Trenching: A Necessity for Assessment of N ² Fixation in Field Grown Faidherbia albida Using N-Enrichment Methodology

Abstract

In Senegal at Nioro experimental station the 15 N 2 isotope dilution technique was used at field conditions for estimating fixation of N 2 in Faidherbia albida, mono cultured and mixed with the reference species, Parkinsonia aculeata. Trenched and nontrenched plots within the plantation were labeled with 15 N 2 - ammonium sulphate containing 2 atom % 15 N excess at the equivalent amount of 20 kg N ha -1. The proportion of fixed N 2 was lower than that previously measured in experiments using Parkia biglobosa as a reference tree species: 20.3% vs. 54.7%. This difference might be due to physiological differences between the two reference species. Significant effect of trenching vs. nontrenching to the labeled area was observed on the percentage of N 2 derived from atmosphere (% Ndfa) of F. albida. The % Ndfa was higher in trenched than in nontrenched labeled area (+ 34.3%). There was no difference however in the amount of N 2 fixed in trenched and nontrenched plots although the estimate in the former (6.70 2 g N plant -1) was slightly higher than that in the later (5.60 g N plant -1). Thus trenching the labeled area seems to be necessary for estimating the N 2 fixed by F. albida in the field.