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.

Adapted cultivars must be developed if significant production of soybeans [Glycine max (L.) Merr.] is to be realized in western Canada. Selection and breeding for ability to support symbiotic N2 fixation is important in this development. 15N isotope dilution techniques were used to determine the percent plant N derived from the atmosphere (%Ndfa), i.e., fixation, and actual amounts of N fixed by two promising cultivars, X005 and Maple Presto. Precent Ndfa for the two cultivars was almost identical under comparable conditions. Depending on soil type and plant growth stage, %Ndfa varied from 38 to 70% in lysimeter experiments and maximized at 67% in the field. X005 fixed significantly more N2 (115 kg N/ha) than Maple Presto (82 kg N/ha) in the field because X005 had a higher N yield, possibly due to a slightly longer growing season. The N yield of both cultivars, when inoculated, did not respond to increasing rates of fertilizer N [Ca (NO3)2]. The highest percent fertilizer use efficiency (%FUE) was 51% when uninoculated and 44% when the seed was inoculated. Although N2 fixation occurred in both cultivars when fertilizer up to 160 kg N/ha was provided, the cultivars differed in their tolerance to fertilizer N with respect to N2 fixation. For X005, N2 fixation was constant as N fertilizer increased from 0 to 80 kg/ha but decreased by 24% where 160 kg N/ha was applied. For Maple Presto, N2 fixation was constant as N fertilizer increased from 0 to 40 kg/ha but decreased by 21% where 80 or 160 kg N/ha was applied. Both cultivars had similar %Ndfa and amount of N fixed/ha to soybeans in other countries.

Early maturing soybean cultivars (groups 00 and 000) adapted to western Canada are now available but since the soils contain no indigenous Rhizobium japonicum, it is important to determine if strains selected for efficient N2fixation with American soybean cultivars are also efficient with Canadian cultivars and if single strain are superior to multistrain inoculants. Accordingly, a 2‐year evaluation was made of the effect of up to eight single strains and two commercial multistrain combinations of R. japonicumgranular inoculants on maturity, seed yield, oil and protein content, and N2fixation (as determined for the first time by 15N isotope dilution) for three promising soybean lines. Rhizobium japonicumstrains 61A148 (USDA 142) and 61A118 (USDA 138) were consistently superior in N2fixation with all soybean cultivars, resulting in seed yields and seed protein higher than those of the other strains. Strain 61A101 was consistently an inefficient N2fixer. These differences in efficiency between strains were not related to documented ability to take up H2evolved by nitrogenase because, although strain 61A148 is considered Hup+, strain 61A118 is Hup–. Several strains resulted in percent plant N derived from the atmosphere exceeding 50% with N2fixed as high as 151 kg N ha−1for strain 61A148 on King Grain line X005. However, over all strains, Maple Amber was superior for supporting N2fixation, averaging 91 kg while X005 had 83 kg and Maple Presto 60 kg N fixed ha−1. Line strain combinations resulting in lower levels of N2fixation assimilated more soil N such that total N yields averaged over lines and strains were generally similar. These strains selected for U. S. soybean cultivars are also acceptable for Canadian soybeans. Interstrain competition occurred with one multistrain inoculant, to the detriment of plant yield. A commercial mix containing four strains (61A101, 61A118, 61A124, 61A148) had low N2fixation similar to that of strain 61A101. This suggested that strain 61A101 was highly competitive and may have formed a majority of the nodules thereby excluding the highly efficient strains 61A118 and 61A148. However, a new commercial mix containing three strains which were clones of 61A101, 61A118, and 61A124 did not exhibit any detrimental interstrain competition. Thus, the new multistrain soybean inoculant can be recommended for Canadian soybean cultivars.

Growing soybeans [Glycine max(L.) Merr.] under irrigation requires the use of herbicides and seed‐applied fungicides to control weeds and root diseases. Soils in southern Alberta contain no indigenous Rhizobium japonicumand thus inoculation is essential to the establishment of a successful N2‐fixing symbiosis in soybeans. Unfortunately, these chemicals may be incompatible with survival of the inoculated rhizobia. Accordingly, an evaluation was made of the effects of three seed‐applied fungicides, captan {N‐[{(trichloromethyl) thio]‐4‐cyclohexene‐l,2‐dicarboximidef}, thiram [bis(dimethylthiocarbamoyl)disulfide], and carbathiin (5,6,‐dihydro‐ 2‐methyl‐l,4‐oxathiin‐3‐carboxanilide); four herbicides applied as preplant incorporated, chloramben (3‐amino‐2,5‐dichlorobenzoic acid), linuron [3‐(3,4‐dichIorophenyl)‐l‐methoxy‐l‐methylurea], metribuzin [4‐amino‐6‐tert‐butyl‐3‐{methylthio)‐as‐triazin‐5(4H)‐on], and trifluralin (α,α,α‐trifluoro‐2,6‐dinitro‐N,N‐dipropyl‐p‐toluidine); and one post‐emergence herbicide, diclofop {2‐[4‐(2,4‐dichlorophenoxy) phenoxy]propanoic acid}, on shoot N, seed yield, nodulation, acetylene‐reducing activity (ARA), and 15N‐determined N2fixation of soybean line ‘King Grain X005’ inoculated with granular rhizobial inoculant. The soil was an irrigated Chin loam (Typic Haploboroll). None of the fungicides or herbicides affected seed yield. Thiram and captan applied at 1.0 g kg−1seed reduced shoot N yield at anthesis but a lower rate of thiram (0.54 g kg−1seed) increased N yield. None of the fungicides affected ARA. Metribuzin applied at recommended rates caused erratic germination. Linuron significantly reduced ARA whereas trifluralin stimulated it in 1 year but had no effect in the other year. This alteration in ARA was not reflected in N or seed yield. 15N isotope dilution at levels of 15N natural abundance showed the plant received approximately 50% of its N from N2fixation at anthesis, and in agreement with ARA, showed no effect of chemical when proper rates were applied. We conclude that these herbicides and seed‐applied fungicides have no effect on nodulation and N] fixation under these experimental conditions when granular inoculant is applied.

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