Topography and grazing effects on storage of soil organic carbon and nitrogen in the northern China grasslands

Abstract

Soil on the earth has been estimated to sequester around 5.4% of total carbon (C), most of it as soil organic carbon (SOC), but the exact amount of and relevant mechanisms for the stock of SOC are far from clear. In this study, we assessed how topography and grazing influenced distribution of SOC, and soil total nitrogen (STN) in grasslands in the hilly and mountainous regions of northern China. Soil parameters (soil organic C and N content, pH value, gravel content, and bulk density in 50 cm soil depth) and topographic elements (slope aspect, position and inclination) were investigated at 171 sites. Results from model selection of generalized additive model (GAM) procedure showed the model with integration of topographic factors had strong predictability on the distribution of SOC and STN. This indicated that topography was an important driving factor for distribution of SOC and STN. Both SOC and STN storage were observed higher on shadow slopes and at the middle position of slopes. Although slope inclination had no direct effects on SOC and STN storage, it showed significant effects on soil pH value, gravel content and bulk density, which were significantly negatively correlated with stock of SOC and STN. Besides, the long history of grazing throughout the study area was also found to have significant negative effects on storage of SOC and STN, suggesting weakened relationship between topography and SOC and STN stocks. Our results indicate that topography, in combination with human activities such as grazing, has significant effects on storage of soil C and N either directly or indirectly via changing soil physico-chemical properties. Our study can help to understand the pattern of the topography effects on the storage of SOC and STN in grasslands with patchy distributions in the hilly and mountainous regions of the north China.