Effects of Irrigation Water Temperature and Nitrogen Application Rate on Soil Hydrothermal Environment and Cotton Growth and Yield Under Mulched Drip Irrigation

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.

A two-factor completely randomized experimental design 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)). The effects of nitrogen application on soil hydrothermal environment, cotton growth and yield, and water and nitrogen use efficiency under different irrigation water temperature were analyzed.

The 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 with 15 ℃ of conventional irrigation water temperature, the warming irrigation significantly increased the soil temperature by 0.58-3.30 ℃, and soil water storage was reduced by 1.2%-7.2%, while 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 rate, 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 under 350 kg·hm^-2 nitrogen fertilizer application. 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 seed cotton yield and water use efficiency reached the maximum under T2F2 treatment, which were 6 652.3 kg·hm^-2 and 1.17 kg·m^-3, respectively. But the nitrogen fertilizer bias productivity was significantly greater under T2F2 treatment (22.17 kg·kg^-1) than that under T2F3 treatment (18.80 kg·kg^-1).

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, a suitable combination of irrigation water temperature of 25 ℃ and nitrogen application rate of 300 kg·hm^-2 were recommended in northern Xinjiang.