the existence of several soil organic matter pools (active, slow, and passive) with different turnover times (Parton et al., 1987). Decomposition rates of the active, slow and passive soil organic matter pools affect short- and long-term soil productivity since active soil organic matter is composed of readily decomposed forms and slow and passive pools are more stable forms of soil organic matter. The difference in decomposition rates among the pools may account for nutrient release patterns over time and residual effects of organic additions. Management factors affecting soil organic matter transformations in the Century model include the quantity and quality (i.e., the lignin-to-N ratio) of the organic matter added, placement of the organic input (surface vs. incorporation), fertilizer addition, removal of organic materials and tillage (Parton et al., 1987, 1989). In this paper, we use the term 'pool' to refer to the conceptual categories of SOM in the models, and the term 'fraction' to refer to the SOM category that can be directly measured by physical or chemical fractionation.

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The processes which govern SOM dynamics include those that promote the synthesis of SOM from organic inputs and those that decrease SOM via decomposition. The management of SOM dynamics in different land-use systems, can be achieved via manipulations of organic inputs and/or the factors that influence the transformation of these inputs into the various SOM fractions (Sanchez et al., 1989). For example, Swift et al. (1991) list the following ways in which SOM dynamics may be influenced by management: changing the soil environment by tillage, irrigation, mulching, fertilization, and liming influences the biological processes of decomposition and mineralization, manipulating the quantity and quality of organic inputs by changing the amount and diversity of litters, residues, or plant biomass inputs, modifying the timing, and placement of organic inputs as a means of influencing synchrony between release of nutrients from SOM and plant demand, manipulating soil fauna (agents of decomposition) via tillage, surface mulches, or the use of pesticides. -

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-

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3.1. Changing the soil environment by mulching, tillage, and fertilization Placement of organic inputs affects the fate of organic inputs in soil and its effects on the soil environment. Mulching and minimum tillage decreases soil erosion, soil crusting, the rate of surface evaporation and soil surface temperatures by maintaining organic inputs on the soil surface (Wade and Sanchez, 1983; Inoue, 1991; Prasad and Power, 1991; Ball-Coelho et al., 1993; Nill and Nill, 1993). In Niger, Geiger et al. (1992) also observed that surface mulch entrapped more fertile wind-blown soil and reduced wind erosion which helped to increase millet yields. In addition, surface mulches can reduce weed growth perhaps as a function of the soil surface area covered (Wade and Sanchez, 1983; Fernandes et al., 1993a), although Mt. Pleasant et al. (1992) found no consistent effect of mulching on weed suppression. In comparison to mulching or minimum tillage, incorporation of organic inputs results in increased rates of decomposition and greater short-term nutrient availability (Holland and Coleman, 1987). Accumulation of soil organic matter under mulching or minimum tillage is restricted to the surface while incorporation of organic inputs may affect soil organic matter levels and other soil properties to a greater depth increment (Davelouis, 1990; Prasad and Power, 1991). For example, Wade and Sanchez (1983) observed greater decreases in soil acidity when kudzu was incorporated versus when it was mulched. Management decisions on placement of organic inputs are frequently based on selection of sites which offer higher potential returns for investment of labor and limited nutrient supply. Examples of such sites are home-gardens which,

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due to their proximity to the household and their importance as a food source, often receive higher organic inputs. In the semiarid region of India, cash crops and irrigated sites frequently receive higher organic inputs than other crops and rain-fed areas (Motavalli et al., 1994a). Concentration of organic inputs on a small land area is, therefore, one strategy to influence soil organic dynamics on a manageable scale. The combined use of both fertilizers and organic inputs has been recommended as a means of maintaining high crop productivity and stable yields (Nambiar and Abrol, 1989). A need for supplementing organic wastes with fertilizers may occur when organic inputs with a high C : N ratio are applied which have a high risk of short-term N immobilization. For example, application of N fertilizer with several high C : N farm wastes in India decreased N immobilization in soils grown to sugar cane and improved sugar cane yields (Yadav and Prasad, 1992). Kang (1993) obtained higher maize yields over a 10-year period in southern Nigeria by the combination of adding fertilizers and retaining crop residues. Nutrients contained in the levels of organic inputs available for land application may not be adequate for crop growth or may not be available for crop uptake of annual crops at critical growth stages. For example, the success of crops in semiarid regions depends on rapid crop growth with initial rains. If sufficient nutrients are not present at that critical time then crops have a greater risk of failure. Combining fertilizer with organic inputs may be a management alternative in regions where fertilizer is available and affordable, where risk of crop failure is low, and where information on proper fertilizer use is available (Babu et al., 1991).

