Inhibition effect of sulfur on Cd activity in soil-rice system and its mechanism

Highlights: • Sulfur can reduce the mobility of Cd in the soil under flooding conditions. • Sulfur promoted the generation of soil labile Fe(Ⅱ), S(-Ⅱ) and DOC, thus reducing Cd availability. • The effect of Fe(Ⅱ) on the mobility of Cd was stronger than that of S(-Ⅱ) and DOC on the mobility of Cd. • Sulfur promoted the formation of iron oxidized cadmium in soil and reduced the effective form of Cd in soil. • Different forms of sulfur had different inhibitory effects on Cd mobility. Sulfur (S) can regulate the mobility of cadmium (Cd) in soil and reduce Cd uptake by rice. However, the mechanism of how S affects soil properties and then regulates Cd mobility in rice growth through multiple factors is unclear. Diffusive gradients in thin films (DGT) and other in situ detection techniques, were innovatively used in this study. The effect of S on soil Cd mobility under flooded condition was analyzed in situ and the related mechanism was discussed. The results showed that the addition of S increased the level of dissolved organic oxygen (DOC), increased the concentration of Fe(Ⅱ) and S(-Ⅱ), and decreased the concentration of labile Cd in soil. Multiple regression analysis showed that Fe(Ⅱ), S(-Ⅱ), DOC were negatively correlated with labile Cd. S promoted the reduction of sulfate and iron and accelerated the formation of CdS and iron complexed Cd. The results of path analysis showed that the synergistic effect of S and Fe significantly inhibited the mobility of Cd. In particular, the mobility of Cd at tillering stage was greatly affected by S and Fe. Mercapto-palygorskite can effectively increase the concentration of Fe(Ⅱ),S(-Ⅱ) and DOC in soil, and reduce the harm of Cd to rice better than S⁰ and Na₂SO₄.