Rice paddy soil, as a typical ephemeral wetland, is known to be a significant environment for Hg methylation. The methylation of inorganic mercury (IHg) in rice paddy soil constitutes a key step in the cycling of Hg in this special terrestrial ecosystem, especially within Hg mining area. However, the systematic survey of the Hg methylation process and the biogeochemical controls on Hg methylation in paddy soil are remaining poorly understood. Thus, an understanding of the factors that control the MeHg production is necessary to underpin more reliable risk assessment and appropriate strategies to remediate contaminated soil.

Financed by the National Key Basic Research Program of China (2013CB430004) and the National Natural Science Foundation of China (41173126, 41203091, and 41473123), the research group headed by Prof. FENG Xinbin at Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) studied the distribution patterns and methylation/demethylation processes of Hg in rice paddy in Hg mining area.

Researchers compared the source and distribution of Hg species in different compartments of the rice paddy during a complete rice growing season at two different typical Hg-contaminated mining sites in Guizhou province, China: an abandoned site and a current-day artisanal site. In situ specific methylation/demethylation rate constants in rice paddy soil were qualified using stable Hg isotope addition technique, simultaneously. The possible biogeochemical controls on MeHg production within the rice paddy ecosystem season were further discussed in the study.

Their results showed that the dominant source of MeHg to paddy soil was in situ methylation of IHg. Active net Hg methylation happened in the soil surface layer at the artisanal Hg mining; however, net production of MeHg at the abandoned Hg mining site was limited. MeHg levels in rice paddy soil were a function of both methylation/demethylation processes and the net methylation potential in the rice paddy soil reflected the measured MeHg. Sulfate stimulating the activity of sulfate-reducing bacteria was a potentially important metabolic pathway for Hg methylation in rice paddies.

The researchers suggest that bioavailable Hg derived from new atmospheric deposition appears to be the primary factor regulating net MeHg production in rice paddies.

The results shown above are recently published in Environmental Pollution and Biogeosciences, respectively.