Interplay of Phosphorus and Rhizobium Inoculation on Common Bean (Phaseolus vulgaris L.) Across Variable Indigenous Rhizobia Soils in Southern Ethiopia

Soil fertility constraints, particularly N and P deficiencies, limit the productivity of common bean (Phaseolus vulgaris L.) in Ethiopia. This study is the first to systematically evaluate the combined effects of phosphorus (P) fertilization and Rhizobium inoculation on biological nitrogen fixation (BNF) and yield across soils with varying levels of indigenous rhizobia populations in southern Ethiopia. The aim of the study was to evaluate the effects of P fertilization and Rhizobium inoculation on nodulation, growth, yield, nutrient concentration, and BNF in soils characterized by high, moderate, and low indigenous rhizobia populations. Field experiments were conducted over two years using four P rates (0, 10, 20, and 30 kg P ha−1) and five Rhizobium strains (uninoculated, 102CB, 106CB, 44CB, and HB-429). P application significantly improved nodulation, growth, yield, nutrient concentration, and N2 fixation, with 20 kg P ha−1 consistently resulting in superior performance. At this rate, grain yields reached 2.51, 2.25, and 2.31 t ha−1 in soils with high, moderate, and low indigenous rhizobia populations, respectively. Inoculation responses depended strongly on indigenous rhizobia abundance: in soils with high indigenous populations, inoculation did not significantly improve growth and yield, whereas in low-population soils, inoculation with strain 102CB produced the highest yield (2.06 t ha−1). For BNF, 20 kg P ha−1 resulted in the highest fixation (44.61 and 36.82 kg N ha−1) in soils with high and moderate indigenous rhizobia populations. Inoculation with 102CB further enhanced N fixation to 44.77 and 36.13 kg N ha−1 in these soils. In low-population soils, the combined application of 102CB and 20 kg P ha−1 significantly increased BNF to 55.34 kg N ha−1. Overall, these findings demonstrate that P fertilization provides universal benefits, while inoculation effectiveness is site-specific. Integrating P fertilization with effective Rhizobium strains offers a practical and sustainable strategy to improve common bean productivity and sustainability of common bean–based farming systems in Ethiopia.