Atmospheric nitrogen (N) deposition is composed of both inorganic N (IN) and organic N (ON), and these sources of N may exhibit different impacts on ecosystems. However, our understanding of the impacts of N deposition is largely based on experimental gradients of INs or more rarely ONs. Thus, the effects of N deposition on ecosystem productivity and biodiversity may be biased. We explored the differential impacts of different IN:ON ratios on aboveground net primary productivity (ANPP) and plant species richness in a typical temperate grassland with a long-term N addition experiment. Our results showed that N addition significantly increased ANPP and reduced species richness. While the IN:ON ratios showed no different effects on ANPP, more species loss occurred with increasing IN:ON ratios. Thus, the evaluation of N deposition on biodiversity might be overestimated if only IN is added or underestimated if only ON is added.