Desulfurization gypsum facilitates soil sodium ion exchange and salt leaching in a heavily saline–sodic sunflower field of northern China

Saline–sodic soils are widely acknowledged as a significant threat to agricultural crop production. Improving these soils is crucial for preserving soil environmental quality and alleviating the global food crisis. Although flue gas desulfurization gypsum (FGDG) has been widely reported to improve saline–sodic soils, the effects of its application on soil salinity and sodium ion dynamics remain poorly understood. The overall effectiveness of this soil amendment has not yet been thoroughly assessed. To address these gaps, we conducted a three–year field experiment from 2022 to 2024 in a heavily saline–sodic sunflower field in the Hetao irrigation district. The UNSATCHEM module of HYDRUS, which accounts for reactions among major ions in soil, including sodium ion exchange and gypsum precipitation/dissolution, was employed to quantify the dynamics of soil salinity and sodium ions, as well as the effectiveness of FGDG applications at rates of 0, 10, 20, and 30 t ha −1 . The results indicate that HYDRUS accurately simulates soil water content ( SWC ), saturated electrical conductivity ( EC e ), and soluble Na + concentration in the field soil under different FGDG application rates. Soil salt dynamics differed clearly across different treatments. In the first year after FGDG application, EC e and Na + under FGDG treatments were higher than those under the non–FGDG treatment. However, in the second and third years after application, soil salt dynamics showed an opposite trend. Furthermore, soil EC e , Na + , and aSAR (ion-activity-based SAR ) decreased by 1.5%, 1.2%, and 1.5%, respectively, with every additional ton per hectare of FGDG applied. Meanwhile, the ion-activity-based Ca-Mg/Na ratio increased by 1.9%. The most significant change occurred at an application rate of 20 t ha −1 , with no noticeable differences beyond 25 t ha −1 . The numerical simulations suggested that the greatest ameliorative effect would be observed in the fifth year (2026), and that the ameliorative effect there would not significantly improve after ten years (from 2032 to 2042). These conclusions and recommendations apply primarily to conditions similar to those prevailing in the Hetao Irrigation District. This study offers a viable solution for arid regions worldwide, demonstrating that FGDG effectively improves heavily saline–sodic soils by enhancing sodium exchange and salt leaching.