Irrigation-dependent soil responses to organic fertilizer and Bacillus–Trichoderma inoculant shape cotton fiber quality and irrigation water productivity

Balancing cotton productivity, fiber quality, and irrigation water use efficiency (IWUE) under contrasting water targets requires understanding how soil amendments interact with root-zone resource conditions. A 16-treatment field experiment was conducted in southern Xinjiang using two organic fertilizer rates (0 and 30 t/ha), four Bacillus–Trichoderma inoculant rates (0, 30, 60, and 90 kg/ha), and two irrigation regimes based on 80% and 100% of crop evapotranspiration (ET c ). Soil responses were evaluated through water status, organic matter, available nutrients, nitrogen-transforming enzyme activities, fungal community structure, and soil quality index (SQI), together with cotton growth, seed cotton yield (SCY), fiber quality, cotton quality index (CQI), and IWUE. Organic fertilizer exerted the broadest and most consistent effects, increasing soil organic matter (SOM) by 11.86%–17.69%, nitrate nitrogen by 22.01%–26.69%, soil nitrate reductase (SNR) by 35.00%–64.71%, soil nitrite reductase (SNIR) by 23.33%–40.74%, and SQI by 160.00%–213.64%. It also increased SCY, CQI, and IWUE by 12.46%–15.91%, 29.41%–69.70%, and 12.21%–16.08%, respectively. Inoculant responses were nonlinear, with 60 kg/ha giving the strongest integrated performance among the tested rates, while fungal community composition showed treatment-dependent restructuring. Full irrigation increased SCY by 3.17%–8.44% and CQI by 2.08%–27.27% but reduced IWUE by 10.49%–15.48%, defining a yield–IWUE trade-off. Random forest analysis identified SOM, SNR, and SNIR as recurrent predictors of SCY and CQI, whereas soil water content was the strongest predictor of IWUE. Borda aggregation ranked the combination of 30 t/ha organic fertilizer and 60 kg/ha inoculant under 100% ET c first, followed by the corresponding treatment under 80% ET c , representing high-output and water-saving options, respectively. These findings support irrigation-specific amendment strategies that balance productive output, fiber quality, and irrigation efficiency.