Overstory #257 - Biological Soil Fertility Management for Tropical Tree-Crop Agroforestry

Tree crop-based agroforestry systems, which function as multistrata systems, are an ecologically and economically important group of land-use systems in the humid and subhumid tropics. They are also found in dry climates in regions with high water availability; however, this aspect is not focused on here. Multistrata systems are widespread in lowland and mountainous areas, often surrounding homesteads and thus referred to as homegardens or forming a transitional zone between cultivated land into forests (Murniati et al., 2001; Schroth et al., 2004a).

By definition, multistrata agroforestry systems are composed of several strata of trees and tree crops. While the simplest systems consist of only two strata—a lower stratum of tree crops such as coffee (Coffea spp.), cocoa (Theobroma cacao), or tea (Camellia sinensis), and an upper, equally monospecific stratum of shade trees—the most complex systems approach the structural complexity of natural forest and may harbor more than a hundred plant species and varieties, being “agroforests,” as defined by Michon and de Foresta (1999).

Groups of plant species typically found in multistrata systems include: shade-tolerant understory tree crops such as coffee, cocoa, and tea; overstory tree crops such as rubber trees (Hevea brasiliensis), large fruit trees such as Brazil nut (Bertholletia excelsa), durian (Durio zibethinus), and overstory palms; timber trees, often in a dominant position, and often remnants of previous forest; smaller “service” trees including leguminous shade trees; midstory species such as citrus (Citrus sp.) and avocado trees (Persea americana), bananas (Musa sp.) and smaller palms. Especially in younger or more open systems; there may also be annual food crops in the understory. Moreover, there may be substantial amounts of spontaneous vegetation in all strata from herbs to emergent trees, depending on the history of the system (e.g., whether it was established on a cleared field or underplanted into thinned forest) and the intensity of management, both present and past.

Soil Fertility Management in Tree-Crop Agroforestry

Despite the economic importance of tree crops in tropical agriculture and commentaries on their ability to maintain and regenerate soil fertility (Sanchez et al., 1985; Schroth et al., 2001a), much less information is available on the biological mechanisms of soil fertility for multistrata systems than for annual crop-based systems, with most research being focused on mineral availability and balances. It is becoming clear, however, that despite many very old and apparently sustainable multistrata systems, such as homegardens (Kumar and Nair, 2004), tree crop-based systems are not safe from soil degradation. The productivity of their soil systems can progressively be undermined, especially over a number of production cycles. This is especially true where, during peaks in commodity prices, tree crops were established on marginally suitable soils, e.g., for cocoa (Ayanlaja, 1987).

Owing to their higher biomass and litter inputs and stronger root systems, tree-crop based systems are better able than some other production systems to regenerate soil conditions, such as soil organic matter levels and soil structure, of land that was previously under annual crops or pasture (see Chapter 21, Uphoff et al., 2006). This is less true for mineral nutrient levels, as evident from the low nutrient levels even in black earth soils under some Amazonian rubber agroforests (Schroth et al., 2004b). However, when tree-crop systems are established on previously forested land, soil characteristics such as organic matter levels and soil structure often decline over time (see review in Schroth et al., 2001a).

Whether a new equilibrium is reached that is still adequate for sustained production depends, among other factors, on soil and climatic conditions, crop species, management practices, and levels of external inputs (the latter three obviously influenced by economic factors). The alternative is reduced soil fertility and crop production, leading to a downward spiral that ends with the abandonment or conversion of the land into less demanding uses (often pasture). The latter scenario is particularly likely on sandy soils where loss of organic matter rapidly leads to degradation of soil structure, which in turn feeds back into diminished plant growth via reduced root development and lessened soil faunal and microbial activity.

