Effect of tree pruning and pruning application to trees on nitrogen fixation by Leucaena and Gliricidia

A microplot study was conducted with Leucaena leucocephala and Gliricidia sepium to determine the effect of pruning the trees and applying the prunings to the soil (as either surface mulch or soil-incorporated green manure) on N-2 fixation by the trees. N-2 fixation was estimated by the N-15 dilution method, and the non-N-2-fixing species Senna siamea was used as a reference species. Number and dry weight of nodules were reduced with applications of prunings in Leucaena, and increased in Gliricidia. For both species, the lowest rate of N-2 fixation (47%) occurred under green manure incorporation after pruning, and the highest (66%) with mulching and no-pruning. Incorporation of green manure into soil significantly depressed N-2 fixation by 19.1% in Gliricidia and 20.6% in Leucaena compared to no-pruning treatment. The deleterious effect of incorporation of prunings into soil also affected the amount of N-2 fixed. During the nine months following pruning, green manure incorporation steadily decreased the percentage of N-2 fixed, whereas with no-pruning, pruning only and pruning with mulching, N-2 fixation increased with time. Reduction of N-2 fixation in pruned plants could imply that they would compete for N with interplanted food crops, causing lower N use efficiency by the latter. Despite lower dry-matter and N yields of Leucaena as compared with Gliricidia, its biological N-2 fixation amounted to 73% of its N requirement as opposed to the corresponding figures of 50% for Gliricidia.