Soil organic carbon (SOC) is an important component of the global carbon cycle, and it has direct effects on the global carbon balance. Better understanding of the controlling factors of SOC and how SOC may respond to environmental changes are important to the accuracy for predicting the consequences of SOC under global change and evaluating the carbon budget. In this paper, the effects of climate, atmospheric composition, vegetation, soil property, land use and management practices on soil organic carbon (SOC) are presented. Based on this, some important tasks are also proposed, including (1) quantifying processes and states of SOC; (2) determining the controlling factors; (3) the effects of nitrogen deposition on SOC; (4) the responses and feedback of SOC to climate variability; and (5) the synthetic simulating SOC dynamics.