Effect of elemental sulfur and gypsum application on the bioavailability and redistribution of cadmium during rice growth.

Recently, concerns over heavy metal contamination of soil have grown. The application of sulfur has been recommended to enhance crop productivity and increase soil cadmium (Cd) immobilization. In this study, a pool experiment was conducted to investigate the effects of two sulfur sources and multiple treatment levels on rice growth and Cd accumulation. The two sulfur forms were elemental sulfur (S) and gypsum, both of which were applied at 0, 0.15, and 0.30 g S kg soil, for a total of five treatments. The results showed that both S and gypsum significantly increased rice biomass compared to the control (CK), and rice yield was increased 2.8-4.8 folds. The effect size was greater for gypsum than S. The application of S reduced the rice grain Cd concentration from 0.61 mg kg (CK) to 0.41-0.46 mg kg, while gypsum reduced the Cd concentration to 0.24-0.43 mg kg. The lower gypsum application level achieved the greatest reduction in rice grain Cd accumulation. This study further demonstrated that the application of S and gypsum led to a decrease in the labile Cd percentage and an increase in the stable Cd percentage. In bulk soil, iron and manganese oxide-bound Cd increased by 6.4-7.3% and 0.7-2.0% for the S and gypsum treatments, respectively. In the rhizosphere, residual Cd increased by >0.6%. Furthermore, this study found that sulfur application reduced Cd transfer from root to shoot, and significantly decreased rice grain Cd accumulation. These findings indicate that sulfur application to paddy soils can promote rice productivity and effectively remediate soil Cd contamination, with a greater effect by gypsum than S.