γ-polyglutamic acid-irrigation coupling strategy to improve soil quality, microbial activity, and cotton yield within arid saline-alkali fields

Soil salinization and drought are major abiotic factors restricting agricultural sustainability and land productivity. γ-Polyglutamic acid (γ-PGA) is an effective soil amendment that can improve water retention and reduce salt accumulation in saline-alkali farmlands. To assess the synergistic impacts of γ-PGA and irrigation on soil quality and seed cotton yield (SY), a two-year field trials were conducted in arid and saline-alkali region of Xinjiang during 2024–2025. Four γ-PGA application rates (P1: 7.5, P2: 15, P3: 22.5, P4: 30 kg ha −1 ) and three irrigation levels (W1: 500, W2: 450, W3: 400 mm) were set. The findings indicated that the interaction between γ-PGA and irrigation markedly enhanced soil nutrients. Compared with W3P1 (conventional treatment), W2P3 elevated soil available phosphorus by 12.60% 13.06%, total nitrogen by 24.17% 29.91%, and total carbon by 61.04% 70.11%, and boosted the activities of soil polyphenol oxidase by 71.60% 74.93%, catalase by 17.36% 17.76%, urease by 33.37% 64.02%, and sucrase by 64.54% 76.73%, while increasing dominant microbial diversity. Redundancy analysis identified key driving factors of bacterial and fungal communities, and structural equation model verified the pathways of γ-PGA and irrigation affecting soil environment and yield. Correlation analysis identified that Proteobacteria , Leotiomycetes , urease, and sucrase acted as core determinants linked to cotton yield improvements. The W2P3 treatment attained the maximum SY and water use efficiency (WUE), reaching 6745.10 kg ha −1 and 12.48 kg ha −1 mm −1 on average, while topping the rankings for both soil quality index (SQI) and global performance indicators (GPI). These results offer a reference for high-efficiency cotton production and water-saving irrigation strategies in arid saline-alkali regions.