Uptake of nitrogen by cocoa (Theobroma cocoaL.) trees derived from soil decomposition of gliricidia (Gliricidia sepiumJacq.) shoots

Abstract

Intercropping between cocoa (Theobroma cocoa L) and legume trees like gliricidia (Gliricidia sepium), that provide shade and fix atmospheric N2 due to the development of a symbiosis with bacteria of the genus Rhizobium, is a promising option for an ecologically sustainable management of N nutrition of cocoa trees. This study, carried out in Ghana (W Africa) under a tropical climate, aimed at determining the rate of decomposition and N release from gliricidia pruning residues, and investigated if cocoa trees benefit from decomposing gliricidia residues in intercropping systems. In a first experiment, litterbags filled either with gliricidia shoots or cocoa leaves were located under cocoa trees in the field and sampled 42, 83, 126, and 216 days after laying (DAL) to determine their remaining amount of N and other mineral nutrients. In a second experiment, 15N-enriched gliricidia shoots were laid as green fertilizer on top of the soil under selected cocoa trees in the field: cocoa shoots were sampled 2, 6, and 9 months later, when cocoa trees were destructively harvested and the N derived from gliricidia shoots determined. Gliricidia shoots had a relatively high N concentration (3.7-4.1% and a C:N close to 10) and the initial amount of N in the litter was almost entirely released within 126 days from the beginning of the experiment, much faster than cocoa litter. Although a relatively low amount of N initially present in the 15N-enriched litter has been recovered in the cocoa trees after one year, results suggest that if a sufficient amount of pruned material of gliricidia is spread around the cocoa trees as green fertilizer, the latter may significantly benefit from N fixed from the atmosphere and then released during the litter decomposition.