Abstract: Atmospheric nitrogen deposition affects forest ecosystems. The ecophysiological process and mechanism of trees in response to nitrogen deposition need further clarification. To explore the changes of photosynthetic traits, biomass accumulation, and carbon and nitrogen allocation in poplar under nitrogen addition, hybrid poplar (Populus ×euramericana ‘Zhongjin2’) seedlings were grown in pots under a rainout shelter in a semiarid area of China. Seedlings were treated with different nitrogen addition levels (0, 3, 6 and 9 g NH₄NO₃·a^-1·plant^-1) for two years. Leaf photosynthetic rate, leaf  functional traits, organ biomass, and carbon and nitrogen allocation were investigated. In the first year, with the increases of nitrogen addition levels, leaf area significantly increased but leaf became thinner, with decreased mesophyll tissue thickness. Leaf area based net photosynthetic rate and stomatal conductance increased firstly and then decreased with increasing nitrogen addition level, while transpiration rate, instantaneous water use efficiency and photosynthetic nitrogen use efficiency did not change. Whole plant biomass and carbon and nitrogen accumulation significantly increased. The carbon investment in stem increased significantly while that in root decreased significantly. In the second year, photosynthetic carbon fixation rate of functional leaves reached saturation more quickly with increasing nitrogen addition level, with toxicity of high-dose addition. The biomass accumulation of individual plant increased significantly and carbon investment to stem was increased in low nitrogen addition, while root growth was inhibited in high nitrogen addition. The enhanced leaf water consumption in response to nitrogen addition may be one of the driving factors of carbon investment to stem, while the decreasing root shoot ratio may reduce the resistance of poplar seedlings to frequent drought in semiarid area.

Key words: nitrogen addition, poplar, leaf functional trait,  , carbon and nitrogen allocation.