3.2. Manipulating the quality and quantiO' of organic inputs Organic inputs into soils of the tropics, comprise a wide range of materials including crop residues (above- and below-ground), green manures, weeds, prunings, and animal, household and agroindustrial wastes. As an example of this diversity, a partial list of organic inputs in use in India is presented in Table 1. Such materials have varying chemical compositions, and depending on handling and processing, different physical forms. Nutrient composition, particularly N content, has been a major selection criterion for organic inputs in situations where organic inputs are relied upon as nutrient sources. Proposed indices for predicting the decomposition rate and effects of such materials on net N mineralization or immobilization include the C to N ratio, lignin to N ratio, or polyphenol to N ratio (Melillo et al., 1982; Palm and Sanchez, 1991; Oglesby and Fownes, 1992; Thomas and Asakawa, 1993). As the lignin or polyphenolic content to N ratio of the plant material increases, the decomposition rate of the material decreases and short-term N availability may decrease. We hypothesize that high lignin-to-N materials will eventually increase the size of the slow and

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Table 1 Selected organic inputs applied in India and their chemical composition (% wet weight basis) (adapted from Gaur et al., 1984) Organic input

N

P205

K20

Animal and human refuse Cattle dung (fresh) Composted cow manure Sheep dung (fresh) Poultry litter Town compost

0.3-0.4 0.5-0.6 1.0-1.6 3.0 1.5-2.0

0.1-0.15 0.15-0.20 0.8-1.2 2.6 1.0

15-0.20 0.5-0.6 0.2-0.6 1.4 1.5

Crop residues Sorghum straw Groundnut straw Sugarcane bagasse

0.4 1.6-1.8 0.25

2.0 0.3-0.5 0.12

2.0 1.1 - 1.7 -

Green manures Sunnhemp Sesbania Cowpea

0.76 0.62 0.71

0.12 0.15

0.51 0.58

Agroindustrial wastes Castor cake Groundnut cake Rape-seed cake Fish meal Cotton-mill dust

5.5-5.8 7.8 5.1 4-10 1.5-2.0

1.8 1.5-1.9 1.8 3-9 0.5-0.6

1.0 1.4 1.0 0.3-1.5 0.6-0.8

passive soil organic matter pools and thereby promote long-term soil productivity. Plant materials from legume species are commonly preferred as organic inputs compared to non-legume species when increased nutrient availability is the principal management objective. This is because legume materials generally have higher N content and lower C ratios than non-legume materials, thereby promoting rapid N mineralization when applied to soil. Benefits to succeeding crops from incorporation of legume residues or green manures (both above- and below-ground materials) have been widely reported (Reddy et al., 1986; Sisworo et al., 1990; Sidhu and Sur, 1993; Thomas and Asakawa, 1993). In general, leaf litter quality is higher and decomposition faster on fertile soils than on acid, infertile soils (Vitousek and Sanford, 1986). Although the aboveground C input has traditionally been viewed as the more important source, there is good evidence that the transfer of C via the rhizosphere is just as important in the formation of SOM (Ladd and Martin, 1983; Martin and Merckx, 1993). Relatively little is known about the quality of root litter, which may differ significantly among species in the proportions of fine to coarse roots, the amount of

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Table 2 Percent C, N, and lignin in shoots and roots of a grass and legume improved pasture Tissue

Low precipitation

High precipitation

C

N

lignin

C

N

lignin

Leaves Brachiaria humidicola Desmodium ot:ali[olium

47.20 48.47

1.42 3.01

4.48 I 1.04

45.43 47.94

1.78 2.66

3.89 10.94

Fine roots Brachiaria humidicola Desmodium ol'alifblium

47.77 48.22

0.78 2.26

11.19 17.70

46.28 47.70

0.82 2.20

10.94 18.95

Brachiaria humidicola Desmodium ocali[blium

47.19 45.61

0.63 0.81

10.28 16.12

47.15 47.25

0.40 1.15

10.96 15.45

Bulk roots (n

46.46

0.46

11.93

Coarse roots

36)

