Effects of combined saline-drought stress on the soil water-salt environment, cotton yield and fiber quality of cotton under drip irrigation

Drought and soil salinity are two major limiting factors on the agricultural productivity in arid northwest China. Particularly, 2.96 million hm² of cultivated land is often subjected to the saline-alkali damage, accounting for more than 40 % among the 7.09 million hm² of cultivated land in Xinjiang. The ratio of the agricultural water to total water consumption is more than 90 %, which seriously impacts on the land productivity. The saline water resources have seriously threaten to the large-scale production, due to the soil secondary salinization. However, it is still unclear on the response mechanisms of the soil water-salt dynamics and cotton growth to the salt-drought stress. This study aims to explore the effects of the combined saline-drought stress on the soil water-salt environment, cotton yield, and fiber quality under drip irrigation. The test material was taken as the conventional cotton variety (Xinluzao 42) widely-grown in Xinjiang Province, China. A pot experiment was conducted using a three-factor (initial soil salinity, irrigation amount, and water salinity) and four-level orthogonal design. 16 treatments were set: four initial soil salinity levels (T1: 2 g/kg, T2: 4 g/kg, T3: 6 g/kg, and T4: 8 g/kg), four irrigation volumes (W1: 3375 m³/hm², W2: 3750 m³/hm², W3: 4125 m³/hm², and W4: 4500 m³/hm²), and four irrigation water salinities (S1: 2 g/L, S2: 5 g/L, S3: 8 g/L, and S4: 11 g/L) in Shihezi City, Xinjiang, China, during 2022. A comparison was then conducted to investigate the soil water and salt content, growth and physiology of cotton, as well as seed cotton yield and fiber quality under combined salinity-drought stress after drip irrigation. Their underlying relationships were constructed to integrate the soil water and salt environment with the yield and fiber quality. Correlation analysis, structural equation modeling, and the entropy weight TOPSIS were applied to evaluate the effects of the salt–drought stress on the soil water–salt dynamics and cotton growth. The results showed that soil water content and soil salinity at different cotton growth stages increased with increasing initial soil salinity and irrigation water salinity. With increasing initial soil salinity, cotton plant height, stem diameter, leaf area index, net photosynthetic rate, transpiration rate, stomatal conductance, seed cotton yield, and water use efficiency exhibited decreasing trends. Fiber length initially increased and then decreased, while the uniformity index, breaking strength, elongation, and comprehensive fiber quality index showed fluctuating trends characterized by an initial increase, followed by a decrease and then another increase. In contrast, the micronaire value exhibited an opposite trend. As irrigation water salinity increased, cotton plant height, stem diameter, leaf area index, seed cotton yield, water use efficiency, uniformity index, and breaking strength declined. Fiber length and the comprehensive fiber quality index decreased initially and then increased, whereas the micronaire value and elongation displayed opposite trends.With increasing irrigation quota, plant height, stem diameter, leaf area index, net photosynthetic rate, transpiration rate, stomatal conductance, fiber length, uniformity index, breaking strength, and micronaire value increased initially and then decreased, while seed cotton yield, water use efficiency, and the comprehensive fiber quality index showed increasing trends. Salt–drought stress was altered the soil water–salt environment, thereby resulting on the cotton growth and development. Specifically, there were the negative effects of the soil initial salinity and irrigation water salinity on the cotton growth, photosynthetic properties, yield, water use efficiency, and fiber quality index. While the irrigation amount shared the positive effects on these five factors. Furthermore, the top three optimal treatments were T1W2S2, T1W1S1, and T2W2S1. While the least three treatments were T4W1S4, T3W2S4, and T4W2S3, respectively. These findings can also provide the theoretical reference to increase the cotton yields in the arid regions of Northwest China.