Contrasting Effects of Desulfurization Gypsum and Aluminum Sulfate Application in Rice Cultivation on Soil Salinity and Sodicity on the Songnen Plain of Northeast China

Soil salinization has become a major threat affecting global arable productivity. Rice cultivation with amendment application is considered an important approach for saline–sodic soil reclamation. Saline–sodic soil without vegetation was selected as the study subject to investigate the effects of amendments in rice cultivation on salinity and sodicity through a pot experiment. The results revealed that the application of desulfurization gypsum combined with aluminum sulfate to saline–sodic soil significantly contributed to decreases in soil salinity and sodicity. The soil pH in the 0–10 cm, 10–20 cm and 20–30 cm soil layers decreased from 9.41–9.84 to 8.06–9.24, whereas the exchangeable sodium percentage (ESP) decreased from 28.98–33.24% to 19.76–30.82%, respectively. The increase in soil exchangeable Ca2+ was accompanied by a decrease in soil exchangeable Na+. Additionally, the application of desulfurization gypsum combined with aluminum sulfate to saline–sodic soil resulted in significant decreases in total alkalinity (TA) and the sodium adsorption ratio (SAR) and an increase in soluble Ca2+. The analysis indicated that soluble Ca2+ derived from desulfurization gypsum is the predominant factor affecting the variation in the soil pH, ESP, SAR, and exchangeable Na+ and Ca2+. The reductions in salinity and sodicity are attributed to the replacement of Ca2+ derived from desulfurization gypsum with Na+ on soil collides. Simultaneously, H+ formed by the hydrolysis of aluminum sulfate neutralizes HCO3− and CO32− in the water layer.