Soil litter dynamics and N cycling in alley cropping systems

Abstract

Many tropical alley cropping systems rely on an efficient transfer of nitrogen from organic residues to food crops. Several reports indicate that N released from decomposing residues is poorly recovered by food crops. This has been ascribed to poor timing between supply and demand of nutrients, and to a temporarily immobilization of N not taken up by the crop. Two studies carried out to trace the fate of N in such systems are reported. In the first (pot) experiment, the accumulation of N in soil litter fractions following a single application of 15N-labelled shoots or roots of Leucaena leucocephala, Flemingia congesta [F. macrophylla] and Dactyladenia barteri [Acioa barteri] was determined. The results show that major shifts in N-distribution are restricted to the first two weeks following application. Furthermore, it is shown that the final distribution of residual N is controlled to a large extent by the biochemical quality of the residues. In a second (field) experiment, microplots were amended with 15N-labelled residues of L. leucocephala, and the fate of N in soil and soil litter fractions was followed. The results again indicate a rapid release of N from applied residues during the first two weeks. After 236 days of decomposition, 8% of Leucaena-N remained in the soil. This residual 15N was largely associated with the more stable, non-litter soil organic matter fraction.