Accumulation, risk assessment of trace elements in soil-herbage systems and effects of elevation on subalpine grassland in the northeast of Tibet Plateau

Ecological environment of remote grassland has become increasingly serious in many countries due to mining, tourism, grazing and other human activities. In this study, a total of 15 pairs of soil-herbage samples were collected in the northeast of the Tibet Plateau to study the relationship between physicochemical properties and content of trace elements in soils and elevation, and to examine the accumulation and fractionation of heavy metals in soil-herbage systems. In addition, the ecological risk of the subalpine grassland was also assessed. The average concentrations of Hg, As, Cu, Zn, Pb, Cd, Cr and Mn in soil were higher than their background values of Gansu soil, but the average concentrations of these heavy metals in herbage satisfied Hygienical Standard for Feeds. The speciation analysis of heavy metals in soil indicated that the exchangeable content of heavy metal was very low, except Pb, Cd, Mn. There was a linear relationship between pH, CaCO3, total phosphorus (TP), organic matter (OM), concentrations of Hg, As, Zn, Pb, Cr and Mn in soils, dry weight of herbage and elevation, while there was a quadratic curve trend between Cu, Cd in soils and elevation. The results of risk assessment showed that there was no obvious ecological risk in the study area.