Gradual fertility loss under tree crops is not always easy to recognize, especially as it affects soil organic matter. For example, in an experiment conducted over 15 years on a Ferralsol in the central Amazon that was previously under forest cover, it was found that African oil palm (Elaeis guineensis) did not respond to nitrogen fertilizer amendments despite reasonably high production levels (Schroth et al., 2000). This could have been interpreted as a comparative advantage of the site for oil palm production; however, another study suggested that primary forest growth in the area responded positively to higher nitrogen levels in this soil (Laurance et al., 1999). The apparent contradiction was resolved by showing that the evident nitrogen sufficiency of the palms was linked to progressive soil organic matter loss (with concomitant nitrogen release) in the topsoil; surplus nitrogen was being leached as nitrate into the subsoil between the palms. The comparative advantage for the area was being lost over time and this would presumably make future rotations less profitable.

From an agronomic viewpoint, the objective of biological soil fertility management in tree-crop agroforestry is essentially to create a favorable environment for the acquisition of soil water and nutrients by the crop plants through a combination of favorable soil structure, high nutrient availability, thorough exploration of the soil by root systems and their mycorrhizas, and absence of disease. The principal biological agents that bring about these conditions, and that are managed either directly or indirectly by farmers, are the plants themselves with their root systems and litter, soil organisms (soil fauna and microbes, including antagonists of soil pests and pathogens), and the supply of soil organic matter. In the following discussion, the principal management interventions will be briefly reviewed, with a focus on recent results from Amazonian tree-crop agroforestry systems.

Managing Soil Microbes and Fauna in Tree-Crop Agroforestry

Root systems represent the “demand side” for soil resources, while soil microbes and fauna contribute to the “supply side” by influencing the decomposition of litter and other organic materials in the soil with associated release of nutrients and both the degradation and stabilization of soil organic matter. Especially fungi and larger soil fauna, the “ecosystem engineers” discussed in Uphoff et al. (2006), also play important roles in creating and stabilizing soil structure, complementing and interacting with the activities of roots, although these interactions have been rarely studied. The role of these organisms in the control of soil pests and diseases in multistrata agroforestry systems is discussed further below.

Much like root distribution and processes, the distribution and activity of microbes and fauna in the soil and litter of multistrata systems tend to be strongly patchy, and the analysis of these patterns provides clues about design and management factors that influence biological processes on and in the soil. The mineralization of soil nitrogen, one of the soil microbial processes that most directly influences plant growth, is a case in point. At different sites in the central Amazon, microbial nitrogen mineralization in the topsoil of tree-crop systems was found to be substantially higher in the interspaces between trees, which were covered with ground vegetation, than in the regularly weeded soil close to the tree crops. Consequently, the vegetation-covered spaces had higher soil moisture and lower bulk density, which in concert with a larger pool of mobile nitrogen in the soil organic matter led to significantly higher nitrogen mineralization rates than in uncovered soil (Schroth et al., 2000; Schroth et al., 2001b).

These differences demonstrate the benefits of a management strategy that maintains permanent soil cover and thereby high microbial activity and rapid nutrient turnover in the soil. In contrast, clean-weeding, even if restricted to circles around trees for localized fertilizer application, may result in markedly reduced nutrient mobilization in the soil, which may then have to be compensated for through mineral fertilizer. This means that fertilization may become, to some extent, self-perpetuating (Schroth et al., 2001b).

To create soil cover in plantations, agronomists often recommend leguminous cover crops. However, this advice has seldom been taken up by smallholder farmers who find it difficult to invest labor in an unproductive crop. Instead, farmers usually establish trees and tree crops in fields of annual or semiperennial food crops. These crops cover the soil while reducing the opportunity cost of having planted tree crops not yet in production. Schroth et al. (2001a) showed that this practice of interplanting food crops may lead to similar or even better tree growth than the use of cover crops. Later in the development of the plantation, when shade prevents successful growth of food crops, aggressive ground-cover species such as grasses can be weeded out to select for a soil cover of unaggressive, broadleaved “noble weeds” (Baker, 2001; Pohlan, 2002).

