title: Dinitrogen fixation by legume shade trees and direct transfer of fixed N to associated cacao in a tropical agroforestry system.
abstract: Natural abundance of (15)N (δ (15)N) 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 N2 fixation and direct N transfer calculations. Bulk and rhizospheric soils contained 72 and 20%, respectively, of whole- system N. No vegetation effect on δ (15)N in rhizospheric soil was detected, probably due to the high native soil N pool. Fine roots of the cacaos associated with inga contained ∼35% of N fixed from the atmosphere (Nf) out of the total N. Leaves of all species had significantly higher δ (15)N than fine roots. Twenty percent of system Nf 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 [Nf] ha(-1) during the 14-year history of the plantation. The conservative estimate of current N2 fixation rate was 41 kg [Nf] ha(-1) year(-1) based on inga biomass only and 50 kg [Nf] ha(-1) year(-1) based on inga and associated trees.
authors.affiliation: Department of Forest Science PO Box 27, 00014 University of Helsinki Finland Current address: Finnish Society of Forest Science, PO Box 18, 01301 Vantaa Finland pekka.nygren@luke.fi.
authors.forename: Pekka
authors.initials: P
authors.lastname: Nygren
authors.affiliation: EARTH University Apartado 4442-1000, San José Costa Rica.
authors.forename: Humberto A
authors.initials: HA
authors.lastname: Leblanc
mesh_terms: D018533:Biomass; D002099:Cacao / Q000378:metabolism*; D007887:Fabaceae / Q000378:metabolism*; D065928:Forests; D038821:Mycorrhizae; D009584:Nitrogen / Q000378:metabolism*; D009586:Nitrogen Fixation; D009587:Nitrogen Isotopes / Q000378:metabolism; D053147:Plant Exudates / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism; D018517:Plant Roots / Q000378:metabolism*; D012987:Soil / Q000737:chemistry; D012988:Soil Microbiology; D014197:Trees / Q000378:metabolism; D014329:Tropical Climate
chemical_list: D009587:Nitrogen Isotopes; D053147:Plant Exudates; D012987:Soil; D009584:Nitrogen
keywords: 15N natural abundance; Inga edulis; N cycle; Theobroma cacao; isotopic fractionation; rhizosphere