Source: Castilla (1992).

lignins and polyphenols present (Castilla, 1992), allelopathic compounds, and nutrient concentrations (Table 2). Conventional indices of quality (Melillo et al., 1982), may therefore not be good predictors of tree root decomposition. Tropical agroforestry systems are characterized by the presence of a variety of tree and shrub species in addition to common annual and perennial crops. Many of these trees and shrubs are managed to provide large quantities of biomass for use as mulches or green manures (Table 3). There is considerable scope for selecting among and within species for varying chemical compositions and decomposition rates of the biomass, for use as mulch or green manure (Fernandes et al., 1993b). This offers a means of manipulating nutrient release from biomass, soil microbial activity, and the rate of conversion of these inputs to SOM. Management activities such as increasing the frequency and intensity of shoot pruning or grazing, result in reductions in shoot regrowth (Duguma et al., 1988; Castilla, 1992). The greater the intensity of shoot pruning, the greater the reductions in fine root biomass, total root biomass, and numbers of active nodules (Fernandes, 1990). Differences in soil chemical effects of legume versus non-legume organic inputs have also been observed for soil properties such as pH, exchangeable A1, A1 saturation and anion adsorption properties (Wade and Sanchez, 1983; Bell and Bessho, 1993; Motavalli et al., 1993). In general, increased soil pH and decreased exchangeable A1 and anion adsorption that has been observed after addition of large amounts of legume materials has been a short-term effect, possibly due to rapid decomposition of legume materials compared to non-legume materials,

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Table 3 Biomass production by tree species in alley cropping on fertile and infertile soils in the humid, sub-humid, and semiarid zones of the tropics (Femandes et al,, 1993b) Species

Trees (No./ha)

Tree age (Mo.)

Prunings (No./yr)

Dry (mg ha-I y r - 1)

Hyderabad, India; rainfall 750 mm/yr; Alfisol, pH 7.0; P = 8 ppm (Olsen) Leucaena leucocephala 2,000 48 n.d. 833 48 n.d.

01.4 1 a 07.4 lw

Ibadan, SW Nigeria; rainfall 1280 mm/yr; Alfisol, pH 6.2, P = 25 ppm Leucaena leucocephala 10,000 36 6

06.5 1

Yurimaguas, Peru; rainfall 2200 mm/yr; Ultisol, pH 4.2-4.6, P = 8 ppm (Olsen) Inga edulis 8,888 11 3 09.6 lw Gliricidia sepium 14/84 5,000 11 3 08.1 lw Gliricidia sepium 34/85 5,000 11 3 01.8 lw Onne, SE Nigeria; rainfall 2400 mm/yr; Ultisol, pH 4.0, P = 50 ppm (Bray-l) Acioa barteri 2,500 48 n.d. Alchornea cordifolia 2,500 48 n.d. Cassia siamea 2,500 48 n.d. Gmelina arborea 2,500 48 n.d.

13.8 lw 14.9 lw 12.2 lw 12.3 lw

Sumatra, rainfall 2575 mm/yr; Oxisol, pH 4.1, P = 4.8-6.8 m g / k g (Melich I) Paraserianthesfalcataria 19,900 09 4 21 4 Calliandra calothyrsus 19,900 09 4 21 4 Gliricidia sepium 10,000 09 4 21 4

04.9 lw 09.7 lw 06.8 lw 10.7 lw 00.6 lw 01.4 lw

Costa Rica, rainfall 2640 mm/yr; Inceptisol, pH 4.3-4.8, P = 8-15 ppm (Olsen) Gliricidia sepium 6,666 24 2 09.6 lw 60 2 15.2 lw Erythrina poeppigiana 555 24 2 07.4 lw 60 2 11.1 lw Western Samoa, rainfall 3000 mm/yr; moderately fertile Inceptisol, no soil data Calliandra calothyrsus 5,000 48 3 3,333 48 3 Glirieidia sepium 5,000 48 3 3,333 48 3

12.1 07.6 10.7 06.5

lw lw lw lw

a l = leaves and green shoots, lw = leaves, green shoots and woody material.

