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 N2 fixation by Gliricidia seplum, nitrogen (N) accumulation by prunings of Gliricidia, Senna siamea (formerly Cassia siamea) and Gmelina arborea, and the N contribution to associated crops of rice and cowpea. Total N accumulated by the hedgerow trees ranged from 297-524 kg N ha-1 on average but varied between tree species and depended on the growing season. Gliricidia seplum accumulated 370 kg N ha-1 on average and more than half of this came from fixation, Senna siamea and Gmelina arborea served as reference trees for estimating N2 fixation. The estimates of N2 fixation using Gmelina as a reference gave higher estimates than those using Senna. Although the dry matter and nitrogen yields of prunings from the hedgerow trees were high, their relative nitrogen contribution to the associated crops was generally low ranging from 5 to 29%. Higher crop yields and nitrogen contribution were observed with Gliricidia seplum prunings. The low N contribution from prunings was attributed to the lack of synchronization between the N released from the prunings and the crop's demand for N.