Abstract
Nitrogen (N) is an important nutrient that determines yield in cocoa trees. Atmospheric N (N2) -fixing plants when involved in intercropping systems may provide additional N sources for agricultural crops, thus reducing the need for mineral fertilization. This study has been carried out in five sites in Ghana (during both the dry and rainy season), where cocoa (Theobroma cacao L.) and gliricidia (Gliricidia sepium (Jacq.) Kunth ex Walp.) trees (a N2-fixing species) were either intercropped or grown as pure stand. The research aims were: 1) estimating the potential of gliricidia plants to fix N2 when used as cocoa intercrop and characterizing both root nodules and rhizobia communities; 2) assessing the rate of release of N and other nutrients from pruning material of gliricidia and leaf litter of cocoa with the aim of improving the efficient use of nutrient resources present in the biomass under cocoa plantation; 3) quantifying the N uptake by cocoa from decomposing gliricidia pruning residues; 4) investigating whether cocoa benefits from gliricidia residues in an intercropping system. The 15N Natural Abundance Method, the litterbag technique and the 15N Enrichment Method were used to achieve objectives 1, 2 and 3 respectively, while objective 4 was achieved by analyzing the leaves 15N abundance of ´close cocoa´, ´distant cocoa´, and gliricidia. Roots nodules from four sites were analyzed in terms of number, weight, and their endophytic bacterial communities. In addition, the amount of N in the annual biomass of gliricidia trees and the rate of decomposition of pruned gliricidia shoots were estimated, while data on 15N of label cocoa trees were collected. In all sites, except one, and in both seasons, δ15N values were significantly higher (with P < 0.05) in cocoa than in gliricidia. About 24 - 53% of the N in gliricidia leaves were derived from N2 fixation thanks to the presence of Rhizobium tropici and Rhizobium etli that occurred in all sites during the rainy season. The estimated annual aboveground biomass of gliricidia was 32.7 kg (DW) tree-1, containing 0.63 kg of N tree-1, on average. Therefore, gliricidia showed a potential to add 0.18-0.42 kg atmospheric N tree-1 year-1 (22-51 kg N ha-1, with a density of 124 trees ha-1). The litterbag experiment showed that when the gliricidia shoots are pruned and moved on top of the soil underneath the cocoa trees, about 50% of pruned gliricidia shoots would decompose within 83 days and over 95% within 126 days. The N released from the decomposed gliricidia pruning residues were taken up by the cocoa fine roots with most absorbed N contributing to new growth of cocoa pods and shoots. Finally, when cocoa trees are grown close to the nearest gliricidia tree (at least 5 m) they would benefit from the N deriving from the gliricidia residues. In conclusion, the N needs of cocoa is often in the range 15-70 g tree-1, therefore, it would be enough planting one gliricidia tree every 4-20 cocoa trees to fulfil the cocoa need for external N.