MINERAL N DYNAMICS IN BARE AND CROPPED LEUCAENA-LEUCOCEPHALA AND DACTYLADENIA-BARTERI ALLEY CROPPING SYSTEMS AFTER THE ADDITION OF N-15-LABELED LEAF RESIDUES

In tropical cropping systems with few external inputs, efficient manag ement of mineral N derived from added organic residues is essential fo r the proper functioning of the system. We studied the dynamics of min eral nitrogen (N) in the top 100 cm of soil with a system of tensiomet ers and suction cups after applying N-15-labelled Leucaena leucocephal a and Dactyladenia barteri residues to bare and cropped microplots ins talled in the respective alley cropping systems, and followed the fate of the N for two maize-cowpea rotations (1992 and 1993). Fifty days a fter applying the residues (DAA), 20% of the added residue N was found in the soil profile of the bare Leucaena treatment, and 5% under Dact yladenia, compared with 5% and 1%, respectively, where cropped. All va lues decreased to about 1% after 505 days. In the cropped soil, no min eral N derived from the residues was lost by leaching during the first 6 weeks. As the maize grew, the soil profile was gradually depleted o f nitrate to near zero in the Dactyladenia treatment, whereas during t he cowpea season the amount of nitrate N increased to 36 kg N ha(-1) f or the Leucaena treatment, and 26 kg N ha(-1) for the Dactyladenia tre atment. The soil of the bare microplots contained substantially more n itrate N (98 and 47 kg N ha(-1) during the first year on average, unde r Leucaena and Dactyladenia, respectively) than that of the cropped mi croplots, except during the 1993 cowpea season. Nitrate residing in th e subsoil (80-100 cm) in the bare treatments was not readily leached t o deeper soil. The risk of losses of native mineral N was greatest dur ing the first 50 DAA and to a lesser extent during the cowpea seasons. Improved management of the hedgerows could increase the potential of the hedgerow trees to recycle mineral N.