Nitrogen‐15‐Determined Nitrogen Fixation in Field‐Grown Chickpea, Lentil, Fababean, and Field Pea 1

Field‐determined estimates of N 2 fixation by 15 N 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 15 N 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 N 2 fixation. When inoculated, N 2 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 N 2 ‐fixing activity. Inoculation increased N 2 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 N 2 fixation by 15 N 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 15 N‐determined fertilizer use efficiency of these controls was similar to that of the N 2 ‐fixing legumes, estimates of N 2 fixation by N balance were not significantly different from those obtained by 15 N isotope dilution.