Field-aged rice hull biochar stimulated the methylation of mercury and altered the microbial community in a paddy soil under controlled redox condition changes

Mercury (Hg) contaminated paddy soils are hot spots for methylmercury (MeHg) which can enter the food chain via rice plants causing high risks for human health. Biochar can immobilize Hg and reduce plant uptake of MeHg. However, the effects of biochar on the microbial community and Hg (de)methylation under dynamic redox conditions in paddy soils are unclear. Therefore, we determined the microbial community in an Hg contaminated paddy soil non-treated and treated with rice hull biochar under controlled redox conditions (< 0mV to 600mV) using a biogeochemical microcosm system. Hg methylation exceeded demethylation in the biochar-treated soil. The aromatic hydrocarbon degraders Phenylobacterium and Novosphingobium provided electron donors stimulating Hg methylation. MeHg demethylation exceeded methylation in the non-treated soil and was associated with lower available organic matter. Actinobacteria were involved in MeHg demethylation and interlinked with nitrifying bacteria and nitrogen-fixing genus Hyphomicrobium. Microbial assemblages seem more important than single species in Hg transformation. For future directions, the demethylation potential of Hyphomicrobium assemblages and other nitrogen-fixing bacteria should be elucidated. Additionally, different organic matter inputs on paddy soils under constant and dynamic redox conditions could unravel the relationship between Hg (de)methylation, microbial carbon utilization and nitrogen cycling. Environmental implication Microbial-mediated transformation of mercury into methylmercury (MeHg) is hazardous for the environment and human health. Certain biochars can immobilize Hg in soils. However, regulating effects of biochar on Hg methylation and microbial community composition under fluctuating redox conditions is poorly understood. Therefore, we analyzed the microbial community in a Hg contaminated paddy soil non-treated and treated with rice hull biochar under controlled redox condition changes. Microbial assemblages were important in Hg transformation providing available organic matter and stimulating Hg methylation in the biochar-treated soil. In the non-treated soil, lower available organic matter fostered MeHg demethylation by Actinobacteria and nitrogen-fixing bacteria.