Most tropical soils are highly weathered (Sanchez 1976) and have a low ability to retain nutrients. Nutrients in rain forest ecosystems are retained primarily in the soil organic matter, where under undisturbed conditions the nutrients are gradually released in a soluble form, or are transferred directly from decomposing litter to roots through mycorrhizae (Herrera et al. 1978). Once a forest or even a small patch of forest is cleared, organic matter input into the cleared area is reduced or eliminated. The litter and humus on the forest floor and the organic matter within the soil quickly disappear. With the disappearance of the soil organic matter, the ability of soil to recycle nutrients is quickly lost, soil fertility rapidly declines, and the ecosystem loses its productive capacity (Jordan 1985). Thus management techniques that preserve litterfall inputs into the soil are especially important in tropical rain forests (Van Wambeke 1992).
For example, in some types of agroforestry systems, leaf litter is used to increase crop yield, especially when leguminous tree leaves are used as mulch. Mulches can protect soils against erosion, decrease weed growth, release nutrients to the soil via decomposition, and moderate soil moisture loss and temperature fluctuations (Montagnini et al. 1993). Farmers frequently use leaf litter as mulch when inorganic fertilizers are too expensive and livestock manure is not available (Byard et al. 1996). Clear-cuts, especially if followed by burning of the accumulated woody debris, are particularly harmful to the ability of the ecosystem to sustain productivity. Therefore, if a tree plantation or agricultural system follows a clear-cut, management has to be geared towards nutrient conservation and replenishment in order to sustain productivity in the long term. More details on nutrient management and conservation in plantations and agroforestry are given in Chapter 6.
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