Synergistic enhancement of cotton yield and water–nitrogen use efficiency in saline–alkali soils through optimized water–nitrogen timing under mulched drip irrigation

In arid saline–alkali regions, cotton productivity is constrained by water scarcity and salinity. However, water–nitrogen management remains input-oriented and overlooks the synchronization of irrigation and nitrogen supply across growth stages. The experiment comprised three irrigation regimes (60%, 80%, and 100% crop evapotranspiration (ETc)) combined with three nitrogen application timings (1/2N–1/2 W; 1/4W–1/2N–1/4 W; and 3/8W–1/2N–1/8 W). Cotton growth traits and resource use efficiency indicators were quantified, and causal relationships were evaluated using a partial least squares path model. The results demonstrated that the regulatory effect of nitrogen timing was governed by a distinct irrigation threshold, with optimal performance occurring at 80% ETc. Under this condition, the W2F2 treatment improved root morphology, leaf area development, organ nitrogen accumulation, and dry matter production, resulting in a seed cotton yield of 6583 kg·ha−1, a water productivity of 1.67 kg·m−3, and a nitrogen partial factor productivity of 21.94 kg·kg−1. Path analysis further indicated that water and fertilizer management exerted significant direct effects on system output. It also indirectly enhanced yield and resource efficiency by promoting biomass accumulation through optimized root morphology, with biomass demonstrating the strongest overall contribution (total effect = 0.537). Overall, the integration of 80% ETc irrigation with the “1/4W–1/2N–1/4 W” nitrogen timing strategy effectively synchronized cotton yield formation with water–nitrogen use efficiency in saline–alkali soils by optimizing root morphology and nitrogen uptake, providing a practical and mechanistic basis for water- and fertilizer-saving green cotton production in arid saline regions.