Isotopic Fractionation and Source Appointment of Methylmercury and Inorganic Mercury in a Paddy Ecosystem

Bioaccumulation of methylmercury (MeHg) in rice grains has been an emerging issue of human health, but the mechanism of bioaccumulation is still poorly understood. Mercury (Hg) isotope measurements are powerful tools for tracing the sources and biogeochemical cycles of Hg in the environment. In this study, MeHg compound-specific stable isotope analysis (CSIA) was developed in paddy soil and rice plants to trace the biogeochemical cycle of Hg in a paddy ecosystem during the whole rice-growing season. Isotopic fractionation was analyzed separately for MeHg and inorganic Hg (IHg). Results showed distinct isotopic signals between MeHg and IHg in rice plants, indicating different sources. delta Hg-202 values of MeHg showed no significant differences between roots, stalks, leaves, and grains at each growth stage. The similar Delta Hg-199 values of MeHg between rice tissues (0.14 +/- 0.08 parts per thousand, 2SD, n = 12), soil (0.13 +/- 0.03 parts per thousand, 2SD, n = 4), and irrigation water (0.17 +/- 0.09 parts per thousand, 2SD, n = 5) suggested that the soil-water system was the original source of MeHg in rice plants. Delta Hg-199 values of IHg in the paddy ecosystem indicated that water, soil, and atmosphere contributed to IHg in grains, leaves, stalks, and roots with varying degree. This study demonstrates that successful application of MeHg CSIA can improve our understanding of the sources and bioaccumulation mechanisms of MeHg and IHg in the paddy ecosystems.