Alley cropping of maize with calliandra and leucaena in the subhumid highlands of Kenya - Part 2. Biomass decomposition, N mineralization, and N uptake by maize

A major challenge in developing agroforestry approaches that utilize tree-l eaf biomass for provision of N to crops is to ensure synchrony between the N released from decomposing prunings and N demand by crops. A study was con ducted in the subhumid highlands of Kenya to assess the rate of decompositi on and mineralization of soil-incorporated Calliandra calothyrsus Meissner (calliandra) and Leucaena leucocephala (Lam.) de Wit (leucaena) tree biomas s and maize roots (Zea mays L.) both in an alley cropping and a sole croppi ng system. The amount of mineralized N peaked four weeks after planting (WA P) maize in all the treatments during both seasons of 1995. Cumulative mine ralized N at week 20 ranged from 114 to 364 kg N ha(-1) season(-1), the abs olute control treatment giving the lowest and the prunings-incorporated tre atments giving the highest amounts in the two seasons. Total N uptake by ma ize, ranging from 42 to 157 kg ha(-)1 season(-1), was lowest in the 'alley- cropped, prunings-removed' treatments, and highest in the 'non alley-croppe d-prunings-incorporated' treatments. The apparent N recovery rate by maize was highest in the fertilizer applied treatments in the two seasons. Decomp osition rate constants (kD) ranged from 0.07 to 0.21 week(-1), and the rate s among the different plant residues were as follows: leucaena < calliandra < maize roots. Nitrogen release rate constants (kN), ranging from 0.04 to 0.25 week(-1), followed a similar pattern as the rate of decomposition with leucaena releasing the highest amount of N followed by calliandra and last ly by maize roots.