Even more than microbial activity in the soil, the fauna in soil and litter of multistrata plots responds to small-scale variations in microclimate and litter quantity and quality, which are created by plant cover and management practices, with the potential to enhance litter decomposition, nutrient release, and soil structure (Lavelle et al., 2003). The influence that soil cover (and thus microclimate) can exert on soil and litter fauna was highlighted by a baiting experiment for wood-feeding termites in a multistrata system and monoculture of peach palm (Bactris gasipaes) in the central Amazon. After 5 months in the field, the percentage of wood-stick baits (buried in the topsoil), which had been infested by termites, decreased significantly in this order: soil under cover crop (Pueraria phaseoloides) in the multistrata plots (45%). uncovered soil in multistrata plots at about 2 m distance from the previous location (18%). peach palm monoculture where all soil was uncovered because of the intensive shade (2.5%) (Hanne, 2001).

Another study at this site showed that several litter-dwelling invertebrate groups formed a diversified landscape on a plot with four local tree crops and ground vegetation of legumes and grasses; the area had distinct patches of more or less suitable habitat depending on the group of fauna (Vohland and Schroth, 1999). Several fauna groups including earthworms and millipedes showed a preference for the fleshy litter of peach palm and the soft, nutrient-rich litter of the legume cover crop and annatto (Bixa orellana) trees, while avoiding the hard, recalcitrant litter of Brazil nut and cupuaçu (Theobroma grandiflorum) trees. Certain other species preferred the litter of Brazil nut trees (caterpillars) or grasses (bugs and thrips).

Overall, faunal density and biomass were strongly determined by the quantity of litter produced by the different plant species. From the data it appears that a minimum quantity of 3 t/ha of litter, but preferably twice that amount, is necessary to maintain an active and diverse litter fauna in an agroforestry plot. Based partly on these data, Lavelle et al. (2003) have suggested that, for biological soil fertility management, agroforestry practices should ensure a supply of about 2 t/ha/yr of labile organic matter from litter for digestive assimilation by the soil fauna, while maintaining 3–6 t/ha of litter on the soil surface. Research is needed to determine the quantity and quality mix of litter that will achieve these values at a range of sites.

The patchiness of faunal distribution in tree-crop agroforestry systems provides further arguments for high planting densities, and for intimate mixing of species with less favorable litter characteristics with species that produce high litter quantity and quality, so that larger patches with hostile conditions for microflora and fauna, and the soil biological processes they drive, do not develop.

Soil Organic Matter Management in Tree-Crop Agroforestry

Soil organic matter has a profound influence on chemical, physical, and biological soil properties, and its loss from cultivated soil is a widely used indicator of soil degradation. Compared with other forms of agriculture, tree-crop agroforestry systems offer far better opportunities to conserve soil organic matter since they maintain permanent soil cover, which also stimulates faunal and microbial activity; produce more biomass and thus above- and belowground litter, especially at high planting densities, which also increase the efficiency of soil exploration by root systems; seldom require the use of fire, which destroys biomass and litter; and do not employ soil tillage, which stimulates soil organic matter breakdown and destroys faunal populations and structures in the soil.

However, tree-crop systems usually lose soil organic matter if established on forest soil, as shown above, but the extent of loss differs between tree species. By associating different kinds of trees and tree crops, land users can capitalize on the ability of some species to retain higher levels of organic matter in the soil than do other, perhaps commercially, more valuable species. This buffers the system against organic matter and fertility loss. For example, in a 40-year-old multistrata system in Nigeria, cola trees (Cola nitida) retained higher organic matter levels in the soil than did the cocoa trees with which they were interplanted (Ekanade, 1987). Similarly, in a 7-year-old association of four local tree crops and a legume ground cover in the central Amazon, two tree species with recalcitrant litter, Brazil nut and cupuaçu, had similar organic matter levels in the top 10 cm of soil as adjacent rainforest, while the species with higher litter quality with which they had been interplanted (peach palm, annatto, and a leguminous cover crop) showed a tendency for lower topsoil carbon levels even after this relatively short time period (Schroth et al., 2002). This suggests that tree crops with recalcitrant litter can serve, to a certain extent, as “insurance” against soil organic matter loss in multistrata systems, in addition to their production role (Schroth et al., 2002), while other species with more nutrient-rich, high-quality litter will have a more beneficial effect on litter (and soil) fauna and essential soil microbial processes, as shown above.

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