Organic and inorganic carbon in the topsoil of the Mongolian and Tibetan grasslands: pattern, control and implications

Soil carbon (C) is the largest C pool in the terrestrial biosphere and includes both inorganic and organic components. Studying patterns and controls of soil C help us to understand and estimate potential responses of soil C to global change in the future. Here we analyzed topsoil data of 81 sites obtained from a regional survey across grasslands in the Inner Mongolia and on the Tibetan Plateau during 20062007, attempting to find the patterns and controls of soil inorganic carbon (SIC) and soil organic carbon (SOC). The averages of inorganic and organic carbon in the topsoil (0-20 cm) across the study region were 0.38% and 3.63 %, ranging between 0.00-2.92% and 0.32-26.17% respectively. Both SIC and SOC in the Tibetan grasslands (0.51% and 5.24% respectively) were higher than those in the Inner Mongolian grasslands (0.21% and 1.61 %). Regression tree analyses showed that the spatial pattern of SIC and SOC were controlled by different factors. Chemical and physical processes of soil formation drive the spatial pattern of SIC, while biotic processes drive the spatial pattern of SOC. SIC was controlled by soil acidification and other processes depending on soil pH. Vegetation type is the most important variable driving the spatial pattern of SOC. According to our models, given the acidification rate in Chinese grassland soils in the future is the same as that in Chinese cropland soils during the past two decades: 0.27 and 0.48 units per 20 yr in the Inner Mongolian grasslands and the Tibetan grasslands respectively, it will lead to a 30% and 53% decrease in SIC in the Inner Mongolian grasslands and the Tibetan grasslands respectively. However, negative relationship between soil pH and SOC suggests that acidification will inhibit decomposition of SOC, thus will not lead to a significant general loss of carbon from soils in these regions.