Field‐determined estimates of N2 fixation by 15N isotope dilution have not been determined in irrigated annual grain legumes in North America. Nor does knowledge exist as to which nonfixing control plants are most appropriate for these grain legumes when using 15N isotope dilution methods. Within a crop species grown on two Typic Haploboroll soils for 2 yr, lentil (Lens culinaris Medik), fababean (Vicia faba L. minor), and pea (Pisum sativum L.) cultivars adapted to western Canada did not differ in their ability to benefit from symbiotic N2 fixation. When inoculated, N2 fixed averaged 176,84, 216, and 185 kg N ha−1 for chickpea (Cicer arietinum L.), lentil, fababean, and pea, respectively. The percent plant N derived from the atmosphere averaged 82, 67, 85, and 79%, respectively, for the same crops. Response to inoculation was dependent on the crop even though all were infected by Rhizobium leguminosarum Uninoculated chickpea had no nodules or N2‐fixing activity. Inoculation increased N2 fixation in fababean by 19 to 67% and in lentil by 5 to 16% depending on the site. There was a response to inoculation in pea at only one of the two sites. The presence of indigenous soil R. leguminosarum precluded the use of uninoculated treatments as nonfixing controls (except for chickpea) for estimating N2 fixation by 15N isotope dilution. Barley (Hordeum vulgare L. ‘Galt’) or wheat (Triticum aestivum L. emend. Thell ‘Columbus’) appeared to be appropriate control plants for these grain legumes. Because the 15N‐determined fertilizer use efficiency of these controls was similar to that of the N2‐fixing legumes, estimates of N2 fixation by N balance were not significantly different from those obtained by 15N isotope dilution.
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Rennie et al. (1986) studied this question.