Abundance and Symbiotic Efficiency of Indigenous Rhizobia Nodulating Faba Bean and Common Bean in Southern Ethiopia

Tadele Geremu^1,2*Girma Abera³Bekele Lemma⁴Frank Rasche^5,6

• ¹Department of Plant and Horticulture Science, College of Agriculture, Hawassa University, Hawassa, Ethiopia
• ²Natural Resource Research Directorate, Oromia Agricultural Research Institute, Addis Ababa, Ethiopia
• ³Natural Resources Development and Climate Program, Ethiopian Agricultural Transformation Institute (ATI), Addis Ababa, Ethiopia
• ⁴Chemistry Department, College of Natural science, Hawassa University, Hawassa, Ethiopia
• ⁵International Institute of Tropical Agriculture (IITA), Nairobi, Kenya
• ⁶Institute of Agricultural Sciences in the Tropics (Hans-Ruthenberg-Institute), University of Hohenheim, Stuttgart, Germany

The symbiotic association between legumes and indigenous rhizobia is crucial for enhancing legume productivity. However, inconsistent results and suboptimal performance of rhizobia inoculation in promoting legume nodulation have been observed. In this regard, we assessed the abundance and symbiotic efficiency of indigenous rhizobia nodulating faba bean and common bean, as well as the soil factors affecting rhizobia abundance in southern Ethiopia. The study also compared the performance of indigenous rhizobia with commercial strains and mineral nitrogen treatment plants. A total of 132 soil samples were collected from barley, wheat, maize, potato, common bean, faba bean, intercropped common bean and maize, enset, and grazing land. Indigenous rhizobia was isolated and enumerated from these samples. Faba bean (FB) and common bean (CB) rhizobia population ranged from 0.0 to 1.7 x 10 4 and 1.7 x 10 1 to 1.7 x 10 7 cells g -1 soil, respectively. Rhizobia populations were significantly influenced by soil pH, EC, OC, TN, CEC, exchangeable acidity, aluminium, and the host crop occurrence. The isolated indigenous rhizobia demonstrated significant potential in enhancing nodulation, shoot dry weight, and TN accumulation in plants. Symbiotic efficiency indices revealed that over 95% of the indigenous rhizobia were effective in nodulation and shoot dry matter accumulation, indicating that naturally occurring rhizobia are efficient and may reduce the need for commercial inoculants in areas with abundant indigenous populations. However, in areas where rhizobia populations are low, strains isolated from faba bean (33FB, 84FB) and common bean (44CB, 102CB), which outperformed commercial strains should be further evaluated. The results suggest that soil rhizobia population levels should be assessed prior to inoculation to optimize nodulation and crop performance. To this end, it is emphasized to evaluate soil rhizobia strains to assess their stability and competitiveness relative to commercial inoculants across different agroecological conditions.