Evaluating the impacts of long-term saline water irrigation on soil salinity and cotton yield under plastic film mulching: A 15-year field study

In the context of dwindling freshwater resources, it is imperative to comprehensively evaluate crop productivity and soil resource sustainability when considering the use of saline water for irrigation (SWI). This study aimed to investigate the effects of SWI on soil salinity variation and sustainability of flat sowing cotton (Gossypium hirsutum L.) through a comprehensive 15-year field experiment conducted in the North China Plain, encompassing six levels of irrigation water salinity (ECiw of 1.3, 3.4, 7.1, 10.6, 14.1, and 17.7 dS m−1, respectively). The results of the 15-year SWI revealed an increase in soil salinity within the cotton root zone, with increases of 5%, 20%, 39%, 67%, 116%, and 156%, respectively, observed across the six treatments. In treatments with higher ECiw values (14.1 and 17.7 dS m−1), salt accumulation extended to a depth of 3.0 m. Long-term SWI has a risk of causing soil sodicity and alkalinity. Sodium adsorption ratio (SAR) and pH in the treatments with ECiw ≥ 7.1 dS m−1 increased significantly (P < 0.05) after cotton harvest in the 15th year of continuous SWI. Cotton yield performance exhibited a distinct response to varying levels of ECiw. The treatment with an ECiw of 3.4 dS m−1 yielded the highest average cotton yield of 3419 kg ha−1, while cotton yields in treatments with an ECiw of 14.1 and 17.7 dS m−1 were more than 13% and 20%, respectively, lower than the maximum yield (P < 0.05). The sustainability index of cotton yield was at its lowest (0.68) in the treatments with an ECiw of 7.1 dS m−1, with values between 0.72 and 0.75 in the other treatments. Additionally, long-term SWI resulted in changes in cotton quality, including a decrease in micronaire value and an increase in elongation index. A quadratic correlation analysis identified a threshold ECiw of 6.8 dS m−1 for optimum cotton yield. In summary, it is recommended that the ECiw should not exceed 6.8 dS m−1 to ensure the sustainability and safety of long-term SWI for cotton production in this research region.