Mercury (Hg) pollution in soils and sediments receives considerable concern because Hg, especially its methylated form methylHg (MeHg), can be readily accumulated by plants, threatening the health of consumers. Knowledge regarding the mechanism of mobilization and methylation of Hg in soils is fundamental to manage the risk of this metal, particularly in paddy soils. Seasonal flooding of paddy fields leads to frequent changes of redox potential (E_H), and drives series of biogeochemical reactions, thereby affecting the Hg mobilization and methylation. Therefore, we aim to investigate the mobilization and methylation of Hg in a polluted soil under dynamic redox conditions using an advanced automated biogeochemical microcosm system. The experiment was conducted under stepwise variation from reducing (around −380 mV) to oxidizing conditions (around +310 mV). The soil suspensions were regularly collected from the microcosm at the defined E_H windows (-300 mV, -200 mV, -100 mV, 0 mV, +100 mV, +200 mV, +300 mV), subsequently centrifuged, and filtrated (separated into dissolved and colloid phases). The filtrated samples were analyzed for total Hg (THg), MeHg, dissolved organic carbon (DOC), iron (Fe), manganese (Mn), sulphate (SO₄²⁻), and chloride (Cl⁻). The colloidal phase was collected for TEM-EDX analysis. The sediment-phase was analyzed for Fe K-edge X-ray near edge structure (XANES) spectroscopy, MeHg, phyto-available Hg, and for microbial communities.

The increase of redox potential caused an increase of soil pH. Dissolved concentrations of THg, MeHg, Fe, Mn, DOC, and SO₄²⁻ increased under reducing conditions, while Cl^- increased under oxic conditions. The increase of THg concentration under reducing acidic conditions might be due to the reductive/acidic dissolution of Fe/Mn oxides, by which the sorbed Hg might be mobilized. The increase of MeHg concentration at low E_H indicates an enhanced Hg methylation, likely due to the presence of Hg-methylation microorganisms such as Desulfitobacterium, as demonstrated by 16S rRNA sequences analysis. Also, the higher THg concentration might provide more inorganic Hg substrate for microbial methylation. The ln(MeHg/THg) ratio (Hg methylation potential) indicates the net production of MeHg normalized to total dissolved Hg (THg) concentration. This ratio increases with rising DOC to THg ratio (ln(DOC/THg) ratio) (R²=0.59, P < 0.01), while decreases with ln(Cl^-)(R²=0.66, P < 0.01). The colloidal concentration of MeHg and THg increased under moderately oxic and oxic conditions, respectively, which might be due to the colloid particles formed at high pH adsorbed Hg or MeHg in soil suspension. The colloidal phase consisted of Fe, Mn, Mg, Al, and Si aggregates, as characterized by TEM-EDX analysis. The Fe K-edge XANES analysis for the sediment phase showed that the dissolution of Ferrihydrite occurred at reducing conditions, confirming the mobilization of Fe oxides. Our results conclude that the redox potential changes affect the mobilization and methylation of Hg through related changes in DOC, Cl-, Fe, and Mn, as well as microbial community structures. This knowledge may be helpful for managing the risk of Hg in soil.