Bacillus subtilis Mobilizes Phosphorus by Reshaping P Pools, Sorption, and Enzyme Activities

Low phosphorus (P) availability constrains crop production in saline‐alkaline soils of Xinjiang, China. We tested whether Bacillus subtilis ( B. subtilis ) mobilizes soil P by altering P fractions, sorption–desorption, and P‐cycling enzymes in a saline‐alkaline cotton soil. Under conventional fertilization, B. subtilis was applied at 0 (B0), 15 (B1), 45 (B2), and 75 kg ha −1 (B3), with an additional no‐P treatment (CK). Soil P fractions were quantified by sequential extraction, sorption–desorption was assessed using Langmuir and Freundlich models, and soil organic matter, pH, electrical conductivity, and enzyme activities were measured; relationships and key drivers were explored using correlation and redundancy analysis. Stable P dominated the soil (88.12%–91.94% of total P). Relative to B0, inoculation increased labile and moderately labile inorganic P: H 2 O‐Pi and NaHCO 3 ‐Pi increased by 14.53%–74.36% and 33.56%–59.56%, respectively, and NaOH‐Pi increased by 5.08%–21.98%, whereas NaHCO 3 ‐Po and NaOH‐Po declined by ~10% under B2–B3. Sorption capacity decreased (Qm and maximum buffering capacity declined by 9.8%–11.7% and 18.6%–19.3%, respectively; Kf decreased by ~21%–22%), while desorption increased across initial P concentrations. Enzyme activities increased, especially alkaline phosphatase (31.16%–66.18%) and phosphodiesterase (46.24%–64.34%). Soil organic matter and enzyme activities were positively associated with labile inorganic P, whereas pH, electrical conductivity, and sorption parameters were negatively associated. Overall, B. subtilis mobilized soil P by promoting organic P mineralization and weakening sorption, with effects plateauing at 45 kg ha −1 under the present field conditions.