Use of Sinorhizobium (Ensifer) fredii for soybean inoculants in South Spain

The symbiotic potential of fast-growing soybean rhizobia isolated from diverse provinces of China, including S. fredii SMH12 from Vietnam, has been evaluated. All of them produced nitrogen-fixing nodules with the American soybean cultivar Osumi under greenhouse and field conditions. Two years of field experiments conducted in two localities in South Spain with alkaline and moderately acid pH soils showed that seed yields of soybean plants inoculated with fast-growing strains SMH12, WH1, WH8 and WW4 were similar or higher than those obtained with soybean inoculated with the reference slow-growing Bradyrhizobium japonicum strain USDA110. Competitiveness studies between S. fredii SMH12 and B. japonicum USDA110 demonstrated that soybean cultivar and pH of soil determined the outcome of the competition. Alkaline pH (7.8) favoured SMH12 to occupied nodules with American soybean cultivars while USDA110 was more competitive in soils with a moderately acid pH (6.6). The Asiatic cultivar Peking was almost exclusively nodulated by SMH12 in both types of soils. On the other hand, the percentage of nodules occupied by fast-growing soybean inoculants applied into a soil containing indigenous Bradyrhizobium population of 4 × 103 rhizobia/g was inferior to 18%. SMH12 occupied a 7% and 17% of the nodules with Osumi and Kochi soybean cultivars, respectively.