Impact of historic grazing on steppe soils on the northern Tibetan Plateau

Few studies have focused on changes in the physical and chemical properties of soils that are induced by grazing at high altitudes. Our aim was to identify potential responses of soil to grazing pressure on the semiarid steppe of the northern Tibetan Plateau and their probable causes. Fractal geometry to describe soil structure, soil dynamics, and physical processes within soil is becoming an increasingly useful tool that allows a better understanding of the performance of soil systems. In this study, we sampled four experimental areas in the northern part of the Tibetan Plateau under different grazing intensities: ungrazed, lightly grazed, moderately grazed and heavily grazed plots. Fractal methods were applied to characterise particle-size distributions and pore patterns of soils under different grazing intensities. Our results reveal a highly significant decrease in the fractal dimensions of particle size distributions (D (1) ) and the fractal dimensions of all pores (D (2) ) with increasing grazing intensity. Soil organic carbon (SOC), total N and total P concentrations increased significantly with decreasing grazing intensity. We did not find differences in soil pH in response to grazing. Grazing induced a significant deterioration of the physical and chemical topsoil properties in the semiarid steppe of the northern Tibetan Plateau. Fractal dimensions can be a useful parameter for quantifying soil degradation due to human activities.