Iron–Manganese (Oxyhydro)oxides, Rather than Oxidation of Sulfides, Determine Mobilization of Cd during Soil Drainage in Paddy Soil Systems

The preharvest drainage of rice paddy fields during the grain filling stage can result in a substantial mobilization of Cd in soil and, consequently, elevated grain Cd concentration. However, the processes controlling the mobilization of Cd remains poorly understood. Using 12 field-contaminated paddy soils, we investigated the factors controlling the temporal changes in Cd solubility in paddy soils that were incubated anaerobically for 40 d followed by a 20 d oxidation period. Soluble and extractable Cd concentrations decreased rapidly upon flooding but increased during the oxidation phase, with Cd solubility (aqueous Cd/soil Cd) largely depending upon porewater pH. Furthermore, inhibiting sulfate reduction or inhibiting oxidation dissolution of Cd-sulfides had little or no effect on the mobilization of Cd in the subsequent oxidation phase. Both sequential extraction and X-ray absorption spectroscopy (XAS) analyses revealed that changes in Cd solubility were largely dependent upon the transformation of Cd between the Fe–Mn (oxyhydro)oxide fraction and exchangeable fraction. Mobilization of Cd upon soil drainage was caused by a decrease in soil pH resulting in the release of Cd from Fe–Mn (oxyhydro)oxides. Taken together, Fe–Mn (oxyhydro)oxides play a critical (and prevalent) role in controlling the mobilization of Cd upon soil drainage in paddy systems.