Transient methylmercury formation during wetland soil rewetting is governed by organic matter availability rather than coagulant history.

Rewetting of previously drained wetland soils can induce rapid redox shifts that influence mercury (Hg) cycling and methylmercury (MeHg) production. We investigated how prior exposure to common metal-salt and biopolymer coagulants influences Hg transformations during wetland soil rewetting and whether organic matter (OM) availability modulates these effects. Wetland soils (∼300 µg kg-1 total Hg) were incubated in triplicate soil-water microcosms for 16 days following rewetting. Treatments included soils previously exposed to ferric sulfate, polyaluminum chloride, or a chitosan-based coagulant, alongside untreated controls. Upon rewetting without additional OM, redox potential declined to ∼250 mV, consistent with manganese-reducing conditions, and MeHg concentrations remained relatively low in water (0.05-0.18 ng L-1) and soils (1-2 µg kg-1) across all treatments. In contrast, addition of algal-derived OM (Spirulina powder) rapidly decreased redox potential to 0-100 mV and produced a short-lived pulse of MeHg formation in all treatments, with aqueous MeHg peaking at ∼30-40 ng L-1 and soil MeHg at ∼6-8 µg kg-1 on days 2-4 before declining. Coagulant treatments increased transient inorganic Hg concentrations in overlying water (45-75 ng L-1) relative to controls (∼30 ng L-1) but did not enhance higher MeHg production. Results demonstrate that OM-driven redox dynamics, rather than prior coagulant exposure, control transient MeHg production during wetland soil rewetting, highlighting how episodic carbon inputs can regulate mercury transformation in managed and natural wetlands.