Effect of organic materials and sulfate addition on cadmium bioavailability and accumulation of rice in cadmium sulfide nanoparticles- and cadmium chloride-treated soil.

Water-soluble and extractable cadmium (Cd) govern Cd bioavailability. However, cadmium sulfide nanoparticles (CdSnp) bioavailability remains unclear. Pot experiments investigated how organic materials (1 % wheat straw and pine needles) and sulfate (30 mg S/kg) affect Cd bioavailability and accumulation in rice in CdCl₂- and CdSnp-treated soils. Sulfate addition alone immobilized Cd through sulfate-reducing bacteria-mediated Cd-precipitation in CdCl₂-treated soil, thereby reducing Cd accumulation in brown rice. However, it promoted CdSnp dissolution and Cd transfer, increasing Cd accumulation in CdSnp-treated soils. Organic materials alone promoted Cd re-release from immobilized Cd through sulfur-oxidizing bacteria (SOB)-induced oxidation and dissolved organic matter (DOM)-assisted complexation to increase Cd availability in CdCl₂-treated soil at the maturity stage. Consequently, organic materials alone increased brown rice Cd accumulation in CdCl₂-treated soil, while combination with sulfate had no effect. In CdSnp-treated soils, organic materials promoted CdSnp dissolution through driving SOB, decreasing pH, and increasing DOM complexation, which increased Cd availability and transfer, promoting brown rice Cd accumulation. CdSnp exhibited lower Cd bioavailability and accumulation than CdCl2 due to lower solubility and slower Cd release, suggesting CdSnp smaller contributions to rice Cd accumulation. Therefore, sulfate addition alone is recommended to reduce Cd accumulation, with organic materials only used in combination with sulfate.