Role of Soil Organic Matter in Nutrient Conservation

Conservation of soil organic matter is important for sustainable management of tropical forests (Van Wambeke 1992). Soil organic matter provides additional exchange sites for cations in the soil, thereby decreasing leaching potential. Organic nitrogen is bound in the soil organic matter and is released more in synchrony with plant needs than it would be as nitrates or ammonia, the common forms of inorganic nitrogen in mineral soil (Brady and Weil 2002). Soil organic matter also makes the soil less susceptible to erosion. Organic compounds synthesized by soil microorganisms help bind clay particles together into aggregates that render the soil more permeable to water (Stock-dale et al. 2001). Soil organic matter is also the source of energy for microorganisms whose activity renders the soil more permeable to roots (Jordan 1998).

Because of the importance of soil organic matter for recycling cations, maintaining phosphorus availability, sequestering nitrogen, and preventing soil erosion, management activities should emphasize the maintenance of the litter and humus on the soil surface. Clear-cuts are especially deleterious because the increase in solar radiation reaching the forest floor results in a quick decomposition of the litter. Fire is even more damaging to tropical forest ecosystems when it destroys the top layer of litter and soil organic matter. Fire can instantaneously transform the phosphorus from an organic form in the leaf, where it is slowly released in synchrony with the demand by plants, to an inorganic form where it is bound up and made insoluble by iron and aluminum. Fire converts calcium and potassium to easily leached ash, and volatilizes the nitrogen in tree biomass.

While all fires destroy soil organic matter, the organic matter lost in small-scale shifting cultivation can be quickly replaced by litterfall and tree fall from surrounding forest. However, large-scale burning, as is used to maintain pasture in the Amazon region, can result in an almost permanent loss of organic matter, and consequent long-term loss of productivity (Buschbacher 1986).

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