Nitrogen contribution by multipurpose trees to rice and cowpea in an alley cropping system in Sierra Leone

The integration of leguminous trees into an alley cropping system in Njala, Sierra Leone, to improve agricultural sustainability through increased soil nitrogen was studied. The nitrogen contribution of three hedgerow tree species to associated food crops (rice and cowpea) and the proportion and amounts of nitrogen fixed were determined. Total nitrogen accumulated by the hedgerow trees ranged from 297-524 kg/ha, depending on the species and growing season. For one species (Gliricidia sepium), more than half the nitrogen came from fixation. The relative contribution of prunings to soil nitrogen was low. G. sepium is a significant source of soil nitrogen for food crops in agroforestry systems.