Uncovering the Geochemical Fractionation of Newly Deposited Hg in Paddy Soil Using A Stable Isotope Tracer

Newly deposited mercury (Hg) is more readily methylated to methylmercury (MeHg) and accumulates in rice than ambient Hg in soil. However, the geochemical fractionation of newly deposited (“new” Hg) Hg in the soil is still unknown. In this study, enriched stable Hg isotope was employed to trace the fate of “new” Hg in paddy soils during the rice-growing period. We showed that the majority of “new” Hg was distributed as organic matter bound Hg , followed by residual Hg, Fe/Mn oxides bound Hg, soluble and exchangeable Hg, and carbonates bound Hg in paddy soils. Redistributions of newly deposited Hg in paddy soils were observed, and were shown as decreases in soluble and exchangeable Hg, carbonates bound Hg, organic matter bound Hg, and residual Hg, but increases in Fe/Mn oxides bound Hg. We further revealed that the coupling of autochthonous dissolved organic matter and poorly crystalline Fe (oxyhydr)oxides played predominant role in controlling the redistribution of “new” Hg in paddy soils. Aging processes of newly deposited Hg were absent. This study implies that other Hg pools (e.g., organic matter bound Hg) should be considered instead of merely soluble Hg pools when evaluating the environmental risks of Hg from atmospheric depositions