Iron-manganese (oxyhydro)oxides, rather than oxidation of sulfides, determine the mobilization of Cd during soil drainage in paddy soil systems.

The pre-harvest 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 twelve 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 (oxyhydr)oxides. Taken together, Fe-Mn (oxyhydr)oxides play a critical (and prevalent) role in controlling the mobilization of Cd upon soil drainage in paddy systems.