Optimizing irrigation amount and salinity level for sustainable cotton production and soil health

Brackish water is usually abundant and regarded as potential irrigation water resource in arid regions such as Xinjiang Province, China. However, although brackish water can alleviate water stress for crop growth, it may bring salt into soil, resulting in potential threats of soil secondary salinization and crop salt stress. To use the brackish water safely and effectively, experiments were conducted in 2023 and 2024 to explore the impact of brackish water quantity and quality on the soil salt spatiotemporal distribution, cotton growth and yield in the film mulched drip irrigation fields in Southern Xinjiang. It includes three irrigation amounts (W1: 75 % I, W2: 100 % I and W3: 125 % I, where I is crop irrigation water requirement) and three irrigation salinities (S1: 1.5 g/L, S2: 3.5 g/L and S3: 5.5 g/L). Results show that under excessive irrigation (W3), higher irrigation water salinity would limit the root water uptake, and result in higher soil water content in the root zone. A single deficit irrigation with brackish water at 3.5 g/L showed no significant difference in soil salt distribution before and after irrigation, but increasing irrigation amount was beneficial for mitigating soil salt accumulation in the root zone under the film. Under sufficient irrigation, 5.5 g/L saline water irrigation also caused salt accumulation in the deep soil beneath the film and the bare soil between the films around the wetting front. Compared with deficit irrigation, increasing irrigation amount with 5.5 g/L saline water neither promote cotton growth, nor significantly inhibit it. Higher irrigation salinity lead to a significant decline in cotton yield, but the effect of salt stress was mitigated under water deficit condition. Considering soil salt accumulation in the root zone under the film, cotton yield and water productivity (WP), the optimal irrigation amounts were 1.25 I for freshwater irrigation at 1.5 g/L, 1.35 I for brackish water irrigation at 3.5 g/L, and 1.06 I for saline water irrigation at 5.5 g/L. Our research provides guidance for the rational use of irrigation with different salinities and the sustainable development of irrigated agriculture in arid and semi-arid regions.