Nitrogen is one of the important constraining factors affecting vegetation growth, especially in arid and semi-arid areas of natural grasslands. In a natural grassland ecosystem without fertilizer inputs, the vegetation nutrients mainly come from plant litter decomposition and the mineralization of organic nutrients. Soil nitrogen storage is the result of the historical accumulation of plant litter and the long-term weathering of parent rock in the grassland ecosystem. It is an important nutrient source for vegetation growth, is closely related to vegetation productivity, and is an indicator of ecological service function change. In recent years, the ecological and economic problems caused by grassland degradation and grassland soil nutrient depletion have become the focus of governments and researchers due to climate change and human activity disturbance. Although climate change has important effects on grassland ecosystems, it is difficult to control and manage at the smaller spatial-temporal scales. Human activity is one of the main influencing factors affecting grassland ecological systems. Therefore the control and management of human activities can effectively protect grassland ecosystems and promote sustainable grassland utilization.
Grazing is a significant human disturbance of grassland ecosystems, and is one of the important factors affecting the grassland nitrogen cycle. In order to examine the impact of grazing on soil N storage, we created a grazing pressure index model using the statistical annual grazing livestock data (1990-2011) from counties and MODIS-NDVI (1000 m resolution, 2001-2011), which combined grazing capacity and vegetation productivity per unit area. We used the pressure index model to assess the spatial distribution of grazing pressure in typical steppe of Inner Mongolia, China, and used field sampling point data in 2010 and 2011 across study area to analyze soil N storage and other several related soil properties at low grazing pressures (LG), moderate grazing pressures (MG), and high grazing pressures (HG). The results showed that grazing pressure had a significant effect on soil bulk density (BD), soil total carbon content(TC) and soil total nitrogen content(TN), especially in the surface soil layer (0-10cm). The BD, and TC and TN contents significantly differed between LG, MG, and HG. The TC and TN contents decreased with increasing grazing pressure, and the BD increased as the grazing pressure increased. Clay content (CC) did not significantly differ between the three grazing pressures. Soil N storage showed similar variation characteristics with TN content, but decreased with increasing of soil depth and grazing pressure. In the surface soil, in 2010 and 2011, the soil N storage showed significant differences among LG, MG, and HG. Within the range of 0 to 50 cm, the soil N storage decreased with increasing grazing pressure, and significantly differed among LG, MG and HG (2011, P < 0.05; 2010, P < 0.01). Therefore, grazing has no significant impact on the clay content, and the clay content variation is not the reason for the reduction in soil N storage in the study area. Heavy grazing is one of the main influence factors affecting nitrogen loss in the steppe ecosystem, and thus, reducing grazing pressure helps to reduce the loss in soil N storage and restores vegetation productivity.