Abstract
Aims: The objective was to estimate the fixation of atmospheric nitrogen (Ndfa) by legume (Gliricidia sepium) trees for the benefit of cocoa (Theobroma cacao) trees in agroforestry systems. Methods: Four sites where cocoa and gliricidia were intercropped and one site where gliricidia, cocoa, and orange trees were grown as separate stands were selected in Ghana. Ndfa in gliricidia and cocoa leaves (from all sites) and in shoot axes (in one site only) was assessed by the 15N natural abundance technique. Cocoa trees distant (less than 10 m) from the closest gliricidia were used as reference plants. Results: With few exceptions, leaves of gliricidia and cocoa trees growing in proximity had similar δ15N, whereas the foliar δ15N value of gliricidia was lower than that of distant cocoa trees. The Ndfa in gliricidia leaves ranged from 22 to 50% of total leaf N and was 48% in the shoot axis. Root nodules, found only after the wet season, always showed the inner red color indicating effective N2 fixation and the occurrence of Rhizobium tropici and Rhizobium etli. The annually produced shoots of gliricidia, theoretically suitable to become green manure after pruning, contained 31.4 to 38.0 kg N ha−1 derived from the atmosphere. Conclusions: Gliricidia sepium trees are able to take advantage of the association with rhizobial symbionts to fulfill, at least in part, the N needs of their rapidly growing shoots. In mixed-stand agroforestry systems, with intercropped gliricidia and cocoa trees, the amount of N derived from the atmosphere that could enter the soil if the pruned shoots of gliricidia trees are used as green manure could diminish the need for N fertilizers for cocoa trees.