中文摘要：

碳氮作为陆地生态系统最关键的两大生源要素，它们在自然界的循环过程中不仅各自对全球变暖做出重要贡献，而且两者的循环过程显著耦合，互相影响各自的作用和效果。从氮元素对植物光合作用、呼吸作用以及土壤呼吸作用影响的角度入手，综述了氮输入对陆地生态系统碳固定和碳排放这两个碳循环关键过程的影响特征和机理，分析了陆地生态系统碳源汇对氮素变化响应的不确定性，在此基础上对未来的相关重点研究方向进行了探讨和展望。

英文摘要：

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.