Trenching: A necessity for assessment of N-2 fixation in field grown Faidherbia albida using N-15-enrichment methodology

In Senegal at Nioro experimental station the N-15 isotope dilution techniqu e was used at field conditions for estimating fixation of N-2 in Faidherbia albida, monocultured and mixed with the reference species, Parkinsonia acu leata. Trenched and nontrenched plots within the plantation were labeled wi th N-15-ammonium sulphate containing 2 atom% N-15 excess at the equivalent amount of 20 kg N ha(-1). The proportion of fixed N-2 was lower than that p reviously measured in experiments using Parkia biglobosa as a reference tre e species : 20.3% vs. 54.7%. This difference might be due to physiological differences between the two reference species. Significant effect of trench ing vs. nontrenching to the labeled area was observed on the percentage of N-2 derived from atmosphere (%N dfa) of F. albida. The %N dfa was higher in trenched than in nontrenched labeled area (+ 34.3%). There was no differen ce however in the amount of N2 fixed in trenched and nontrenched plots alth ough the estimate in the former (6.70 g N plant(-1)) was slightly higher th an 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 fiel d.