Saline Water Irrigation Changed the Stability of Soil Aggregates and Crop Yields in a Winter Wheat–Summer Maize Rotation System

Irrigation using saline water is extensively used in areas of agricultural production where freshwater is scarce. However, saline water irrigation adversely impacts soil’s physicochemical characteristics and crop productivity. In this study, we established irrigation water with five salinity levels (ECiw, 1.3, 3.4, 7.1, 10.6, 14.1 dS·m−1) to investigate how these salinity levels influenced grain yields as well as soil salinity, alkalinity, sodicity, and aggregate stability in the 0~20 cm soil layer of a wheat and maize rotation field (in 2022–2023). Tukey’s test, entropy-weighted TOPSIS, and the least squares method were used to analyze the significance analysis, comprehensively evaluate the soil aggregate stability and soil index comprehensive score (SICS), and achieve linear fitting, respectively. The results showed that when ECiw > 3.4 dS·m−1, there was a significant increase in the soil salinity, pH, and sodium adsorption ratio. When ECiw > 7.1 dS·m−1, a significant reduction in soil aggregate stability was observed. When ECiw ≤ 3.4 dS·m−1, there was no significant reduction in the grain yields of wheat and maize. Furthermore, the annual grain yields of wheat and maize decreased by 5% and 10%, respectively, resulting in a change in ECiw values from 2.98 to 4.24 dS·m−1, based on the linear regression analysis of SICS and ECiw, as well as the annual grain yields and SICS. Under uniform irrigation conditions, the soil salinity, alkalinity, and sodicity were lower, and soil aggregate indexes were more stable at the maturity stage of maize.