Tree-biomass decomposition, nitrogen dynamics, and maize growth under agroforestry conditions in subhumid highlands of Kenya

Declining crop yields, consequent to continuous cropping without external addition of adequate nutrients, is a major problem facing smallholder farmers in the tropics. The high cost of inorganic fertilizers has led to increased interest in the use of leafy biomass from woody species as a source of nutrients to crops. This dissertation embodies the results of a study conducted in the subhumid highlands of Kenya on the influence of soil-incorporated leafy biomass of two agroforestry tree species (calliandra and leucaena) on maize yield and soil fertility.;The results revealed that inclusion of leucaena as hedgerows in crop fields (alley cropping) increased crop yields during the four-year study-period, whereas alley cropping with calliandra had an adverse effect. In maize fields, soil incorporation of leafy biomass obtained from trees grown outside the field, improved crop yield and decreased the rate of soil nutrient depletion. However, the applied biomass contributed more to long-term build up of soil nutrient status than to meeting the nutrient requirements of the current season's crop.