Dominant species as biological indicators to predict the changes of trace element in different types of rangeland

Alpine meadow, typical steppe and desert are globally important rangeland ecosystems. However, the seasonal variations in concentrations of trace elements, which are important nutrients for grazing livestock, are unclear in these types of rangeland and it is difficult to prevent disorders in grazing livestock caused by trace element excesses and deficiencies. In order to identify which forage species might serve as indicators of the trace element status of livestock forage, we investigated the seasonal trends in Fe, Mn, Cu, and Zn concentrations in the plant communities and dominant species from diverse rangeland types in northwest China in relation to livestock sufficiency levels. The trace element concentrations in the plant communities changed significantly with the season, with the highest Cu, Mn and Zn concentrations and the lowest Fe and Mn concentrations detected in spring and summer, respectively. The seasonal mean temperature, seasonal precipitation, and above-ground biomass significantly negatively correlated with the seasonal variation of Fe and Mn concentrations in the plant community. However, there was almost no significant correlation between these factors and the seasonal Cu and Zn concentrations. The plant concentrations of Fe and Mn were much higher than the recommended levels for livestock in all rangeland types, but the concentrations of Cu in the alpine meadow and the Zn concentrations in all the rangelands barely met the nutritional requirements for animal growth. Based on these deficiencies, we suggest that some forage species could be used as biological indicators to predict changes in the concentrations of trace elements. A decrease in the relative biomass of Kobresia capillifolia and Kobresia humilis in alpine meadows implies a decrease in the levels of Cu in the plant community. A decrease in the relative biomass of K. humilis in alpine meadows, an increase in Stipa bungeana in typical steppes and an increase in Nitraria tangutorum and Salsola passertine in desert rangelands lead to decreased concentrations of Zn in the plant community.