Mechanisms of POM-mediated Hg immobilization and MeHg suppression in paddy soil: a perspective of coupled geochemical and microbial processes under water management.

Particulate organic matter (POM) plays a critical role in Hg biogeochemistry, yet its immobilization mechanisms and interactions with water management remain unclear. This study investigated POM-mediated Hg immobilization under non-flooded and flooded conditions through a 120-day incubation experiment, combined with batch adsorption, multi-technique characterization, and microbial community analysis. POM addition significantly reduced soil available Hg and promoted the redistribution of Hg to operationally defined less-labile or residual fractions, with available Hg reduction reaching 82% under flooded conditions, compared to 61% under non-flooded. Mechanistically, POM immobilized Hg2+ through physical entrapment within its porous structure and chemisorption-dominated surface complexation via sulfur and oxygen-containing functional groups, with reactive organic sulfur as the most active binding site. Indirectly, POM addition increased SOC, and structural equation modeling (SEM) supported a statistical pathway consistent with SOC contributing to Hg transformation under flooding (path coefficients: 0.872-0.902). Additionally, POM dose-dependently suppressed MeHg accumulation by reducing bioavailable Hg pools and decreasing hgcA gene abundance, with microbial community restructuring further contributing to this suppression through enrichment of keystone taxa such as Vicinamibacter. Overall, POM amendment combined with flooding may mitigate Hg bioavailability in contaminated agricultural soils under controlled conditions, but field validation is required before recommending this as a practical strategy.