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

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.

Key Words: Dinitrogen fixation • Acetylene reduction

² Research scientists, Research Station, Agric. Canada, Lethbridge, Alberta, Canada T1J 4B1.

Received for publication August 17, 1982.