Nitrogen recovery by alley-cropped maize and trees from N-15-labeled tree biomass in the subhumid highlands of Kenya

The effectiveness of tree-leaf biomass as a source of N to crops in agrofor estry systems depends on the rate at which crops can obtain N from the biom ass. A study was conducted to determine the fate of N-15 labeled, soil-appl ied biomass of two hedgerow species, Calliandra calothyrsus Meissner (calli andra) and Leucaena leucocephala (Lam.) de Wit (leucaena), in the subhumid highlands of Kenya. Labeled biomass obtained from N-15 fertilized trees was applied to microplots in an alley cropping field and maize planted. N upta ke and recovery by maize and hedgerow trees was periodically determined ove r a 20-week period during the short rain (1995) and the long rain (1996) gr owing seasons. In maize crop from treatments that received leucaena biomass , higher N uptake and recovery were recorded than in maize from the plots t hat received calliandra biomass. However, N uptake and recovery were higher in calliandra tree hedges than in leucaena hedges, indicating differences in N uptake by the two tree species. The largest fraction (55-69%) of N in the applied tree biomass was left: in the soil N pool, 8-13% recovered by m aize, 2-3% by tree hedges, and 20-30% could not be accounted for. Some of t he unaccounted for N may have been left in the wood and root portions of th e tree hedges and in the bulk soil below the 20-cm depth. The study shows t hat only a small fraction of the N contained in the N-rich biomass that is applied to the soil is taken up by the current season's crop, suggesting th at a major benefit may be in the build-up of the soil N store.