Soil amendments with ZnSO or MnSO are effective at reducing Cd accumulation in rice grain: An application of the voltaic cell principle.

Cadmium (Cd) contamination in paddy soil often results in elevated Cd concentrations in rice grain, which is a serious concern threatening food safety. Most of the Cd accumulated in rice grain is derived from its remobilization in paddy soil during the grain filling period when paddy water is drained. We have previously shown that the voltaic cell effect controls the oxidative release of cadmium sulfide (CdS) during the drainage period. Metal sulfides with lower electrochemical potentials than CdS can suppress the oxidation of CdS. In the present study, we tested whether amendments of ZnSO or MnSO could enhance the suppressive voltaic effect on Cd release and subsequent accumulation in rice grain. The one-time addition of ZnSO (75 kg/ha Zn) decreased CaCl-extractable Cd concentrations in soils by 32-64% in pot experiments and by 16-30% in field trials during the drainage period. Consequently, Cd concentrations in brown rice were reduced by 74-87% and 60-72% in pot experiments and field trials, respectively. Importantly, this effect persisted in the second year without further addition. The amendment of MnSO had similar effects in decreasing soil extractable Cd and Cd concentrations in brown rice. These effects were not attributed to the addition of sulfate. A single application of such doses of ZnSO or MnSO (e.g. 75-150 kg/ha Zn or Mn) only caused a marginal increase in soil Zn or Mn concentrations and had no significant impact on grain yield. Taken together, amendments of ZnSO and/or MnSO (at the rate of 75-150 kg/ha Zn and or Mn) formed a protective voltaic cell partner against the oxidative dissolution of CdS and thus were highly effective in reducing Cd accumulation in rice grain. This work provides a simple but effective method to decrease soil Cd availability during soil drainage and mitigate Cd accumulation in rice to ensure food safety.