Water-fertilizer management combined with Bacillus subtilis enhances cotton productivity in saline-alkali soils in Xinjiang, China

Soil salinization and drought constrain agricultural sustainability and land productivity. Bacillus subtilis is an effective microbial amendment for improving soil quality and crop productivity in saline–alkali soils. To assess the synergistic impacts of Bacillus subtilis on soil quality, resource-use efficiency, and seed cotton yield, a two-year field experiment was conducted with three irrigation levels (I52: 5200, I45: 4500, and I38: 3800 m3·ha⁻1), three nitrogen application rates (N300: 300, N255: 255, and N210: 210 kg·ha⁻1), and three Bacillus subtilis application rates (B0: 0, B15: 15, and B30: 30 kg·ha⁻1) were set. The findings indicated that integrated water–nitrogen management with Bacillus subtilis improved soil structure, enhanced soil water retention, and reduced soil salinity. Compared with I52N300B0, integrated management increased the proportion of macroaggregates (＞2 mm) by 4.77%–16.92% and decreased the proportion of microaggregates (＜0.25 mm) by 13.07%–40.41%. It also decreased soil bulk density by 2.84%–15.01% and increased total porosity by 12.39%–58.62%. In addition, Bacillus subtilis enhanced soil enzyme activities and promoted nitrogen transformation. Structural equation modeling further showed that the improved soil quality promoted root development and resource capture, thereby increasing cotton yield, water use efficiency (WUE), and nitrogen partial factor productivity (NPFP). The I45N300B30 and I52N255B30 treatments achieved the highest soil quality index (SQI) values of 0.69 and 0.75, respectively, with corresponding seed cotton yields of 6943.8 and 7063.6 kg·ha⁻1. Overall, integrating appropriate water–nitrogen management with Bacillus subtilis can provide a practical reference for active irrigation in saline–alkali agroecosystems.