Soil C sequestration has been a research area of increasing significance in study of Earth surface processes and global change. The understanding of capacity, biophysical potential and mechanisms involved in soil C sequestration has been demanded by the assessment of global biophysical potential of GHGs mitigation for the post-Kyoto period. In this paper, the main research achievements in soil C sequestration study are overviewed focusing C capacity and protection mechanisms. While much has been dedicated to the C pools and C sequestration, there is a good number of studies showing that chemical binding, transformation of OC are both involved in C capacity and stabilization as well as the physical protection in coarse particle size fractions. Interaction of soil-plant(crop)-microbes in soil crop system may exert significant role in C sequestrat ion in cropland soils which will become a frontier of C study by holistic approaches in the near future. Finally, a conceptual model of C sequestration of agricultural soils is proposed and suggestions for further study of C sequestration in China are given with emphasis on the processes under interact ive effect of land management and crop production trends in the framework of global change biology systems.