The pedosphere, due to its larger organic carbon storage, higher respiration rate and shorter turnover time in the carbon exchange with the atmosphere, plays an important role in the processes of global carbon cycle. China's grasslands with various types constitute an important part of the global grassland ecological system, which provides ideal test fields for the study of soil organic carbon content and its variation. Soil organic carbon (SOC) content is controlled by many factors (such as climate, parent material, vegetation, and soil physicochemical-biological properties) that have complex interactions. Primary productivity controlled by the climate influences the formation of soil organic matter, while soil physicochemical-biological properties are responsible for the decomposition rate of soil organic matter. In this study, we collected soil samples (n=374) from alpine grassland in the Qinghai-Tibet Plateau and temperate grassland in the Inner Mongolia Plateau and measured the organic carbon contents, bulk density, texture, moisture, total nitrogen, pH and roots of these soil samples. Also, we applied stepwise multi-regression models to evaluate the combined effects of soil environmental factors on SOC content. The results showed that the SOC content in the alpine grassland (20.18 kg C/m²) was higher than that in the temperate grassland (9.23 kg C/m²), indicating that SOC contents probably were more dominated by soil physicochemical-biological factors. In the study region, there were different relationships between SOC contents and different soil factors. For example, SOC contents were significantly negatively correlated with soil bulk density, and significantly positively with moisture, silt contents, roots, total nitrogen and the C/N ratio, respectively. The correlations of the SOC contents with pH and clay contents were not significant. Total nitrogen, moisture and roots in the soils were key factors on SOC contents change (top 10 cm) of the grassland ecosystem. The combined contribution of total soil nitrogen and roots to the variation of SOC contents in the alpine grassland represented 87.84 %, while it was lower (75%) to the variation of SOC contents (top 10 cm) in the temperate grassland. The contribution of soil moisture to the SOC contents variation in the temperate grassland was up to 33.27%. Therefore, soil moisture was the most important factor for the SOC variation in the temperate grassland. The C/N ratio in the alpine grassland was significantly higher than that in the temperate grassland, indicating that slower decomposition of soil organic matter in the alpine grassland resulted from lower microbial activity.
Responding to the global environmental change, the variation of SOC reservoirs was determined by a combination of physical processes (weathering processes, water cycle processes, soil formation processes and biological processes) on the earth surface and anthropogenic activities. Therefore,it deserves efforts to investigate how the SOC reservoirs of grassland ecosystems in China, located in sensitively regions to climatic change, will respond to these changes.