Sulfur Enhances Rice Cadmium Accumulation in Organic Deficient Soil: The Significance of Incorporation with Straw

Application of wheat straw could contribute to a sulfur-driven reduction in cadmium (Cd) bioavailability under reducing conditions induced by organic matter degradation. A pot experiment was conducted in organic matter deficient paddy soil under waterlogged conditions to assess the effects of sulfur (S, 30 mg kg^-1), wheat straw (W, 1.0%), and their combination (WS) on Cd availability and accumulation in rice (Oryza sativa L.). Sulfur application alone increased Cd uptake in rice, whereas straw addition significantly reduced Cd accumulation, with WS achieving the greatest reduction. The mitigating effect was attributed to CdS precipitation and co-precipitation with FeS/FeS₂ under straw amendment, as well as enhanced iron plaque formation on roots, which restricted Cd uptake. In contrast, in OM-deficient soil, sulfate promoted Cd mobilization in pore water due to limited electron supply for sulfate reduction. Compared with other sulfur forms, sulfate is more readily absorbed by rice, thereby synergistically enhancing Cd uptake by rice and promoting Cd translocation in different rice tissues. However, straw amendment supported reduction in sulfate, reducing Cd uptake by rice compared with S supplement alone. Overall, wheat straw amendment enhanced sulfur-mediated immobilization of Cd and effectively decreased Cd accumulation in rice.