T h e p h y s i c a l f o r m o f the o r g a n i c i n p u t c a n h a v e a s i g n i f i c a n t e f f e c t o n b i o l o g i c a l d e c o m p o s i t i o n ( S w i f t et al., 1979). F o r e x a m p l e , l a r g e r - s i z e d c r o p r e s i d u e f r a g m e n t s g e n e r a l l y h a v e a s l o w e r rate o f d e c o m p o s i t i o n t h a n c h e m i c a l l y

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, mechanically or biologically processed wastes such as farmyard manure or distillery sludge. Further research is needed in this area to determine the interaction between organic input quality and physical form on decomposition processes. Considerable research has been done on the use of animal manures as organic amendments in tropical soils (Sanchez and Miller, 1986; Motavalli et al., 1994a). Besides being a source of nutrients, animal manure has also been used to reduce soil acidity (Lungu et al., 1993). The chemical composition of animal manures is influenced by diet, storage, and subsequent handling. Manures that are applied to the soil in dried, aerobically decomposed forms, generally have a low nitrogen content (Mugwira and Mukurumbira, 1984). During aerobic decomposition, organic materials of high stability and low inorganic N are formed (Kirchmann, 1985). Anaerobic decomposition, however, results in compounds of low molecular weight such as volatile fatty acids (Guenzi and Beard, 1981) and high concentrations of NH4-N. As a result, the dynamics of C and N mineralization of organic manures can be greatly influenced by management factors such as storage, handling, and application.

3.3. Modifying the timing, and placement of organic inputs The release of organic nutrients to coincide with plant growth demand or synchrony (Anderson and Ingram, 1993) can be affected by several management factors, including timing of application, the quality and quantity of organic inputs and method of placement. The different patterns of nutrient uptake demand and root development among crops are also important considerations in selecting management practices to increase nutrient availability and uptake from organic inputs. Several studies have observed different patterns of N release among a wide range of litter material or green manures (Palm and Sanchez, 1991; Oglesby and Fownes, 1992; Thomas and Asakawa, 1993). Initial recovery of organic N applied in organic inputs by a succeeding crop under tropical conditions can be low, amounting to between 10 to 28% (Sisworo et al., 1990; Haggar et al., 1993; Sidhu and Sur, 1993). The long-term effects of organic inputs on soil productivity are critical components of sustainable management strategies for agroecosystems. Residual effects of organic inputs may be a result of physically and chemically stabilized organic residues in the soil and may form the slow and passive soil organic matter pools in the Century model (Parton et al., 1987). For example, Sisworo et al. (1990) and Sidhu and Sur (1993) also observed residual effects of applied organic inputs on crop uptake up to the sixth crop after application. A build up or maintenance of soil organic matter levels over time has often been associated with increased soil productivity. Haggar et al. (1993) concluded that long-term maintenance of nutrient availability in an alley cropping system compared to a mono-cropping system may be more important for soil productivity than the

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synchrony of nutrient release from tree mulch to a succeeding crop since crop recovery of mulch N is low. From a practical standpoint, farmers may need to use some chemical fertilizer until native SOM pools are able to supply adequate quantities of nutrients to satisfy crop demand. 3.4. Manipulating soil f a u n a

