Tracing the nitrogen flow between Gliricidia and cocoa trees in intercropping system using the 15N natural abundant method

The accurate estimate of N-2 fixation by legume trees using the N-15 natural abundant method ((15)NNAM) requires the presence of a reference plant, whose delta N-15 reflects that of the soil derived-N. Our study was conducted in four cocoa (Theobroma cacao L.) plantations located in two cocoa growing regions of Ghana. Gliricidia trees (Gliricidia sepium Jacq. Kunth ex Walp.), a legume which fixes N-2 were intercropped in the investigated plantations. The cocoa trees were planted in rows with a density of 3x3 m, while Gliricidia trees were dispersed at variable distances from cocoa (<5 m to >10 m). Our objective was to find out to which extent the presence of Gliricidia in an intercropping system influences delta N-15 of cocoa (no ability to fix N-2) growing at different distances from Gliricidia. We used the (15)NNAM which is based on the difference in delta N-15 values between the soil-mineral N (assessed by the reference plant cocoa) and atmospheric N-2, to assess N-2-fixation. At the end of both the rainy and dry season, we sampled young, but fully expanded leaves of cocoa trees which were at least 12 m away (referred as 'cocoa far') or at 5 m or closer (referred as 'cocoa close') to the nearest Gliricidia tree. Gliricidia leaves were sampled as well. Our data show that, except in one site, the delta N-15 of 'cocoa far' was always higher (p<0.05) than 'cocoa close'. The delta N-15 of 'cocoa close' and Gliricidia were similar in both seasons. Data suggest that 'cocoa close' benefited from the N deriving from residues of the Gliricidia tree due to their proximity to this legume tree, and therefore would not serve as a good reference plant for the (15)NNAM.