N2‐Fixation in Field Beans Quantified by 15N Isotope Dilution. I. Effect of Strains of Rhizobium phaseoli1

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Abstract
Most soils sown to field beans (Phaseolus vulgaris L.) contain indigenous Rhizobium phaseoli; thus, few studies have evaluated the effect of different strains of R. phaseoli on N2 fixation and yield of bean cultivars. Since western Canadian soils contain no indigenous R phaseoli, strains were screened for their ability to fix N2 in two bean cultivars, Aurora and Kentwood. Initially, the N2‐fixing ability of 19 strains of R phaseoli was determined in vitro. The best were then compared with 16 superior Centro Internacional de Agricultura Tropical strains in a 2‐ year International Bean Inoculation Trial in the field in association with the same two small white bean cultivars. 15N isotope dilution at levels of 15N natural abundance in the field showed the % Ndfa (percentage plant N derived from the atmosphere) averaged 50% for Aurora but only 32% for Kentwood over all 16 strains. The % Ndfa did not vary between the two experimental years, but due to different levels of soil N, yield and actual amounts of N2 fixed did vary. Aurora averaged 70 kg N ha−1 and Kentwood 37 kg N ha−1 fixed. Some strains fixed more than 100 kg N ha−1 in Aurora, resulting in dry matter and N yields in excess of uninoculated treatments receiving 40 or 100 kg fertilizer N ha−1. Strains of R phaseoli from high temperature soils of Latin America fixed high amounts of N2 in association with temperate bean cultivars under conditions of low temperature growth (10 to 23°C). CIAT strains 632,899, 640, and 904 and commercial Nitragin were highly efficient N2 fixers with Aurora as were CIAT strains 952, 161, 166, and commercial Nitragin with Kentwood. Thus, R. phuseoli are as efficient as other rhizobia in supplying fixed N2 to their host plant and, in N2‐fixing mode, certain bean cultivars can meet their genetic yield potential in the field without the addition of fertilizer N.