Abstract: To clarify the decomposition characteristics and stability of soil particulate organic carbon in different vegetation types in alpine region, we analyzed soil particulate composition, soil particulate organic carbon content, and organic carbon isotope (δ¹³C) of different sized particulates of different vegetation types in alpine meadow, shrub, forest and farmland in a southern slope of Qilian Mountains. The results showed that sand content was the highest particulate composition in soils, ranging from 58.3% to 78.2%, followed by silt content, ranging from 20.2% to 32.2%, and the clay content was the lowest, ranging from 1.6% to 6.3%, across different vegetation types. The organic carbon content of soil particulates increased with the increases of particulate size. The organic carbon content was from 21.6 to 74.5 g·kg^-1 for sand particulates, between 64.8 and 163.9 g·kg^-1 in silt particulates, and ranged from 60.3 to 196.4 g·kg^-1 in clays. With decreasing soil particulate size, the δ¹³C values increased gradually, ranging from -27.5‰ to -23.8‰. Those results indicate that the degree of decomposition of organic matter becomes higher, and the rate of decomposition becomes slower as the soil particulate size becomes smaller. The particulate organic carbon content of silt and clay is high, with low decomposition rate. These two kinds of particulates are important factors affecting the stability of organic carbon pools.

Key words: stable carbon isotope, soil particulate organic carbon, soil organic carbon pool stability, the southern slope of Qilian Mountains.