Soil organic matter fractions and soil carbon management index in grasslands with different fencing ages The distribution of soil organic carbon as affected by landforms in a small watershed of gully region of the Loess Plateau Effects of long-term fertilization on labile organic matter and carbon management index(CMI) of the typical soils of China Effects of gap size in Pinus massoniana plantations on different soil labile organic carbon fractions The changes of soil organic carbon and carbon management index in alpine steppe