Soil organisms have evolved to cope with three major constraints: (1) to move in a compacted and dense environment, (2) to feed on low-quality resources; and (3) to tolerate the drying or flooding of the porous space (Lavelle, 1988). The decomposition of organic residues is closely related to microbial activities. Although soil microbial communities are both numerous and diverse, their activities are generally limited to micro-sites due to their inability to move in the compact soil. They depend on roots and soil fauna to have access to new feeding substrates (Lavelle and Spain, 1994). Although there are many types of soil invertebrates (microfauna, mesofauna, and macrofauna), it is the earthworms and termites of the macrofauna group that are the more readily accessible and potentially manageable 'ecosystem engineers' (Lavelle, 1988). Earthworms and termites not only ingest large amounts of litter and soil, but also actively move around in the soil and thereby play a major regulatory role in the dynamics of litter and SOM. Earthworms and termites are able to efficiently digest woody material and low quality organic residues. With earthworms, the addition of mucus to the soil serves as a priming effect on soil organic carbon by greatly increasing soil microbial activity (Martin et al., 1987). There is evidence that the enzymes, cellulase and mannase (important for the digestion of root materials), are actually produced by microorganisms in the gut lumen of the earthworms (Zhang et al., 1993). Termites are helped by the enzymes produced by their rich gut microflora to digest Cole et al., 1987). Lavelle (1994) has described the scale effects of earthworms and termites on SOM dynamics. The passage of soil and SOM through the gut of an earthworm strongly increases SOM mineralization. Over a few hours, earthworm digestion can result in the fragmentation of organic debris and the release of significant amounts of mineral nitrogen and phosphorus (Barois et al., 1987). This mineralization may continue for several hours in freshly deposited casts due to enhanced microbial biomass. Lopez-Hernandez et al. (1993) reported that part of the P that is normally absorbed in soil, may be desorbed after transit through earthworm guts. The medium-term effect (days to months) on SOM dynamics is related to the structures created by earthworms and termites. A few days following their deposition, mineralization rates decline rapidly in the casts of earthworms. Over several months, mineralization is actually inhibited in the compact structure of the casts. Martin (1991), however, found evidence that while coarse organic fractions had been protected, the mineralization of the

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finest fractions was increased. Termites have also been reported to increase the potential for N-mineralization in the structures they create (Abbadie and Lepage, 1989). In comparison to temperate regions, termite activity in tropical environments may reduce the length of time organic inputs are maintained on soil surfaces, due to their effective mixing of organic inputs in soil (Anderson and Flanagan, 1989). Where conditions are suitable for their activities, macrofauna, and especially earthworms and termites, become major regulators of microbial activities (Lavelle, 1994). Both earthworms and termites, however, are quite sensitive to soil disturbance and the use of pesticides. Lavelle and Pashanasi (1989), reported a drastic decline in the biomass of soil macrofauna in a mechanized, high-input cropping system relative to low-input cropping and agroforestry. In another study of grazing pressures on below-ground pasture productivity, Castilla (1992) measured a significant decline in endogeic (soil feeding) earthworm populations as pasture productivity decreased and soil bulk density increased due to overgrazing and trampling. Initial results from trials on earthworm inoculations in tropical soils that have been subjected to slash-and-burn cultivation, are positive. Soils that have been degraded tend to have low populations of soil fauna, and a significant correlation was observed between the increase in plant productivity and the biomass of introduced earthworms (Lavelle et al., 1992). Cause and effect relationships are more difficult to establish, however, as the dynamics of macrofauna communities during cropping rotations or long-term soil disturbances are still unknown. Lavelle et al. (1993) reported that the major constraint to the successful establishment and growth of introduced earthworms in degraded soils, was the lack of fresh organic inputs to feed the earthworms. We hypothesize those management activities such as mulching and no-till, will promote diverse and active soil fauna communities relative to high-input, mechanized systems. 3.5. Measuring SOM pools and their response to management Although various SOM pools have been proposed and named, there is as yet no single fractionation method that allows accurate determination of all the pools. Soil microorganisms are responsible for the mineralization of nutrients, decomposition, and degradation of toxic compounds. Being a labile fraction of soil organic matter, the microbial biomass can reflect changes in soil organic matter and soil development. Several researchers have found microbial biomass to be a useful index for assessing the contribution of SOM to soil aggregate stability (Gupta and Germida, 1988; Haynes and Swift, 1990). The microbial biomass in soils contains considerable quantities of nutrients, which depending on the stage of growth, can be a source or a sink of plant nutrients (Duxbury et al., 1989). Research has shown that as much as 34% of applied N fertilizer (Ladd and Foster, 1988), and between 5 and 16% of applied P (McLaughlin and

