生物固氮是自然条件下氮素进入生态系统的主要途径。随着全球大气氮沉降的急剧增加，其对生物固氮速率产生了重要影响。然而，目前关于氮沉降影响生物固氮速率的阈值（阈值下限：氮沉降高于该值时生物固氮速率受到抑制；阈值上限：氮沉降高于该值时生物固氮速率不再变化或者停止）和机制还不清楚，尤其是在包含多种固氮类型的森林生态系统。本研究拟以燕山北部山地典型的杨桦天然林为研究对象，通过设置10种添加量的高密度氮添加梯度野外实验，旨在揭示氮添加对森林共生固氮和自由固氮速率影响的阈值；通过设计多个氮添加室内控制实验，借助分子生物学技术（PCR、T-RFLP法），分析固氮菌群落的数量、组成和结构的变化。最后，综合分析野外和室内实验结果，从种间关系和资源限制角度，阐明氮沉降影响森林生物固氮的过程和机制，为氮沉降影响森林生物固氮的区域评估和生态模型改进提供基础。

Biological nitrogen fixation (BNF) is the primary natural input of N to ecosystems. With the rapid increase of global N deposition, it has an important effect on the rate of BNF. However, the threshold value above which N2 fixation is inhibited and the mechanism of N deposition effecting on the rates of BNF are not clear, especially in the forest ecosystem with many types of BNF. In this study, we set an N addition gradient experiment with ten treatments in typical natural forests in the northern Yanshan Mountain, to explore the threshold values of N addition effecting on BNF in different types including symbiotic N2 fixation and free-living N2 fixation. Moreover, we conduct some N addition experiments in the laboratory, and analyse the change of the quantity, composition and structure of N2-fixer communities after N addition. Finally, by comparing and analysing the results of field experiments and laboratory experiments, we explore the process and mechanism of the effects of N deposition on BNF from the perspectives of species interactions and resource limitation. This study will provide a basis for the regional assessment of the effect of N deposition on forest BNF, and also for the improvement of ecological models.