Abundance of Indigenous Soybean-Nodulating Rhizobia in Relation to Soil Properties and Cropping Patterns in a Midland Agro-Ecology of Southern Ethiopia

Estimating indigenous rhizobial populations is crucial for understanding soil rhizobia abundance, determining the potential need for inoculation, and evaluating the performance of introduced inoculant strains. However, in southern Ethiopia, information on the population abundance of soybean-nodulating rhizobia is limited. To address this gap, the present study was conducted to evaluate the population abundance of indigenous soybean-nodulating rhizobia and to assess the influence of cropping history and soil properties on rhizobial abundance. The study was conducted across five sites suitable for soybean cultivation in southern Ethiopia: Arsi-Negelle, Boricha, Dore, Hawassa, and Wondo Genet. The study sites represented a range of cropping systems, including sole maize, sole tobacco, sole haricot bean, maize–potato intercropping, and crop rotation. Composite soil samples were collected from a depth of 0–20 cm, and rhizobial abundance was determined using the most probable number (MPN) technique. Indigenous rhizobial populations ranged from 0 to 1.7 × 101 cells g−1 of dry soil. Overall, the population levels were low, suggesting that inoculation with effective rhizobial strains would likely improve nodulation and biological nitrogen fixation. Relatively higher rhizobial population densities were observed at Arsi-Negelle under haricot bean cropping history. Statistically significant positive correlations were found between rhizobial abundance and cation exchange capacity, organic carbon, and organic matter. In general, native rhizobial populations across all study locations were below levels considered sufficient to support effective soybean nodulation and nitrogen fixation, indicating the need for inoculation to enhance soybean productivity in the study areas.