COMPETITION FOR NODULATION OF SOYBEAN BY BRADYRHIZOBIUM-JAPONICUM 123 AND 138 IN SOIL CONTAINING INDIGENOUS RHIZOBIA

We characterized field isolates of Bradyrhizobium japonicum from soybeans grown in Missouri soil and measured competitiveness of peat-formulated B. japonicum 123 (serogroup 123) and 138 (serogroup c1) in this soil. Nodule occupancy by serogroup 123, but not serogroup c1, varied with plant age in uninoculated. field-grown soybeans. Serogroups c1 and 38-115 were found in 37% of all nodules of these plants. Serogroups 76, 122, 123. c3 and 94 were also present, and rhizobia of unknown serology occupied 17% of the nodules. Inoculation with either strain 123 or 138 produced similar numbers of nodules. Strain 123 as an inoculant increased nodule occupancy by serogroup 123 13-fold, while a 2-fold increase in serogroup c1 nodule occupancy was obtained with strain 138 inoculation. Increases in nodule occupancy by the inoculant was at the expense of serogroups other than 123 and c1. Inoculation of either strain resulted in increased nodule occupancy in first-formed nodules in unsterile soil. Dual inoculation with both strains in unsterile soil allowed serogroup c1 to occupy twice the number of nodules as did serogroup 123. In sterilized soil, strain 123 outcompetcd strain 138, occupying 74% of all nodules. The effect of inoculant carrier on competitiveness is discussed. © 1990.