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Alston, 1986), can be retained in the microbial biomass. It is important to note, however, that high microbial biomass values do not mean high soil fertility or plant productivity. The turnover rather than the amount of microbial biomass is important to soil fertility and related to soil management (Sparling and Ross, 1993). To predict the effects of management on soil organic matter, however, both the size and the turnover of the soil microbial biomass must be known. Several researchers have recently attempted to measure the SOM pools in soils of the tropics (Luizao et al., 1992; Haggar et al., 1993; Mazzarino et al., 1993; Motavalli et al., 1994b). Results from these studies have not shown a clear relationship between measurements of microbial biomass with the active pool and light fraction C with the slow pool. This is possibly because biological lability is not organic matter, but also on the total soil environment, including interaction of soil organic matter with inorganic surfaces and temperature and moisture effects (Duxbury et al., 1989). Use of microbial biomass and light fraction measurements may be more appropriate for comparing similar tropical agroecosystems over time rather than different types of ecosystems (Swift and Woomer, 1993). Chemical fractionation has not proven very useful in following SOM dynamics (Duxbury et al., 1989). Physical fractionation is considered less destructive and Christensen (1992) hypothesized that the resulting fractions will be responsive to management. Cambardella and Elliott (1992) used a method based on chemical dispersal of the soil combined with physical separation by particle size to isolate the particulate organic matter (POM) fraction from grassland soils in Nebraska. This fraction, which is similar to the slow SOC pool, appears to be sensitive to management and was found to be depleted in cultivated soils. In addition, an enriched labile fraction (ELF) associated with the mineral portion of the soil was isolated from macroaggregates. The POM and ELF fractions together accounted for up to 30% of the total N in the soil. The significance of these fractions in forest soils or soils that have been cultivated for a long time is not known. Although the SOM fractions obtained via physical fractionation methods are likely to be responsive to management, there are presently no SOM dynamics models based on such fractions. Limited experience with physical fractionation methods and a general lack of data on the dynamics of the physical fractions, are probably the major reasons for the absence of 'physically fractionated SOM' models.

4. Conclusions The predictable management of SOM dynamics in tropical agroecosystems is constrained by the lack of appropriate methodologies to isolate SOM pools that are management-responsive. The current set of parameters used to define the quality of organic inputs, needs further testing with a variety of organic inputs,

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under different management, and in different soil and climate conditions. The limited information available on the contribution of root litter in agroecosysterns, is also a major information gap. It may be that we need to redefine the parameters of the postulated pools, in order to make it possible to routinely isolate different management-responsive SOM pools. Recent advances in the use of physical fractionation (Elliott and Cambardella, 1991) may provide a breakthrough in the search for agronomically meaningful SOM pools. Ecosystem approaches have provided a framework for integrating soil processes, driving variables and soil properties (Elliott et al., 1993). Simulation models based on ecosystem concepts, such as the Century model, offer a dynamic conceptual framework to examine the effects of long-term management (Cole et al., 1987). However, several limitations (e.g., exclusion of soil biotic effects) limit the Century model's applicability to tropical environments (Woomer, 1993). Development of ecosystem models with greater management options, such as offered by Version 4.0 of the Century model (Metherell et al., 1993) are necessary for further evaluation of the effects of management strategies with organic inputs. The problem remains, however, that the pools in currently available SOM dynamics models (like Century) are defined on the basis of the degree of their physical or chemical stability (turnover time), and that, with the exception of microbial biomass, they cannot be directly measured by physical or chemical fractionation (Paustian et al., 1992). Hassink (1995), has suggested that if the methods being developed for physical fractionation, yield management-responsive SOM fractions, then it may be more efficient to develop SOM models based on these fractions rather than conceptual pools. Despite increasing scientific understanding of the natural processes involved in soil organic matter dynamics, there are several factors that limit the adoption of management strategies to maintain or increase soil organic matter in tropical agroecosystems. A major constraint is often an inadequate supply of organic inputs due to competing uses for organic materials as fodder, fuel and building materials (Prasad and Power, 1991). Difficulties imposed in handling organic inputs such as transport, tillage and seasonal labor a n d / o r mechanical requirements are also major limitations (Wade and Sanchez, 1983). Although such constraints will vary from region to region within the tropics and from one land-use system to another, it is important to emphasize that management options for organic inputs in tropical land-use systems are often limited by social and economic factors.

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