Abstract: A total of 175 soil samples were collected from surface soil layers at Yushu and Maduo, in the three rivers source region (Yangtz River, Yellow River and Lancang River) from July to September in 2012 for analysis of the chemical properties and Hg contents.On such a basis, relationships of Hg content with soil pH and total organic carbon (TOC) were investigated, and distribution characteristics of mercury in the soil of Yushu and Maduo were discussed. Results show that the soil in the studied area tended to be slightly alkaline and rich in TOC. The soil in Maduo was is higher in pH, but slightly lower in TOC than the soil in Yushu. The arithmetic average of mercury contents in the soil of the studied area was 0.034 mg•kg-¹, which was lower than the average backrground value of the country, but obviously higher than the average background value of Qinghai Province in 1990. Soil Hg content in the studied area was obviously on the high side, and much higher in gray-cinnamon soil than in the other four types of soils, which differed very slightly in average Hg content. The spatial distribution of Hg contents demonstrates that Hg content in the surface soil in Yushu was much higher than in Maduo, which is related to degree of urbanization and industrial structure of the two areas. High soil Hg content was found mainly in the areas with dense population and busy traffic. Correlation was found between Hg content and pH, while significantly positive relationship was observed between Hg content and TOC (r = 0.605, n = 90, P<0.05). Compared with Yushu, Maduo, relatively higher in pH and lower in TOC, was lower in soil Hg content, of which the specific cause waits needs to be further studied through continuous observation of changes in soil environmental quality in the area.

Key words: soil, mercury, spatial distribution, Yushu County, Maduo County