Water–nitrogen synergistic optimization: Model–guided management strategies for enhancing cotton (Gossypium hirsutum L.) productivity in desert-oasis agroecosystems

Global water scarcity and soil degradation from excessive fertilization are critical environmental challenges to sustainable agriculture. Optimized irrigation and nitrogen fertilization regimes are urgently required to meet increasing agricultural demands with limited water and fertilizer resources, thereby achieving synergistic improvements in crop yield and resource use efficiency. Herein, a two-year field experiment (2020–2021) was performed on cotton plants under mulching and drip irrigation in Alaer, Xinjiang. Three irrigation levels (W1: 405 mm; W2: 345 mm; W3: 270 mm) and four nitrogen application rates (total seasonal application: N1: 540 kg·ha−1; N2: 405 kg·ha−1; N3: 270 kg·ha−1; N4: 0 kg·ha−1) were applied. Results indicated that irrigation treatments significantly affected seed cotton yield, cotton lint percentage (2020), number of bolls per plant (2021), mean length of upper part, micronaire, water productivity, and partial factor productivity of nitrogen (PFPN) (P < 0.05), with the W1 treatment demonstrating overall superior performance. Nitrogen treatment significantly influenced seed cotton yield, number of bolls per plant, cotton lint percentage (2020), breaking elongation, water productivity, and PFPN (P < 0.05), with the N1 treatment showing the most favorable outcomes. Water–nitrogen interactions significantly affected boll weight (2020), cottonseed yield (2021), fiber length, and micronaire (P < 0.05), with the W1N1 interaction presenting the most pronounced effects. Field experiments identified the W1N1 treatment as optimal, increasing plant height by 3.51 %–18.71 % and dry matter accumulation by 4.03 %–18.92 % compared to lower-input treatments. The W1N3 treatment achieved the highest PFPN but yielded suboptimal cotton yield and WP, underscoring the need for multiobjective optimization to balance resource efficiency and productivity. The DSSAT model demonstrated high accuracy in predicting yield (NRMSE < 10 %) and nitrogen partial factor productivity (R2 = 0.97). Based on principal component analysis, an optimal drip fertigation strategy was identified for cotton in the arid region of southern Xinjiang, which combined 390 mm irrigation, 390 kg·ha−1 nitrogen, 7–10 day intervals, and fertigation in the first seven events, achieving a seed yield of about 7549 kg·ha−1. This study provides a scientific basis for efficient water and nitrogen management in cotton plants cultivated under drip irrigation in southern Xinjiang.