Soil constraints limit the effectiveness of phosphate-solubilizing bacteria in acidic Lixisols of Burkina Faso

Phosphorus (P) deficiency limits crop productivity in weathered tropical soils of sub-Saharan Africa, particularly in Lixisols where P availability is low. Phosphate-solubilizing microorganisms may improve P use efficiency and support sustainable crop production. This study evaluated the phosphate-solubilizing and plant growth-promoting potential of microbial strains under Lixisol conditions. Six distinct strains with contrasting taxonomy and P-cycling traits were assessed for inorganic P solubilization and organic P mineralization in vitro using tricalcium phosphate, sodium phytate, and Burkina Faso phosphate rock (BPR). Their ability to mobilize soil P was tested in a pot experiment with cowpea grown in two contrasting Lixisols amended with BPR. In vitro P solubilization was associated with acidification and the presence of phosphate-cycling genes (gcd, pqqE, aphA, phnX). However, this potential translated poorly into increased soil P availability under greenhouse conditions. Combining phosphate-solubilizing bacteria with BPR showed little improvement over BPR alone, whereas triple superphosphate increased P availability and plant biomass. The limited effectiveness of bacterial inoculation was likely constrained by soil buffering capacity, low organic carbon, and microbial competition. These findings show that in vitro screening alone is insufficient to predict performance in weathered tropical soils and highlight the need for soil-adapted evaluation of P-solubilization strategies.