Persistence and Recovery of <i>Rhizobium japonicum</i> Inoculum in a Field Soil¹

Abstract

Abstract Inoculation of soybean ( Glycine max L.) rarely increases seed yields in fields where this crop has been previously grown. The inability of the inoculum strain to persist in soil could be a factor in this poor response. The persistence of the inoculant strain Rhizobium japonicum USDA 110 was followed for 56 weeks in a field soil (Typic Haplaquoll) using a fluorescent antibody technique. Statistically significant increases in the population of R. japonicum strain USDA 110 in soil were observed for the first 7 weeks after inoculation. When USDA 110 was applied at 2 × 10 6 cells g −1 soil the population of this organism in soil reached 1 × 10 7 cells g −1 soil 1 week after inoculation; a value significantly higher than observed in other inoculation and fertilization treatments. The population observed for the uninoculated control varied nonsignificantly from 6 × 10 4 to 2.6 × 10 5 cells g −1 during both growing seasons. No significant differences due to inoculation were observed for seed yield, or total aboveground N in either year. Even though the soil population was increased significantly, inoculation of strain USDA 110 at high rates produced no significant alteration in strain distribution in the nodules of soybeans in either year. Rhizobium japonicum strain USDA 110 was observed mostly in doubly infected nodules with strain USDA 123 which may have minimized the effect of strain USDA 110 on N 2 fixation and seed yield.