Appraisal of the Nitrogen‐15 Natural‐Abundance Method for Quantifying Dinitrogen Fixation

Several investigators have questioned the use of the 15 N natural‐abundance method of estimating N 2 fixation because of variability in soil δ 15 N and small differences between the δ 15 N of soil N and atmospheric N. Investigations were conducted to compare the 15 N natural‐abundance and 15 N‐isotope‐dilution methods for estimating N 2 fixation of field‐grown pea ( Pisum sativum L.) and lentil ( Lens culinaris Medik.). Spatial variability was assessed at three sites by determining the δ 15 N of non‐N 2 ‐fixing plants. Seasonal variation in δ 15 N for spring and winter wheat ( Triticum aestivum L.), flax ( Linum usitatissimum L.), barley ( Hordeum vulgare L.), rape ( Brassica napus L.) and lentil was determined at one site. Comparisons between δ 15 N and 15 N‐enriched isotope‐dilution methods for estimating N 2 fixation by lentil were conducted at several sites over a 3‐yr period. Variability in δ 15 N of the reference plant was site dependent: the δ 15 N ranged from 2.8 to 9.3 at the first site, 3.4 to 8.8 at the second site, and 3.5 to 6.2 at the third site. The average δ 15 N of four of the five non‐N 2 ‐fixing plants increased from 5.4 at 42 d after planting to 6.9 at the final harvest. The fifth non‐N 2 ‐fixing plant, rape, accumulated most of its N during the first 42 d after planting, and its δ 15 N value declined from 8.1 at 42 d after planting to 7.3 at the final harvest. The δ 15 N values for lentil were similar at 42 and 63 d after planting to the δ 15 N values of the four reference plants, but did not increase after 63 d. Estimates of N 2 fixation were not significantly different in 18 out of 21 comparisons; in two comparisons the δ 15 N method and in one comparison the 15 N‐enriched method provided higher estimates of N 2 fixation. Overall, both methods appeared to provide equally reliable estimates of N 2 fixation for lentil.