Mechanism of distribution of soil organic matter(SOM)with depth was studied based on SOM content,SOM ¹⁴C,SOM ¹³C,clay content and sporopollen composition of two soil profiles at the forest vegetation zone and shrub-meadow zone in the Dinghushan Biosphere Reserve.Results indicated that distribution of SOM with depth was related to the evolution of soil profiles.With the depth SOM sources decreased but SOM loss increased due to SOM decomposition during evolution of the soil profiles and SOM content decreased gradually while SOM ¹⁴C apparent age increased.Obvious correlationship was observed between variation of the SOM ¹³C value with the depth and that of the SOM contents with the depth.All these were the result of regular decomposition of SOM in different replacement periods during the development of the soil profiles.Vertical distribution of clay reflected the characteristics of pedological illuviation of the soil profiles,indicating that the soil profiles had undergone a long period of pedogenic weathering.The above characteristics of the soil profiles are the results of continuous deposition and soil formation during evolution of the soil profiles,suggesting a significant restraining effect on distribution of SOM with the depth.