Organic amendment-induced reducing conditions facilitate mercury methylation in paddy soils: Insights from the iron/sulfur reduction

Organic amendments in paddy fields, although agronomically beneficial, can potentially stimulate mercury (Hg) methylation due to distinct shifts in soil properties triggered by different types of amendments. Identifying the specific conditions that lead to heightened Hg methylation is thus essential for risk mitigation and sustainable management. Through incubation experiments with rice straw (RS), pig manure (PM), cow dung (CD) and chicken manure (CM), key drivers of Hg methylation were identified by linking methylation extent to amendment-induced dynamics in soil pH, Eh, Fe/S, and organic matter. Results revealed amendment-specific temporal dynamics governed by organic matter composition and reactivity. Rice straw and PM triggered a rapid Eh decline to –300 mV, concurrently accelerating Fe/S reduction and causing early methylmercury (MeHg) peaks on day 10 (7.51 ± 1.16 ng/g) and 18 (3.94 ± 0.32 ng/g), respectively. In contrast, CD and CM exhibited delayed Eh minimization and MeHg peaks (1.75 ± 0.58 and 12.94 ± 0.88 ng/g, respectively) until day 70. This differential MeHg production pattern was mainly driven by Eh-induced Fe/S reduction, which enhanced Hg bioavailability through reductive iron-oxides dissolution and subsequent soluble Hg-S complexes formation, as well as increased abundance of Hg-methylating microbes. These findings highlight that organic materials decomposition kinetics regulate MeHg formation via reductive Fe-S cycling, supporting targeted contamination mitigation strategies in agriculture.