The critical load concept facilitates communication between scientists and policy makers and land managers by translating the complex effects of air pollution on ecosystems into unambiguous numbers that can be used to inform air quality targets. Anthropogenic atmospheric nitrogen (N) deposition adversely affects a variety of ecosystems, but the information used to derive critical loads for North American ecosystems is sparse and often based on experiments investigating N loads substantially higher than current or expected atmospheric deposition. In a four-year field experiment in the northern Great Plains (NGP), where current N deposition levels range from ~3 to 9 kg N ha^-1 y^-1, we added 12 levels of N, from 2.5 to 100 kg N ha^-1 y^-1, to three sites spanning a range of soil fertility and productivity. Our results suggest a conservative critical load of 4 to 6 kg N ha^-1 y^-1 for the most sensitive vegetation type we investigated – Badlands sparse vegetation, a community that supports plant species adapted to low fertility conditions – for which N addition at this rate increased productivity and litter load.  In contrast, for the two more productive vegetation types characteristic of most NGP grasslands, a critical load of 6 to 10 kg N ha^-1 y^-1 was identified. For these vegetation types, N addition at this level altered plant tissue chemistry and increased non-native species. These critical loads are below the currently suggested range of 10 to 25 kg N ha^-1 y^-1 for NGP vegetation and within the range of current or near-future deposition, suggesting that N deposition may already be inducing fundamental changes in NGP ecosystems.