Abstract: Mercury (Hg), a highly toxic heavy metal pollutant, has been recognized as a pollutant with priority control by China, the United Nations Environment Program (UNEP), World Health Organization (WHO), European Union (EU) and the United States Environmental Protection Agency (USEPA). The toxicity of Hg depends on its chemical speciation, in which methylmercury (MeHg) is the most toxic compound. Inorganic mercury (IHg) can be transformed into MeHg in certain condition, and then accumulated and biomagnified in the food chain, posing a potential threat to human health. Paddy soil is an important site for MeHgproduction. Rice field is an important source of MeHg in terrestrial ecosystems. Numerous studies have reported the MeHg contaminated rice worldwide. Recent studies have confirmed that rice consumption is the major pathway of MeHg exposure in Hg mining area and in certain Hg contaminated areas. Biogeochemical processes of Hg in paddy field ecosystem have been well understood in the past decades. This review systematically summarized the biogeochemical processes of Hg in paddy field ecosystems, including: (1) status of Hg pollution in paddy field ecosystem; (2) Hg methylation in rice paddy and its primary controlling factors; (3) the mechanism of Hg accumulation in rice plant; (4) health risks of human exposure to MeHg through rice intake; (5) management strategies to reduce MeHg in rice. With those results gathered, we summarized the shortcomings of the previous studies and prospected the future researches as well.

Key words: priming effect, carbon isotope, soil layer, organic carbon mineralization, no-tillage., black soil