APPRAISAL OF THE N-15 NATURAL-ABUNDANCE METHOD FOR QUANTIFYING DINITROGEN FIXATION

Several investigators have questioned the use of the 15N natural-abundance method of estimating N2 fixation because of variability on soil δ15N and small differences between the δ15N of soil N and atmospheric N. Investigations were conducted to compare the 15N natural-abundance and 15N-isotope-dilution methods for estimating N2 fixation of field-grown pea (Pisum sativum L.) and lentil (Lens culinaris Medik.). Spatial variability was assessed at three sites by determining the δ15N of non-N2-fixing plants. Estimates of N2 fixation were not significantly different in 18 out of 21 comparisons; in two comparisons the δ15N method and in one comparison the 15N-enriched method provided higher estimates of N2 fixation. Overall, both methods appeared to provide equally reliable estimates of N2 fixation for lentil.