Abstract: 【Objective】The response mechanism of soil hydrothermal environment and cotton growth to irrigation water temperature and nitrogen application rate under mulched drip irrigation was explored to determine the reasonable irrigation water temperature and nitrogen application rate of drip irrigation cotton in northern Xinjiang.【Method】A two-factor experiment was conducted with "Xinluzao 42" cotton as the experimental material with four irrigation water temperature levels (15 ℃ (T0), 20 ℃ (T1), 25 ℃ (T2), and 30 ℃ (T3)) and three nitrogen application levels (250 kg·hm^-2 (F1), 300 kg·hm^-2 (F2), and 350 kg·hm^-2 (F3)). A two-factor completely randomized experimental design was used.【Result】The results showed that conventional irrigation water temperature and low nitrogen treatment reduced soil temperature, inhibited cotton growth, decreased boll number per plant and seed cotton yield. Suitable irrigation water and nitrogen application could improve the soil's hydrothermal environment, promote cotton growth and development, and improve seed cotton yield and water and nitrogen utilization. Compared to 15 ℃ of conventional irrigation water temperature, warming irrigation significantly increased the soil temperature by 0.58-3.30℃, soil water storage was reduced by 1.2%-7.2%, and soil respiration rate was significantly increased by 5.7%-28.0%; Cotton plant height, leaf area index, and above-ground dry matter accumulation increased and then decreased with the increase of irrigation water temperature, and reached the maximum at 25℃. With increasing nitrogen application, soil water storage decreased by 3.3%-6.7%, soil respiration rate increased significantly by 3.6%-9.5%, cotton plant height increased significantly by 3.2%-4.9%, leaf area index increased significantly by 5.8%-11.0%, and above-ground dry matter accumulation increased significantly by 1.2%-2.2%, these indicators all reached the maximum at 350 kg·hm^-2. Water use efficiency, nitrogen fertilizer bias productivity, and seed cotton yield all increased and then decreased with the increase of irrigation water temperature, and showed a trend of “increasing, decreasing, and increasing” with the increase of nitrogen application. The path analysis showed that soil temperature directly affected seed cotton yield, while nitrogen application indirectly affected seed cotton yield by promoting cotton growth. The maximum seed cotton yield was 6652.3 kg·hm^-2 in T2F2 treatment and the water use efficiency was 2.03 kg·m^-3, which was not significantly different from the maximum (T2F3) of 2.06 kg·m^-3, and the nitrogen fertilizer bias productivity was significantly greater in T2F2 treatment (22.17 kg·kg^-1) than in T2F3 treatment (18.80 kg·kg^-1).【Conclusion】Considering the effects of irrigation water temperature and nitrogen application on soil temperature, soil respiration rate, cotton growth, yield, and water and nitrogen utilization rate, we recommended a suitable combination of irrigation water temperature of 25℃ and nitrogen application rate of 300 kg·hm^-2 in northern Xinjiang. This study provides a theoretical basis for alleviating the hazards of low-temperature water irrigation, improving cotton water use efficiency, and increasing yield for the cotton industry in the northern cotton region of Xinjiang.

Key words: mulched drip irrigation, cotton, irrigation water temperature, nitrogen fertilizer, yield, water and fertilizer use efficiency