氮沉降大范围的影响着陆地生态系统的结构与功能，目前主要有光限制和离子毒害两种假说解释氮沉降影响群落多样性和生产力变化的机制，但亟待有力的实验验证。物种和群落光合作用是植物生长和干物质积累的基础，并与氮素密切相关。本项目以内蒙古呼伦贝尔草甸草原作为研究对象，通过不同形态、不同剂量含氮化合物添加的大型野外控制实验，对不同氮添加处理下优势种和群落的光合生理特征、植物生长性状以及土壤理化性质（尤其是锰和铝离子）等进行同步观测，并对上述因素进行整合关联分析，验证光限制假说和离子毒害假说，同时解析不同氮添加处理下光限制和离子毒害效应对群落光合作用影响的相对贡献，从而揭示氮沉降导致草原植物多样性和生产力变化的光合作用机制，为草原生物多样性保护和生产力提升的可持续管理提供理论依据。

Nitrogen deposition deeply affects the structure and function of terrestrial ecosystems. At present, light limitation and ion toxicity are the main hypotheses for interpreting the mechanism of nitrogen deposition effecting on the change of community diversity and productivity. However, powerful experimental test of these hypotheses is limited. Photosynthesis of species and community is the basis of plant growth and dry matter accumulation, and it is closely related to nitrogen. This proposal will apply different forms and doses of nitrogen compounds to add large-scale field experiment in Hulun Buir meadow steppe in Inner Mongolia. The photosynthetic physiological characteristics of dominant plants and plant community, plant growth traits and soil physical and chemical properties （especially manganese and aluminum ions） will be measured synchronously under different nitrogen added treatments. The results from integrated analysis of above indexes are to test light limitation and ions toxicity hypotheses. In the meantime, the relative contribution of light limitation and ion toxicity influencing on community photosynthesis under different nitrogen deposition treatments will be clear. Accordingly, the photosynthetic mechanism of nitrogen deposition resulting in altering of plant diversity and productivity will be revealed, and it will provide theoretical evidence for the sustainable management of biodiversity protection and productivity improvement in grassland.