Dinitrogen fixation by legume shade trees and direct transfer of fixed N to associated cacao in a tropical agroforestry system

Natural abundance of N-15 (delta N-15) was determined in bulk soil, rhizospheric soil and vegetation in an organically managed cacao (Theobroma cacao L.) plantation with Inga edulis Mart. legume trees (inga) as the principal shade for studying the nitrogen (N) cycle in the system. Cacao without contact with legumes in an adjacent plantation was used as the reference for N-2 fixation and direct N transfer calculations. Bulk and rhizospheric soils contained 72 and 20%, respectively, of whole-system N. No vegetation effect on delta N-15 in rhizospheric soil was detected, probably due to the high native soil N pool. Fine roots of the cacaos associated with inga contained similar to 35% of N fixed from the atmosphere (N-f) out of the total N. Leaves of all species had significantly higher delta N-15 than fine roots. Twenty percent of system N-f was found in cacao suggesting direct N transfer from inga via a common mycelial network of mycorrhizal fungi or recycling of N-rich root exudates of inga. Inga had accumulated 98 kg [N-f] ha(-1) during the 14-year history of the plantation. The conservative estimate of current N-2 fixation rate was 41 kg [N-f] ha(-1) year(-1) based on inga biomass only and 50 kg [N-f] ha(-1) year(-1) based on inga and associated trees.