Optimizing water and nitrogen thresholds for drip-irrigated cotton under mulch in Xinjiang based on water-nitrogen synergy

To optimize irrigation and nitrogen management for mulched drip-irrigated cotton in Xinjiang, a field experiment investigated varying irrigation amounts, nitrogen application rates, and fertilization frequencies on cotton growth, yield, water and nitrogen use efficiency, and fiber quality. The experiment involved three factors: irrigation amount (360 and 450 mm, denoted as W₁ and W₂), nitrogen application rate (420 and 560 kg/hm², denoted as N₁ and N₂), and fertilization frequency (5 and 8 times, denoted as F₁ and F₂). A control treatment (W₁N₀F₀) without nitrogen application was also included. Nine treatments with three replications were established. Different treatment significantly affected cotton growth traits, yield, water and nitrogen use efficiency, and fiber quality. Compared with W₁N₀F₀, the W₁N₁F₁ treatment demonstrated particularly strong performance. Plant height, stem diameter, and fiber elongation increased significantly by 16.83%, 20.63%, and 11.11%, respectively (p<0.01). In addition, water use efficiency was 23.64% higher than that of W₁N₀F₀, and nitrogen fertilizer productivity reached 42.86 kg/kg, indicating relatively optimal levels. For yield maximization alone, the W₁N₁F₂ treatment increased yield by 36.86% over W₁N₀F₀. It also significantly increased boll weight and number. However, its water use efficiency and nitrogen fertilizer productivity were lower than in W₁N₁F₁. Principal component analysis comprehensively evaluated cotton growth traits, yield components, water and nitrogen use efficiency, and fiber quality. The W₁N₁F₁ treatment received the highest comprehensive score. This treatment balanced high yield with efficient resource use, achieving synergistic improvements in yield, efficiency, and fiber quality. Therefore, under these conditions, the W₁N₁F₁ treatment (irrigation amount of 360 mm, nitrogen application of 420 kg/hm², and fertilization frequency of five times) is recommended as optimal for high yield and quality in mulched drip-irrigated cotton in Xinjiang. This optimization model not only ensures the efficiency of water utilization and nutrient balance, but also provides key technical references for cotton production in Xinjiang and similar ecological regions.