Following the conversion of forest ecosysterm to agricultural ecosysterm, the introduction of C4 plant to a soil,previously developed under C3 vegetation,brings to a different origin of soil organic carbon.In consequently it leads to the isotopical change in the composition of organic carbon. The results showed that the proportion of soil organic carbon derived from the forest still predominates after several decades of cultivation in maiz fields in Guizhou Maolan Karst Forest. The soil organic carbon orig...