Dissolved organic matter (DOM) – Driven variations of cadmium mobility and bioavailability in waterlogged paddy soil

Cadmium (Cd) mobility and bioavailability in paddy soils are strongly influenced by dissolved organic matter (DOM), yet the mechanisms remain unclear. This study conducted a 90-day waterlogged soil incubation with DOM / sulfate amendments under varying Cd levels. Key parameters, including dissolved organic carbon (DOC), pe+pH, Fe/S - related parameters, alongside indicators of Cd mobility and bioavailability, were monitored. Results revealed that DOM addition increased Cd mobility on the 3rd day of incubation (DOI), irrespective of sulfate application, due to Cd desorption from iron oxides and DOM-Cd complexation. After the 10th DOI, DOM addition reduced Cd mobility and bioavailability mainly due to facilitation of sulfide-mediated Cd sequestration driven by Fe-S related reducing bacteria. The combined application with sulfate strengthened this effect. However, in low-Cd soils, DOM addition increased Cd bioavailability since the 45th DOI, likely due to the low Cd/DOM ratio, which limited sulfide immobilization. Nevertheless, sulfate application mitigated this effect. Furthermore, DOM supplementation generally decreased Cd mobility, but increased Cd availability at the 45th DOI in high-Cd soils due to competitive adsorption and Fe transformation. This study demonstrates the dual role of DOM in regulating Cd dynamics and its interaction with sulfate, offering insights for Cd contamination management in paddy soils.