Three-year dynamics of methylmercury production in Hg2+-spiked paddy soils: Mercury speciation, microbial communities, and rice contamination

We investigated three-year changes in soil mercury (Hg) pools, methylmercury (MeHg) production, rice contamination, and microbial communities after a single Hg²⁺ addition to two soils (Soil I and Soil II). In Soil I, total Hg (T-Hg) concentration of brown rice grain was 0.150 ± 0.023 mg/kg (n = 143) in 2015 and increased to 0.233 ± 0.080 (n = 135) and 0.240 ± 0.118 mg/kg (n = 225) in 2016 and 2017. In Soil II, T-Hg declined from 0.530 ± 0.101 (n = 130) in 2015 to 0.124 ± 0.059 (n = 213) and 0.168 ± 0.059 mg/kg (n = 200) in 2016 and 2017. Variations in T-Hg concentrations in rice grains cultivated in the two soils showed a relationship with soil MeHg concentrations within the same soil, but not between different soils. Sequential extraction, which partitioned soil Hg into seven fractions, indicated that Soil II contained a higher proportion of water-extractable Hg. This finding suggests that the mobility of Hg may have influenced the level of Hg contamination in rice grains. The proportion of Hg sulfide peaked approximately one month after the addition of Hg²⁺ in both soils, then decreased over time. In contrast, the fractions of organic-bound and elemental Hg tended to increase over time. In soil II, where DNA extraction was successful, microbial communities showed no clear differences at the phylum level between the Hg-added and non-added samples, but distinct shifts were observed at lower taxonomic levels. Metagenomics showed that the MeHg/T-Hg ratio correlated positively with hgcAB gene abundance (r = 0.85, P < 0.05), while merA/merB showed no clear relationship.