Field assessment and genetic stability of Sinorhizobium fredii strain SMH12 for commercial soybean inoculants

The fast-grower Sinorhizobium fredii SMH12 has been evaluated as a soybean inoculant in alkaline soils (pH 8.2) of the Guadalquivir Valley. We also have investigated S. fredii SMH12 for the occurrence of changes after prolonged subculturing in tryptone-yeast extract media. Field experiments over three different seasons showed that soybean inoculated with S. fredii SMH12 produced seed yields that were not different from those produced by soybeans inoculated with Bradyrhizobium japonicum USDA110, a highly effective soybean inoculant. Soybeans inoculated with a mixture of S. fredii SMH12 and B. japonicum USDA110 formed nodules that only contained the S. fredii co-inoculant. All the analysed bacterial traits remained unmodified after 500 bacterial generations. These results indicate that S. fredii SMH12 appears to be genetically stable and that, at least in the soils tested, is a valuable soybean inoculant.