Detection of nitrogen transfer from N2-fixing shade trees to cacao saplings in 15N labelled soil: ecological and experimental considerations

Theobroma cacao seedlings were grown alone (TCA) or associated with saplings of N-2-fixing shade trees Gliricidia sepium and Inga edulis in 200 l of N-15 labelled soil within a physical root barrier for studying direct nitrogen transfer between the trees and cacao. Root:shoot partitioning ratio for sapling total N was lower than biomass root:shoot ratio in all species. Sapling total N-15 was partitioned between root and shoot in about the same ratio as total N in cacao and inga but in gliricidia much higher proportion of N-15 than total N was found in roots. Thus, whole plant harvesting should be used in N-15 studies whenever possible. Average percentage of fixed N out of total tree N was 74 and 81% for inga estimated by a yield-independent and yield-dependent method, respectively, and 85% for gliricidia independently of estimation method. Strong isotopic evidence on direct N transfer from trees to cacao was observed in two cases out of ten with both tree species. Direct N transfer was not correlated with mycorrhizal colonisation of either donor or receiver plant roots. Direct N transfer from inga and gliricidia to cacao is conceivable but its prevalence and the transfer pathway via mycorrhizal connections or via reabsorption of N-rich legume root exudates by cacao require further study. Competition in the restricted soil space may also have limited the apparent transfer in this study because the trees accumulated more soil-derived N than cacao in spite of active N-2 fixation.