Key factors of mercury methylation and methylmercury accumulation in paddy soils

Rice consumption serves as a primary pathway for human dietary exposure to methylmercury (MeHg), a potent neurotoxin predominantly formed in paddy soils. Although soil physicochemical properties exert a strong influence on MeHg production and accumulation, soil heterogeneity has hindered the identification of consistent controlling factors. In this study, a systematic meta-analysis of 22 peer-reviewed datasets was conducted to evaluate environmental regulators of MeHg contamination in paddy soils, with a specific distinction made between sites located within and outside Hg mining areas. The results revealed significantly elevated Hg and MeHg levels in paddy soils associated with Hg mining activity. Across both site types, soil pH, organic matter (OM), and total Hg (THg) were identified as key determinants of MeHg dynamics. MeHg accumulation was highest under weakly alkaline conditions (pH 7.0-8.0), while both low (<10 g/kg) and high OM (>60 g/kg) OM levels were unfavorable for MeHg accumulation. In Hg mining area paddy fields, maximum %MeHg occurred at OM levels of 30-40 g/kg, whereas in ordinary paddy fields, the optimal range was 20-30 g/kg. In addition, rice cultivation significantly absorbs soil MeHg. In acidic paddy soils, modest pH reduction coupled with appropriate OM amendments may suppress MeHg production. These findings offer important insights for the targeted management and remediation of Hg and MeHg in paddy soil.