Recovery of Introduced Rhizobium japonicum Strains by Soybean Genotypes

Soybean [Glycine max (L.) Merr.] seed yields and seed protein percentages have been increased significantly with inoculation where soybeans were not grown previously. However, when the soil contains Rhizobium japonicum from previous inoculation, significant differences in seed yield are usually not obtained from inoculation. Generally, about 5 to 10% of the nodules are produced by the inoculant applied to the seed with the other 90 to 95% of the nodules produced by R. japonicum already present in the soil. Field studies were used to identify soybean genotype-it, japonicum strain combinations which might enhance soybean yields through more effective biological N₂ fixation. Soil types in the experiments were Waukegan silt loam (Typic Hapludoll), Nicollet silty clay loam (Aquic Hapludoll) and Webster silty clay loam (Typic Haplaquoll). The first approach involved screening soybean genotypes for their ability to recover introduced R. japonicum strains from a mixed indigenous soil population. The second approach involved evaluating over 1,600 soybean lines for their degree of nodulation (relative to commonly-grown cultivars) with the indigenous strains of R. japonicum. Twenty-four soybean lines were selected from the 1,600 for further study in yield trials. Some lines were better nodulated than the commonly-grown cultivars while other lines were very poorly nodulated. High recoveries of inoculant strains usually produced positive yield responses. Two of the tested soybean lines (PI 358.319 and PI 70.017) recovered an introduced strain (R. japonicum USDA 110) at enhanced levels (as high as 55% of the plant nodules were from the inoculated strain) and responded with increased seed yield at both test locations. The seed yields of PI 358.319 and PI 70.017 were increased by as much as 437 and 459 kg/ha, respectively, while the seed yields of commonlygrown cultivars were not increased. Recovery of inoculant strains was found to be independent of the soybean genotype's ability to nodulate with the indigenous R. japonicum strains. Generalizations on the recovery and effectiveness of introduced R. japonicum strains could not be made based on the soybean line's ability to nodulate with the indigenous R. japonicum strains. Each soybean genotype must be examined individually for nodulation with indigeous R. japonicum, for recovery of introduced strains, and for the general effectiveness with introduced strains to increase seed yields.