Natural abundance of 15N in two cacao plantations with legume and non-legume shade trees

Natural abundance of N-15 was sampled in young and mature leaves, branches, stem, and coarse roots of trees in a cacao (Theobroma cacao) plantation shaded by legume tree Inga edulis and scattered non-legumes, in a cacao plantation with mixed-species shade (legume Gliricidia sepium and several non-legumes), and in a tree hedgerow bordering the plantations in Guacimo, in the humid Caribbean lowlands of Costa Rica. The deviation of the sample N-15 proportion from that of atmosphere (delta N-15) was similar in non-legumes Cordia alliodora, Posoqueria latifolia, Rollinia pittieri, and T. cacao. Deep-rooted Hieronyma alchorneoides had lower delta N-15 than other non-N-2-fixers, which probably reflected uptake from a partially different soil N pool. Gliricidia sepium had low delta N-15. Inga edulis had high delta N-15 in leaves and branches but low in stem and coarse roots. The percentage of N fixed from atmosphere out of total tree N (%N-f) in G. sepium varied 56-74%; N-2 fixation was more active in July (the rainiest season) than in March (the relatively dry season). The variation of delta N-15 between organs in I. edulis was probably associated to N-15 fractionation in leaves. Stem and coarse root delta N-15 was assumed to reflect the actual ratio of N-2 fixation to soil N uptake; stem-based estimates of %N-f in I. edulis were 48-63%. Theobroma cacao below I. edulis had lower delta N-15 than T. cacao below mixed-species shade, which may indicate direct N transfer from I. edulis to T. cacao but results so far were inconclusive. Further research should address the N-15 fractionation in the studied species for improving the accuracy of the N transfer estimates. The delta N-15 appeared to vary according to ecophysiological characteristics of the trees.