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Biochar could reshape microbial communities, thereby altering methylmercury (MeHg) concentrations in rice rhizosphere and seeds. However, it remains unclear whether and how biochar amendment perturbs microbe-mediated MeHg production in mercury (Hg) contaminated paddy soil. Here, we used pinecone-derived biochar and its six modified biochars to reveal the disturbance. Results showed that selenium- and chitosan-modified biochar significantly reduced MeHg concentrations in the rhizosphere by 85.83% and 63.90%, thereby decreasing MeHg contents in seeds by 86.37% and 75.50%. The two modified bicohars increased the abundance of putative Hg-resistant microorganisms Bacillus , the dominant microbe in rhizosphere. These reductions about MeHg could be facilitated by biochar sensitive microbes such as Oxalobacteraceae and Subgroup_7. Pinecone-derived biochar increased MeHg concentration in rhizosphere but unimpacted MeHg content in seeds was observed. This biochar decreased the abundance in Bacillus but enhanced in putative Hg methylator Desulfovibrio. The increasing MeHg concentration in rhizosphere could be improved by biochar sensitive microbes such as Saccharimonadales and Clostridia. Network analysis showed that Saccharimonadales and Clostridia were the most prominent keystone taxa in rhizosphere, and the three biochars manipulated abundances of the microbes related to MeHg production in rhizosphere by those biochar sensitive microbes. Therefore, selenium- and chitosan-modified biochar could reduce soil MeHg production by these microorganisms, and is helpful in controlling MeHg contamination in rice. [Display omitted] • Effects of six modified biochars on the enrichment of MeHg in rice were compared • Selenium and chitosan modified biochars greatly reduced MeHg contents in rice. • Bacillus altered soil MeHg production by MeHg degradation under biochar amendment. • Saccharimonadales and Clostridia could increase MeHg contents in rhizosphere. [ABSTRACT FROM AUTHOR]

Over 3000 mercury (Hg)-contaminated sites worldwide contain liquid metallic Hg [Hg(0) l ] representing a continuous source of elemental Hg(0) in the environment through volatilization and solubilization in water. Currently, there are few effective treatment technologies available to remove or sequester Hg(0) l in situ. We investigated sonochemical treatments coupled with complexing agents, polysulfide and sulfide, in oxidizing Hg(0) l and stabilizing Hg in water, soil and quartz sand. Results indicate that sonication is highly effective in breaking up and oxidizing liquid Hg(0) l beads via acoustic cavitation, particularly in the presence of polysulfide. Without complexing agents, sonication caused only minor oxidation of Hg(0) l but increased headspace gaseous Hg(0) g and dissolved Hg(0) aq in water. However, the presence of polysulfide essentially stopped Hg(0) volatilization and solubilization. As a charged polymer, polysulfide was more effective than sulfide in oxidizing Hg(0) l and subsequently stabilizing the precipitated metacinnabar (β-HgS) nanocrystals. Sonochemical treatments with sulfide yielded incomplete oxidation of Hg(0) l , likely resulting from the formation of HgS coatings on the dispersed µm-size Hg(0) l bead surfaces. Sonication with polysulfide also resulted in rapid oxidation of Hg(0) l and precipitation of HgS in quartz sand and in the Hg(0) l -contaminated soil. This research indicates that sonochemical treatment with polysulfide could be an effective means in rapidly converting Hg(0) l to insoluble HgS precipitates in water and sediments, thereby preventing its further emission and release to the environment. We suggest that future studies are performed to confirm its technical feasibility and treatment efficacy for remediation applications. [Display omitted] • Sonochemical oxidation and stabilization of liquid elemental mercury [Hg(0) l ] are evaluated in water and soil. • Sonication promotes the breakup and oxidation of Hg(0) l beads via acoustic cavitation. • Sonication coupled with polysulfide prevents Hg(0) l solubilization and volatilization loss. • Polysulfide is more effective than sulfide in sonochemical oxidation of Hg(0) l and formation of HgS nanocrystals. • Sonochemical treatment offers a potentially effective means for rapidly converting Hg(0) l to HgS in water and soil. [ABSTRACT FROM AUTHOR]

Previous research has found total mercury (THg) and methylmercury (MeHg) levels increase with litterfall decay, thus suggesting litterfall decomposition plays an essential role in the biogeochemical transformation of mercury (Hg). However, it remains unclear how Hg accumulates in the decaying litter, how bacterial taxa networks vary and what roles various microorganisms play during litterfall decomposition, especially nitrogen (N)-fixing, MeHg-degrading and Hg-methylating microbes. Here, we demonstrated as degradation proceeded, a gradually-complex network evolved for litterfall bacteria for the subtropical mixed broadleaf-conifer (MBC) forest, whereas a relatively static network existed for the evergreen broadleaf (EB) forest. N-fixing and MeHg-degrading bacteria dominated throughout litterfall decomposition process, with relative abundances of N-fixing genera and nifH copies maximum and relative abundances of MeHg-degrading bacteria and merAB copies minimum in summer. Hence, N-fixing bacteria likely mediate THg increase in the decomposing litterfall, while MeHg enhancement may be regulated by aerobic MeHg-degrading microbes which can transform MeHg to inorganic divalent Hg (Hg2+) or further to elemental Hg (Hg0). Together, this work elucidates variations of N-fixing and MeHg-degrading microbes in decaying litterfall and their relationships with Hg accumulation, providing novel insights into understanding the biogeochemical cycle of Hg in the forest ecosystem. [Display omitted] • A gradually-complex network evolved for the decaying litterfall of MBC forest. • A relatively static network existed for the decomposing EB forest litterfall. • N-fixing bacteria may mediate the increase of THg levels in the decomposing litterfall. • MeHg enhancement is likely regulated by aerobic MeHg-degrading microbes. • Underlying soil is the source of litterfall THg, while MeHg is transported from litterfall to soil. [ABSTRACT FROM AUTHOR]