Atmospheric nitrogen (N) deposition is sharply increasing, and this may have substantial impact on carbon (C) fixation and allocation of plants. Knowledge of the effect of N addition on C fixation and allocation patterns of assimilated C is therefore critical to understand the effects on C cycles in terrestrial ecosystems. We conducted a field experiment to examine the effects of N addition (0 to 25 g N m(-2) yr(-1)) on photosynthetic C fixation and C allocation by Leymus chinensis using (CO2)-C-13 pulse-labeling and measurements of the percentage of assimilation allocated to nonstructural carbohydrates (NSCs), secondary metabolites (SMs), and growth in a temperate semi-arid grassland. Moderate N addition (5 and 10 g N m(-2) yr(-1)) significantly increased the delta C-13 value and C-13 fixation of plant leaves by increasing P-n and biomass of L. chinensis. However, high N addition (25 g N m(-2) yr(-1)) did not result in a further increase in P-n and total biomass, suggesting excess N inputs was harmful to photosynthetic C fixation in plants. Moderate N addition (10 g N m(-2) yr(-1)) also significantly increased the root/shoot ratio and the proportion of assimilated C-13 allocated to roots, but decreased the proportion of leaves and stems allocated, indicating more C was allocated to roots with N addition. We further found that moderate N addition increased the overall NSC and SM concentrations, but C allocation to growth decreased with increasing N, possibly indicating the existence of a trade-off between the C allocation to defense and the allocation to storage and growth. Our findings demonstrated that N addition will alter photosynthetic C fixation and the allocation patterns of photoassimilates, and thus will significantly affect the C cycle and C balance of terrestrial ecosystems under predicted future global changes.