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

Dep. of Soil Science, Univ. of Saskatchewan, Saskatoon, SK, Canada S7N 0W0

* Corresponding author.

ABSTRACT

Several investigators have questioned the use of the ¹⁵N natural-abundance method of estimating N₂ fixation because of variability in soil ¹⁵N and small differences between the ¹⁵N 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 ¹⁵N of non-N₂-fixing plants. Seasonal variation in ¹⁵N 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 ¹⁵N and ¹⁵N-enriched isotope-dilution methods for estimating N₂ fixation by lentil were conducted at several sites over a 3-yr period. Variability in ¹⁵N of the reference plant was site dependent: the ¹⁵N 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 ¹⁵N 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 ¹⁵N value declined from 8.1 at 42 d after planting to 7.3 at the final harvest. The ¹⁵N values for lentil were similar at 42 and 63 d after planting to the ¹⁵N 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 ¹⁵N 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.

S.I.P. no. 618.

Received for publication July 8, 1989.

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