Spatial distribution and ecological risk assessment of heavy metals in alpine grasslands of the Zoige Basin, China

Heavy metals elements are not only related to the functions of ecosystems but also affect human health. To understand the characteristics of heavy metals in the topsoil of the Zoige alpine basin, a total of 252 grass and topsoil samples were collected in May and September 2014. The results showed that only Cd and Pb highly exceeded their background values (BV); in May and September, Cd was 2.02- and 1.55-fold higher than its BV, respectively, and Pb was 2.35- and 2.17-fold above its BV, respectively. The sources of Cd and Pb were homologous. In addition, the comprehensive potential ecological risk index was less than 150, indicating that heavy metal pollution in the study area is currently low. The spatial interpolation indicated that Cd and Pb pollution might be related to tourism and transportation, but the low biological absorption coefficient for all heavy metals showed that heavy metal absorption ability of forage was low and would not impact yak breeding. Finally, the soil was lightly contaminated by Cd and Pb due to the rapid development of the animal husbandry and tourism. The spatial variation of heavy metal in the basin is dominated by structural factors, and the random factors also have an effect on spatial distribution of As, Cd, Cu and Ni. The random factors such as overgrazing can exert an influence on physical structure and the circulation of nutrient substances of meadow soil through livestock grazing and trampling, ultimately affecting the content and distribution of soil heavy metals.

Automated summary

Heavy metals elements not only affect ecosystems but also human health. The study found that only Cd and Pb exceeded their background values in the topsoil of the Zoige alpine basin, with Cd being 2.02- and 1.55-fold higher than its background value in May and September, respectively, and Pb being 2.35- and 2.17-fold higher than its background value in May and September, respectively. The comprehensive potential ecological risk index indicated low pollution levels in the study area. Spatial interpolation suggested that Cd and Pb pollution might be related to tourism and transportation, but the low biological absorption coefficient of heavy metals in forage indicated minimal impact on yak breeding. Overall, the soil in the basin was lightly contaminated with Cd and Pb due to rapid development in animal husbandry and tourism. Spatial variation of heavy metals was primarily influenced by structural factors, while random factors such as overgrazing influenced the content and distribution of soil heavy metals.