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

Pekka Nygren, Humberto A. Leblanc
2008 Agroforestry for Commodity Production: Ecological and Social Dimensions  
Natural abundance of 15 N 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 nonlegumes, in a cacao plantation with mixed-species shade (legume Gliricidia sepium and several non-legumes), and in a tree hedgerow bordering the plantations in Guácimo, in the humid Caribbean lowlands of Costa Rica. The deviation of the sample 15 N proportion from that of atmosphere (d 15 N) was
more » ... lar in non-legumes Cordia alliodora, Posoqueria latifolia, Rollinia pittieri, and T. cacao. Deep-rooted Hieronyma alchorneoides had lower d 15 N than other non-N 2 -fixers, which probably reflected uptake from a partially different soil N pool. Gliricidia sepium had low d 15 N. Inga edulis had high d 15 N 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 d 15 N between organs in I. edulis was probably associated to 15 N fractionation in leaves. Stem and coarse root d 15 N 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 d 15 N 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 15 N fractionation in the studied species for improving the accuracy of the N transfer estimates. The d 15 N appeared to vary according to ecophysiological characteristics of the trees.
doi:10.1007/978-90-481-3365-9_5 fatcat:bwaso5ejqba3vhawatldvmtfau