Short-term shaping of the alfalfa rhizosphere microecosystem in saline-alkali lands driven by pqqE gene-mediated phosphate-solubilizing bacteria

Excessive salt ions in saline-alkali soil exacerbate the precipitation and fixation of phosphorus (P), subjecting plants to combined salinity and P deficiency stresses. Halo-tolerant phosphate-solubilizing bacteria (HT-PSB) regulate the secretion of organic acids through the pqq E gene to release phosphate ions bound to salt ions, offering a near-nature strategy to mitigate these constraints. This 6-month, single-season field study was conducted to evaluate the short-term effects of pqq E-mediated P-solubilization on alfalfa P absorption, soil P transformation, rhizosphere microbial community changes, and the expression of P-cycling functional genes. An HT-PSB strain AWH-NS6, isolated from the rhizosphere of Haloxylon ammodendron in a saline desert, as well as its pqq E knockout mutant (AWH-NS6-Δ pqq E) and complemented strain [AWH-NS6-Δ pqq E( pqq E)], were applied to alfalfa fields with lightly and moderately salinized soil. The results showed that the wild-type and complemented strain inoculant treatments significantly reduced soil pH, and lowered salinity and exchangeable sodium percentage (ESP) by 14.49~50.97% compared to the control treatment. This amelioration was accompanied by a 14.02~34.33% increase in labile P fractions (H 2 O-P i , NaHCO 3 -P i ) and a corresponding 19.43~26.08% reduction in recalcitrant residual P. Moreover, soil alkaline phosphatase and phytase activities were improved by inoculant treatments, key P mineralization genes ( phnP , phoA ) were upregulated, and the rhizosphere microbiome was shown to undergo short-term changes, with enrichment of beneficial taxa such as Pseudomonadota and Actinomycetota. Within the 6-month study period, these edaphic and microbiomic changes were accompanied by improved P absorption by alfalfa, optimized plant K + /Na + homeostasis, and increased biomass and forage quality. Structural equation modeling (SEM) suggested that PSB application may alleviate the inhibitory effects of salinity on P-cycling functional genes and was associated with the transformation of stable P into bioavailable forms. These short-term findings suggest a potential microbial, low-input strategy for saline-alkali soil remediation and sustainable forage production.