EFFECT OF SOIL DEGRADATION ON LEAF DECOMPOSITION AND NUTRIENT RELEASEUNDER HUMID TROPICAL CONDITIONS

Characterization of degradation of an Alfisol caused by clearing and l ong-term cultivation and the effect of the degradation on leaf (Cajanu s cajan) decomposition and nutrient (N and P) release were studied in the humid zone of southwest Nigeria. Degradation of the Alfisol as a r esult of land clearing and 17-year continuous cropping led to loss of soil biota and deterioration of soil physical and chemical properties. Compared with nondegraded soil, the degraded soil was 38% lower in so il organic carbon, 55% lower fn exchangeable bases, 56% lower in micro bial biomass, and had 44% lower microarthropod density, 34% less silt, and 88% less water infiltration; it also had 13% higher bulk density acid 14% more sand. Soil pH was lower in the degraded than in the nond egraded soil. The macrofauna community in the degraded soil was termit e-dominant (Ancistratermes cavithorax), whereas it was earthworm-domin ant (Hyperiodrilus africanus) in the nondegraded soil. Leaf decomposit ion and nutrient release were lower irt the degraded soil than in the nondegraded soil during the 150-day experiment. Higher decreases in le af decomposition and nutrient release in the degraded soil were observ ed when macrofauna were excluded. Macrofauna-mediated leaf decompositi on and nutrient release in the degraded soil were higher than in the n ondegraded soil. Irrespective of soil degradation, macrofauna-mediated decomposition and nutrient release resulted in net nutrient mineraliz ation. (Soil Science 1998;163:897-906).