| Abstrakt: |
Competitive indigenous strains of Bradyrhizobium japonicumoccupy many soils of the Midwest, but they are generally considered to be less efficient in fixing N2than many strains added as inoculants. Inoculant strains will not improve N2fixation in soybean [Glycine max(L.) Merr.], however, unless they are able to form nodules on the host plant. This research evaluated whether strains selected for competitiveness in the greenhouse could compete against indigenous bradyrhizobia in the field in forming nodules when added at rates used for commercial inoculants, and whether higher nodule occupancy resulted in improved plant growth. Plots were established in north‐central Iowa at seven sites on four representative soil series. Seven to 12 strains were introduced in two or three soils each year from 1983 to 1985 at approximately 5.8 log10viable cells per centimeter of row as granular‐peat inoculants. Results showed that average strain recoveries ranged from 0 to 46%. Some introduced strains differed in their competitive ability among the sites tested. The most competitive strains were AN8, AN10, AN11, AN23, AN27, and AN30, which typically occupied at least 20% of the nodules at a given site. When considering all members of each serogroup tested, strains from serogroup 123 were the best competitors. Increased nodule occupancy by the introduced strains did not result in increased plant growth or yield. Nodulating and nonnodulating ‘Clark’ isolines and urea fertilizer treatments were used to evaluate contributions from N2fixation and plant growth with added N fertilizer. |
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