Microbial mechanisms underlying organic phosphorus mineralization and inorganic phosphorus solubilization in arid cotton fields under brackish water drip irrigation and phosphorus fertilization

Phosphorus (P) fertilizer utilization is globally constrained by strong soil fixation, which reduces its effectiveness in improving crop yield and quality—a challenge particularly pronounced in arid regions like Xinjiang, China. In arid regions facing freshwater scarcity and soil salinization, integrating brackish water drip irrigation with P fertilization is a potential strategy to enhance cotton productivity. However, the microbial mechanisms particularly functional genes involved in organic phosphorus mineralization (OPM) and inorganic Phosphorus solubilization (IPS), remain poorly understood. This study, conducted in a fixed-site field experiment with consistent long-term treatments, assessed four treatments: fresh water without P (FW), fresh water with P (FWP), brackish water without P (BW), and brackish water with P (BWP), brackish water irrigation significantly reduced cotton growth with biomass, P uptake, and yield decreasing by 27.1 %, 36.1 %, and 17.2 %, respectively. It also altered soil properties, increasing water content, electrical conductivity, organic carbon, and bulk density, while decreasing pH and porosity. P fertilization mitigated these effects, boosting biomass, P uptake, and yield by 51.3 %, 57.9 %, and 10.4 %, respectively. Microbial analysis revealed that brackish water irrigation increased the relative abundance of Pseudomonadota, Bacteroidota, and the gene phnP , while decreasing phoD, gcd , and pqqC . P fertilization increased the abundance of Pseudomonadota, Bacteroidota, Gemmatimonadota, and Planctomycetota, with regulatory effects on functional genes varying under salinity levels. These findings elucidate the regulatory impacts of long-term brackish water drip irrigation and P fertilization on OPM and IPS associated genes in arid cotton soils, offering theoretical guidance for optimizing P management in saline environments. • Long-term brackish water drip irrigation reduces soil phosphorus availability. • Brackish water irrigation enriches Pseudomonadota and Bacteroidota. • Brackish water irrigation increases phnP abundance but decreases phoD , gcd , and pqqC abundance. • Phosphorus fertilization enriches Gemmatimonadota and Planctomycetota. • Phosphorus fertilization enhances phoD and gcd under brackish water irrigation.