Rice hull biochar enhances the mobilization and methylation of mercury in a soil under changing redox conditions: Implication for Hg risks management in paddy fields

Abstract

Biochar amendment to paddy soils was promising to mitigate mercury (Hg) accumulation in rice; thus, it was applied to reduce human Hg exposure via rice consumption. However, how biochar affects Hg mobilization and MeHg formation in soil under changed redox potential (Eh) conditions remained unknown. Here, we explored the change of dissolved total Hg (DTHg) and dissolved MeHg (DMeHg), and their controlling biogeochemical factors in a soil with(out) biochar amendment under changing Eh conditions using biogeochemical microcosm. Biochar amendment resulted in a widen Eh range (−300 to 400 mV) compared to the control (−250 to 350 mV), demonstrating that biochar promoted reduction-oxidization reactions in soil. Biochar amendment enhanced Hg mobilization by mediating reductive dissolution of Fe/Mn (hydr)oxides. Thus, the increased Hg availability promoted MeHg formation in the soils. Biochar amendment changed the soil organic matter (SOM) composition. Positive correlations between the relative abundance of LIPID (lipids, alkanes/alkenes), ALKYL (alkylaromatics), and suberin and MeHg concentrations indicate that these SOM groups might be related to MeHg formation. Biochar enhanced the releasing and methylation of Hg by promoting the mobilization of Fe(oxyhydr)oxides and alternation of carbon chemistry under dynamic Eh conditions. There is an unexpected environmental risk associated with biochar application to paddy soils under dynamic Eh condition, and one should be aware this risk when applying biochar aiming to minimize human Hg exposure health risks via rice consumption.