Phosphorus Availability Modulates Bradyrhizobium japonicum’s Impact on Soil Micribial Biomass and Soybean (Glycine max (L.) Merrill) Yield in Lubumbashi Tropical Soils

Tropical soils in sub-Saharan Africa exhibit low fertility, high acidity, and limited Phosphorus (P) availability, a key nutrient for agricultural productivity and the sustainability of cropping systems. This deficiency, exacerbated by the strong fixation of P in acidic soils, not only limits plant growth but also the efficiency of the nitrogen-fixing symbiosis between soybeans (Glycine max (L.) Merrill) and Bradyrhizobium Japonicum. Phosphorus indeed plays a central role in nodulation, atmospheric nitrogen fixation, and soil microbial activity. This study was conducted in the tropical soils of Lubumbashi (DRC) to evaluate the effect of different levels of P availability on soil microbial biomass and the yield of soybeans inoculated with B. japonicum. An experimental split-plot design was implemented: Three strains of Bradyrhizobium japonicum, an uninoculated control, and five soybean varieties (Glycine max (L.) Merrill), each repeated three times, were established at each site. Measurements of microbial biomass, nodulation, symbiotic efficiency, and grain yield were taken. The results show that increased P availability significantly stimulates soil microbial biomass, improves nodulation, and increases the efficiency of biological nitrogen fixation, resulting in higher soybean yields. These observations confirm the importance of optimal P management to maximize the agronomic and microbiological benefits of inoculation with B. japonicum in tropical agroecosystems with acidic soils.