External carbon input to soil influences the available carbon content and the microbial carbon cycle, as well as the microbial utilization of other nutritional elements in the terrestrial ecosystem. With globally increasing nitrogen deposition, the nitrogen content of soil in some regions has changed significantly and now exceeds microbial demand. This in turn increases the need for available carbon. Therefore, artificial regulation of available carbon sources might be essential for relieving microbial carbon limitation and improving carbon sequestration capacity of terrestrial ecosystem. In this paper, previous studies concerning the influences of external carbon input on soil carbon emission, litter fall decomposition, soil carbon pools, and the functional microbial mechanisms in these processes were reviewed. This work will provide a reference for improving the carbon sink capacity of terrestrial ecosystems.