Differences and mechanism of dynamic changes of Cd activity regulated by polymorphous sulfur in paddy soil

Sulfur (S) can decrease the bioavailability of Cd in paddy soil, and therefore reduce Cd uptake by rice. However, the dynamic influence mechanism of different forms of sulfur on the bioavailability of cadmium in soil has not been systematically studied. In this study, we used Diffusive gradients in thin films (DGT) technology and soil pore water sampling technology to investigate the effects of different types of S application on the bioavailability of Cd. The three forms of S are elemental sulfur (S⁰), sodium sulfate (SO₄^2-), and mercapto-grafted palygorskite (MP), which have been treated in six ways according to CK, SL, SH, SO₄²L, SO₄²H, MP (L and H are the low and high levels of treatments). The results showed that soluble and labile Cd concentration was quickly fixed after flooding but activated after rice transplantation. Both MP and SL treatments increased the content of dissolved organic carbon (DOC) and significantly reduced the accumulation of Cd in roots and grains. The Cd content of roots treated with MP was only 0.50 mg kg^-1, which was 77.6% lower than the Cd content in CK (2.22 mg kg^-1). S promotes the reduction and dissolution of Fe, and the formation of FeS/FeS₂ is also one of the reasons for the low bioavailability of Cd, while the SH treatment has a greater effect on lowering pH, so the effect of fixing Cd is not obvious. In addition, SO₄^2- treatment delays the soil reduction process, so it cannot effectively fix Cd. At the stage of rice maturity, dissolved Cd increased in S treatment, but MP treatment did not have this phenomenon. The possible reason was the consumption of DOC or the oxidation of CdS which caused Cd to enter the soil solution. Taken together, Fe-S play a critical role in controlling the mobilization of Cd in paddy systems.