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

Abstract

Several investigators have questioned the use of the 15N natural-abundance method of estimating N2 fixation because of variability in 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. Seasonal variation in δ15N 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 δ15N and 15N-enriched isotope-dilution methods for estimating N2 fixation by lentil were conducted at several sites over a 3-yr period. Variability in δ15N of the reference plant was site dependent: the δ15N 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 δ15N of four of the five non-N2-fixing plants increased from 5.4 at 42 d after planting to 6.9 at the final harvest. The fifth non-N2-fixing plant, rape, accumulated most of its N during the first 42 d after planting, and its δ15N value declined from 8.1 at 42 d after planting to 7.3 at the final harvest. The δ15N values for lentil were similar at 42 and 63 d after planting to the δ15N values of the four reference plants, but did not increase after 63 d. 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.