The critical load concept facilitates communication between scientists and policy makers or land managers by translating the complex effects of air pollution on ecosystems into concrete 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 20 kg N/ha/yr, we added 12 levels of N, from 2.5 to 100 kg N/ha/yr, to three natural vegetation types spanning a range of soil fertility and productivity.
Results/Conclusions
Highly variable results among years, as well as inconsistent responses to an increasing dose of N within sites, complicated the derivation of critical loads. A precautionary approach to this derivation, from the standpoint of managers trying to maintain current ecosystem processes and plant communities, suggests a critical load of 4-6 kg N/ha/yr for the most sensitive vegetation type we investigated – Badlands sparse vegetation, a community that supports plant species adapted to low fertility conditions – and a critical load of 6-10 kg N/ha/yr for the two more productive vegetation types, which are characteristic of most NGP grasslands. N addition at these levels increased biomass, litter loads, and in one instance, the abundance of invasive annual grasses. A less precautionary approach to deriving critical loads yielded higher values of 10-38 kg N/ha/yr. The precautionary critical loads are lower than those previously proposed for this region (10-25 kg N/ha/yr) and well within the range of current or projected N deposition rates. Thus, anthropogenic nitrogen deposition may already be affecting NGP ecosystems.