ORGANIC-MATTER IN TROPICAL SOILS - CURRENT CONDITIONS, CONCERNS AND PROSPECTS FOR CONSERVATION

Soil organic matter (SOM) plays a central role in maintaining tropical soil fertility. Its conservation and maintenance in tropical cultivation systems is imperative if soil degradation is to be halted and cropping made sustainable. Around 22-23% of global carbon is present in tropical ecosystems but only 2-3% of this is present in the soil. Fluxes of CO2 to the atmosphere from tropical soils after deforestation contributes to the greenhouse effect and could represent a 1-4% increase in the overall steady-state annual CO2 release to the atmosphere. The composition of SOM is extremely diverse, with active, or labile, fractions decomposing in a matter of months while the more resistant, or nonlabile, fractions remain in soil for many years. SOM plays a central role in providing good soil structure, in supplying nutrients and in providing ion exchange capacity in tropical soils. Organic matter decomposition rates in tropical soils are generally faster than in temperate soils because decay of the nonlabile fraction is generally faster. Changes in tropical land use have profound effects on SOM status. Tropical deforestation causes significant losses of soil carbon and nitrogen which tend to decline further under continuous cultivation. Reasons for declines in soil fertility include accelerated soil erosion, loss of litter influx after canopy removal and enhanced decomposition and nutrient mineralization rates after forest clearance. A wide range of soil management techniques have been designed to conserve and improve SOM stocks in tropical cultivation systems. These include agroforestry techniques, intercropping with legumes and the use of mulches and other organic inputs. While many of these techniques look promising in terms of improved crop yields, there is urgent need for research on how these practices alter the physical, chemical and biological processes and conditions of tropical soils.