Dinitrogen-fixation by three neotropical agroforestry tree species under semi-controlled field conditions

Abstract

Cultivating dinitrogen-fixing legume trees with crops in agroforestry is a relatively common N management practice in the Neotropics. The objective of this study was to assess the N2 fixation potential of three important Neotropical agroforestry tree species, Erythrina poeppigiana, Erythrina fusca, and Inga edulis, under semi-controlled field conditions. The study was conducted in the humid tropical climate of the Caribbean coastal plain of Costa Rica. In 2002, seedlings of I. edulis and Vochysia guatemalensis were planted in one-meter-deep open-ended plastic cylinders buried in soil within hedgerows of the same species. Overall tree spacing was 1 × 4 m to simulate a typical alley-cropping design. The 15N was applied as (NH4)2SO4 at 10% 15N atom excess 15 days after planting at the rate of 20 kg [N] ha−1. In 2003, seedlings of E. poeppigiana, E. fusca, and V. guatemalensis were planted in the same field using the existing cylinders. The 15N application was repeated at the rate of 20 kg [N] ha−1 15 days after planting and 10 kg [N] ha−1 was added three months after planting. Trees were harvested 9 months after planting in both years. The 15N content of leaves, branches, stems, and roots was determined by mass spectrometry. The percentage of atmospheric N fixed out of total N (%Nf) was calculated based on 15N atom excess in leaves or total biomass. The difference between the two calculation methods was insignificant for all species. Sixty percent of I. edulis trees fixed N2; %Nf was 57% for the N2-fixing trees. Biomass production and N yield were similar in N2-fixing and non-N2-fixing I. edulis. No obvious cause was found for why not all I. edulis trees fixed N2. All E. poeppigiana and E. fusca trees fixed N2; %Nf was ca. 59% and 64%, respectively. These data were extrapolated to typical agroforestry systems using published data on N recycling by the studied species. Inga edulis may recycle ca. 100 kg ha−1 a−1 of N fixed from atmosphere to soil if only 60% of trees fix N2, E. poeppigiana 60–160 kg ha−1 a−1, and E. fuscaca. 80 kg ha−1 a−1.