Native rhizobia nodulating soybean (Glycine max (L.) Merr.) performs better than commercial strain across locations...

Soybean (Glycine max (L.) Merr.) is one of the important legume crops, rich in protein, vegetable oil, maintain soil fertility and used as human food and livestock feed. Despite its importance, adoption and productivity of soybean in South Ethiopia is limited by poor agricultural practices and poor access to inputs like biofertilizers. This work aimed at trapping and screening elite rhizobia strains to use as inoculants in farmers' fields for improved soil health and crop productivity. Forty-four rhizobia strains isolated from different locations in South Ethiopia region were screened along with a commercial inoculant Bradyrhizobium japonicum strain (obtained from Menagesha Biotech Industry, Addis Ababa) in greenhouse using modified Leonard jars (MLJ). Three top performing rhizobia strains SB19, SB22, and SB24 based on the MLJ experiment and the commercial inoculant were further evaluated in farmers' fields at different locations with varying eco-physiological conditions because environmental factors differ from place to place, affecting how living things grow, survive and function. MLJ experiment revealed that the new isolates SB19, SB22, and SB24 had significantly higher relative symbiotic effectiveness (SE%) (p < 0.05) than un-inoculated and N-fertilized control plants. Except SB19, the top performing strains did not differ from the commercial strains regarding SE%. Strain SB19 produced 34 to 61 number of nodules, while the commercial strain induced 22 to 49 nodules. In the first field experiment (2023) inoculation of soybean with SB19 resulted in average grain yields of 3.1 and 2.92 tons ha⁻¹ at Arba Minch University (AMU) demo farm and Abaya campus experimental sites to be consistent with, respectively. In experiment 2 (2024), SB19 strain resulted in the average grain yields of 2.39 and 2.45 tons ha^-1 at Abaya and AMU demo sites, respectively. Across all locations, the commercial strain produced an average yield of 2.25 to 2.40 tons ha⁻¹, which was significantly lower (p < 0.05) than that of the native strains, but higher than the control plants, which yielded 1.87 to 2.02 tons ha⁻¹. Among the evaluated strains, SB19 consistently exhibited the most promising performance across all fields and locations compared to the commercial one and others. This finding highlights the presence of highly effective, locally adapted rhizobial strains capable of nodulating soybean in South Ethiopian soils, and demonstrate their promise for selection and improvement into superior inoculant strains to enhance soybean productivity.