NITROGEN CONTRIBUTION BY MULTIPURPOSE TREES TO RICE AND COWPEA IN AN ALLEY CROPPING SYSTEM IN SIERRA-LEONE

In an alley cropping experiment, a study was carried out on N-2 fixati on by Gliricidia sepium, nitrogen (N) accumulation by prunings of Glir icidia, Senna siamea (formerly Cassia siamea) and Gmelina arborea, and the N contribution to associated crops of rice and cowpea. Total N ac cumulated by the hedgerow trees ranged from 297-524 kg N ha(-1) on ave rage but varied between tree species and depended on the growing seaso n. Gliricidia sepium accumulated 370 kg N ha(-1) on average and more t han half of this came from fixation. Senna siamea and Gmelina arborea served as reference trees for estimating N-2 fixation. The estimates o f N-2 fixation using Gmelina as a reference gave higher estimates than those using Senna. Although the dry matter and nitrogen yields of pru nings from the hedgerow trees were high, their relative nitrogen contr ibution to the associated crops was generally low ranging from 5 to 29 %. Higher crop yields and nitrogen contribution were observed with Gli ricidia sepium prunings. The low N contribution from prunings was attr ibuted to the lack of synchronization between the N released from the prunings and the crop's demand for N.