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. δ202Hg values of MeHg showed no significant
differences between roots, stalks, leaves, and grains at each growth
stage. The similar Δ199Hg values of MeHg between
rice tissues (0.14 ± 0.08‰, 2SD, n =
12), soil (0.13 ± 0.03‰, 2SD, n = 4),
and irrigation water (0.17 ± 0.09‰, 2SD, n = 5) suggested that the soil–water system was the original
source of MeHg in rice plants. Δ199Hg 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.
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