Abstract: The influence of selenium (Se) supplementation on mercury (Hg) speciation and its bioavailability in paddy soil was investigated using the sequential selective extraction method. Results showed that organochelated Hg, elemental Hg, and sulfide Hg were dominant, accounting for more than 90% of the total Hg in soils both with and without Se supplementation. The bioavailable portion of Hg in soil was limited, accounting for only 0.27% of the total. Se supplementation had a significant influence on the abundances of organochelated Hg and elemental Hg, but little influence on its other forms. The abundance of organochelated Hg was gradually decreased with the increase of Se supplementation in soils, while the change of elemental Hg was opposite. Se supplementation greatly mitigated the bioavailability of Hg in soils. We postulate that organochelated Hg is the important source for the bioavailable portion of Hg in soils. The shift of the balance between organochelated Hg and bioavailable Hg in soils is probably the main reason for the decreased bioavailability of Hg when the soils were added with Se.

Key words: NDVI maximum, Tibetan Plateau, precipitation change, precipitation concentration index, precipitation centroid