Soil salinity and sodicity changes after a one‐time application of flue gas desulphurization gypsum to paddy fields

Abstract Sodic soil is more likely to deteriorate in paddy fields than in other fields. Flue gas desulfurization (FGD) gypsum can reclaim sodic soils, but how long a one‐time application of FGD gypsum provides benefits in paddy fields is poorly understood. In this study, soil salinity, sodicity, and soluble and exchangeable cation concentrations were evaluated over a 7‐year period to investigate the reclamation of sodic soil in a paddy field by FGD gypsum. After 7 years, compared with those in the initial soil, the pH, sodium adsorption ratio, and exchangeable sodium percentage of the topsoil (0–20 cm) decreased by at least 2 units, 72.2% and 80.9%, respectively. In the process of FGD gypsum reclamation, substantial changes in topsoil salinity and sodicity occurred mainly between the 4th and 5th years of reclamation, representing the boundary of the FGD gypsum dissolution stage, which was followed by the soil reclamation maintenance stage. In the FGD gypsum dissolution stage, soil salinity was dominated by Ca 2+ that accumulated in the 0–10 cm soil layer. Over time, the exchangeable Ca 2+ concentration decreased with soil depth to 40 cm. In the reclamation maintenance stage, FGD gypsum was basically depleted, but the sodicity still showed a significant decrease compared with that in the initial soil. These results indicate that FGD gypsum can significantly and effectively reclaim sodic soil in paddy fields for at least 7 years. Though the FGD gypsum was exhausted 5 years after its application, reclamation continued to be maintained due to the presence of exchangeable Ca 2+ .