Insight Into The Reduction In The Rice Grain Methylmercury Concentrations Following Biochar Amendments

There are growing concern about methylmercury（Me Hg）accumulation in rice grains and the health risks of dietary exposure to Me Hg in mercury（Hg）-contaminated areas.In view of the lack of effective,green and economically feasible technologies for the treatment and remediation of Hg pollution in paddy fields,in this study,microcosm and pot experiments were conducted by adding biochar amendment to the Hg-contaminated paddy soil to explore:（1）the changes of Hg methylation and Me Hg bioavailability in soils;（2）the effect on Me Hg accumulation in rice under different water management conditions;（3）the changes of Me Hg exposure risk with Me Hg-laden rice consumption.The main conclusions are as follows:1.The results of microcosm experiments showed that bamboo-derived biochar had different effects on Hg methylation in soils.The addition of bamboo-derived biochar to soils in Xunyang and Yixing reduced Me Hg concentrations in soil;on the contrary,the addition of bamboo-derived biochar to soil in Wanshan increased Me Hg concentrations in soil.Similarly,cotton stalk-and smooth cordgrass-derived biochar increased Me Hg concentrations in Wanshan soil.However,the fraction of ammonium thiosulfate（（NH₄）₂S₂O₃）extractable Me Hg（percentage of the total）in all soils decreased with increasing dose of biochar.When 0.3%（w/w）biochar was added,the fraction of extractable Me Hg in soil decreased by 15–89%.2.The results of pot experiments showed that the addition of low-dose（0.3%,w/w）biochar reduced Me Hg concentration in rice.The addition of bamboo-derived biochar to soils in Wanshan,Xunyang,and Yixing reduced Me Hg concentration in rice by 56–88%;under flooding conditions for two successive years and intermittent flooding conditions for one year,the addition of bamboo-,cotton stalk-and smooth cordgrass-derived biochar to soil in Wanshan reduced Me Hg concentration in rice by 34–68%and32–49%,respectively.The decrease of grain Me Hg concentration following biochar amendment was attributed to the decrease of Me Hg bioavailability in soil,which may be closely related to the higher content of organic sulfur functional groups in biochar or/and reduced organic sulfur in soil,and further study is needed.3.The addition of low-dose（0.3%,w/w）biochar could reduce the risk of Me Hg exposure caused by Me Hg-laden rice consumption.In short,the present study could provide a scientific basis for the use of low-dose biochar to treat and remediate Hg-contaminated paddy soils,which indicates that biochar could have great potential for remediating of Hg-contaminated paddy fields.And the present study has important practical significance for agricultural safety production and health protection of local residents in Hg-contaminated areas.