The divergent roles of humic- and fulvic-acid DOM in regulating Hg methylation/demethylation in paddy soils.

Understanding mercury (Hg) methylation and demethylation processes in paddy soils is crucial for controlling food contamination and health risks, yet the roles of dissolved organic matter (DOM) fractions remain unclear. A year-long field study, followed by an incubation experiment using Hg isotope tracers (202HgII and Me200Hg), was conducted to investigate how the humic acid (HA) and fulvic acid (FA) fractions regulate these processes. Field correlation results showed significant relationships between DOM properties and MeHg levels (p < 0.001), which varied with the dominant characteristics of DOM in paddy soils. The prevalent humification compounds reduced MeHg production, whereas enriching paddy soil with autochthonous DOM increased Hg methylation and MeHg production. The average methylation rate constants (Km-202HgII) for Hg-bound FA (Hg-FA) and Hg-bound HA (Hg-HA) treatments were 0.91 ± 0.22 and 0.40 ± 0.06 10-3 d-1, respectively, and the average demethylation rate constants (Kd-Me200Hg) were 0.28 ± 0.07 and 0.36 ± 0.11 d-1. Incubation results revealed that Hg-FA, with low aromaticity and molecular weight, had significantly higher methylation rates than Hg-HA (p < 0.05), suggesting greater bioavailability. Conversely, high-molecular-weight HA, with greater aromaticity and humic character, reduced net methylation potential and promoted MeHg degradation. Hg methylation and demethylation in response to the same DOM fraction varied across the studied sites, highlighting the distinct roles of each fraction's properties in paddy soils. This study broadens the understanding of DOM's roles in Hg fate in rice paddies, demonstrating that net MeHg production is governed by the balance between FA-promoted methylation and HA-promoted demethylation, challenging the traditional view that total DOM concentration is the primary controlling factor.