Novel phosphate-solubilizing bacteria enhance phosphorus uptake and growth in Populus nigra under P-deficient soils

Abstract Aims Phosphorus (P) is an essential macronutrient, but in many alkaline and acidic soils it is immobilized in insoluble forms, restricting plant uptake. Such limitations are particularly problematic for Populus nigra, a fast-growing tree widely used in forestry and ecological restoration. This study aimed to assess whether novel phosphate-solubilizing bacteria (PSB) can enhance P availability, nutrient uptake, and growth performance of P. nigra Clone 154.62 under P-deficient conditions. Methods and results Three PSB strains isolated from black poplar rhizospheres—Enterobacter hormaechei subsp. xiangfangensis FREX4, Pantoea dispersa FRPD12, and Pantoea stewartii subsp. stewartii FRPS6—were evaluated through in vitro solubilization assays, biosafety tests, and outdoor inoculation trials. FREX4 and FRPD12 efficiently solubilized tricalcium phosphate (355 and 368.1 µg mL−1), while FRPS6 showed higher activity on hydroxyapatite (66.5 µg mL−1). Inoculation significantly improved seedling height, biomass, and chlorophyll content, with FREX4 consistently producing the greatest effects. Plant P concentration nearly doubled relative to the control (0.39% vs. 0.21%), and available soil P increased from 3.66 to 12.8 mg kg−1. Root colonization assays confirmed superior rhizosphere competence of FREX4. Biosafety analyses indicated that FREX4 and FRPD12 were safe for application, whereas FRPS6 retained virulence traits despite its solubilization ability. Conclusions PSB inoculation improved P acquisition, plant growth, and soil fertility, with FREX4 identified as the most effective and safe biofertilizer candidate for forestry use.