In terrestrial ecosystems, carbon fixation through photosynthesis and carbon release through respiration are the two key processes of carbon cycle, which are also the most important indexes to determine the features of carbon source and sink. For the terrestrial ecosystems with limited nitrogen nutrient, external nitrogen input can affect the two key processes mentioned above, and then indirectly affect the characteristics of carbon source and sink. Here we review the characteristics and mechanisms of the impacts of the external nitrogen input on the carbon fixation and release. Among them, the effects of nitrogen on the photosynthesis, plant respiration and soil respiration including root respiration and heterotrophic respiration are covered. Then, the paper also explores the uncertainties of the responses of carbon source and sink to the nitrogen input. At last, the researches in this field are suggested to strengthen the following aspects: (1) The effects of different nitrogen rates and forms on carbon fixation through photosynthesis and carbon release through soil respiration in different areas and different time to find out the most suitable nitrogen application rate and ratio for efficient carbon sink promotion; (2) The effects of nitrogen on soil microbial processes in rhizosphere, and to acquire some breakthroughs in the relative research techniques and methods; (3) The effects of nitrogen on the key processes of carbon cycle in some other terrestrial ecosystems besides the forest ecosystem, and the coupling of nitrogen and other environment factors, exploring the effects and mechanisms of all the factors on the key processes of carbon cycle in terrestrial ecosystems.