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

Authors: FELIZITAS, BOIE - University Of Wuppertal; Ducey, Thomas; YING, XING - University Of Wuppertal; JIANXU, WANG - University Of Wuppertal; JÖRG, RINKLEBE - University Of Wuppertal

Submitted to: Journal of Hazardous Materials
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2024
Publication Date: 4/27/2024
Citation: Felizitas, B., Ducey, T.F., Ying, X., Jianxu, W., Jörg, R. 2024. Field-aged rice hull biochar stimulated the methylation of mercury and altered the microbial community in a paddy soil under controlled redox condition changes. Journal of Hazardous Materials [online]. 472. https://doi.org/10.1016/j.jhazmat.2024.134446.
DOI: https://doi.org/10.1016/j.jhazmat.2024.134446

Interpretive Summary: Mercury is a heavy metal contaminant frequently identified in rice paddy fields, and poses a human health risk when it is converted in compounds such as methylmercury, taken up by rice plants and enters the human food chain. Biochar is a widely adopted remediation tool for the binding and sequestration of heavy metals, but the understanding of the role biochar might play in sequestering mercuric compounds in rice paddy soil remains unknown. In this study, we investigated the interactions between a rice hull biochar and mercury in a rice paddy soil under varying oxygen conditions. Unfortunately, the influence of biochar amendment to the soils under low oxygen conditions result in shifts to microbial community populations that stimulate the conversion of mercury to methylmercury, while concomitantly inhibiting microbial populations which would remove those methyl groups (in a process known as demethylation). Further work should focus on how to restore microbial groups that can perform demethylation of mercury, as well as understand the exact influences that biochar has on microbial populations that stimulate mercury methylation under low oxygen conditions.

Technical Abstract: Mercury (Hg) contaminated paddy soils are hot spots for the microbially mediated neurotoxin methylmercury (MeHg) which can enter the food chain via rice plants causing high risks for human health. The bioavailability of Hg and its uptake by plants is affected by biochar. However, the interactions between dynamic redox conditions, microbial Hg methylation and biochar is not clear yet. To address this knowledge gap, we investigated MeHg and the microbial community in a paddy soil non-treated and treated with rice hull biochar under dynamic redox conditions (< 0 mV – 600 mV). Hg methylation exceeded demethylation in the biochar-treated paddy soil accompanied by increased aromaticity of dissolved organic matter at certain redox windows. Abundances of the aromatic hydrocarbon degraders Phenylobacterium and Novosphingobium were enhanced in the biochar-treated soil. The rice hull biochar stimulated Hg methylation by providing electron donors. Hg demethylation occurred in the untreated paddy soil due to interactions between putative Hg demethylators and ammonia and nitrite oxidizers. The genus Hyphomicrobium played an important role in the degradation of MeHg. Therefore, we recommend investigations on the demethylation potential of Hyphomicrobium-enriched microbial communities and further studies on Hg methylation related to the microbial utilization of aromatic groups under dynamic redox conditions.