The input and output processes of below-ground carbon of ecosystems are the core components of carbon allocation and transformation in terrestrial ecosystems and directly affect global carbon cycling. Litterfall, root turnover, root exudate, soil organic carbon, soil microbe and soil respiration are important parts of the input and output processes in terrestrial ecosystems. Because the parts are implicated and we have limited techniques and methods to measure the parts, the input and output processes of below-ground carbon of terrestrial ecosystems are still poorly understood. Therefore, results of carbon cycling in terrestrial ecosystems are uncertain. We summarize the different methods used for studying litterfall, root turnover, root exudate, soil organic carbon, soil microbe and soil respiration and the responses of plant parts to climatic changes. We discuss the difficulties in researching the input and output processes of below-ground carbon of terrestrial ecosystems and propose several topics for further research.