INFLUENCE OF PRUNING MANAGEMENT ON P AND N DISTRIBUTION AND USE EFFICIENCY BY N-2 FIXING AND NON-N-2 FIXING TREES USED IN ALLEY CROPPING SYSTEMS

Pruning of hedgerow trees is an important management practice for the successful establishment of an alley cropping system. Although pruning affects biomass production, only meager evidence of this management o n distribution of nutrients among the different plant organs after tre e regrowth is available. This study examined the effect of pruning on the distribution and use efficiency of N and P in a N-2 fixing legumin ous tree species, Gliricidia sepium, and two non-N-2 fixing leguminous tree species, Senna siamea and S. spectabilis, grown in a field on an Alfisol (low in P) at Fashola (Guinea Savanna Zone), Southwestern Nig eria. Four P rates, 0, 20, 40 and 80 kg P ha(-1) as single superphosph ate were used and management treatments included pruned versus unprune d plants. The N-15 isotope dilution technique was used to measure N-2 fixation in G. sepium. Partitioning of total P among different plant o rgans was influenced by plant species and pruning management, but was not affected by P application rates. The distribution of total P in th e various plant organs followed that of dry matter yield while N parti tioning had a different pattern. Pruned plants distributed about 118% more total P to branches and had a higher physiological P use efficien cy (PPUE) than unpruned plants. Leaves were the biggest sink for total N and N allocation in the other plant organs was influenced by plant species and pruning management. G. sepium had relatively more of its t otal N and P partitioned into roots (about double that of the non-N-2 fixing trees) but had a lower PPUE. Unpruned-and pruned G. sepium deri ved 35 and 54% respectively of their total N from atmospheric N2, with about 54% of the fixed N2 being allocated to leaves and roots. Result s showed that N and P pools turned over in the branches during plant r egrowth after pruning but the causative factors associated with this p henomenon were not clear.