A recent paper by Payne et al. examine the response of grassland species to N deposition across Europe. The goal was to identify critical loads of N deposition for individual species.
In Europe, critical loads are considered to be between 10-15 kg N ha-1 (1-1.5 g m-2), which is greater than most loads here in the US.
The 15 kg ha-1 level was about right at the community level. At this level, communities really start to change. But individual species decline at much lower levels.
Their results suggest that any level of N deposition might negatively impact some species.
This isn't a paper that tests alternative hypotheses. They don't analyze covariates. They don't take into account past histories of N deposition (which might have been greater) or other factors that might be influencing species abundance.
The authors recommend:
"It also suggests that less overall ecological damage may result from new sources of reactive N in current high-N deposition regions, where a shift to a more pollution-tolerant community may have already occurred, rather than regions that currently receive low levels of pollution and are sensitive to even small increases in N deposition. Effective pollution control policy should focus on avoiding all new pollution sources in currently unpolluted regions"
Gary Lovett wrote a commentary on this for PNAS. The potential implications for policy at least here in the US are clear:
"Critical loads remain an important tool for assessing the impacts of atmospheric deposition on ecosystems, but the best use of this tool requires effective interplay between science and policy, including periodic reexaminations of the policy to incorporate the most recent scientific advances. In some cases, such as the Payne et al. paper (2), those advances may cause us to reconsider the fundamental concepts that underlie the policy."
The critical scientific question here is whether a critical load of N can be identified. If not, regulations would be necessary to reduce loading as much as possible, relative to the costs of doing so.
The open question here is whether all N deposition is bad. If anthropogenic N deposition were removed, what would happen to ecosystems? What would the consequences be?
In Europe, where N deposition levels are relatively high, a number of ecosystem functions might "improve", but what about where N deposition is lower. Is it possible that there would be negative implications of reducing N deposition?
That's a question few are addressing.
Payne, R. J., N. B. Dise, C. J. Stevens, and D. J. Gowing. 2013. Impact of nitrogen deposition at the species level. Proceedings of the National Academy of Sciences of the United States of America 110:984-987.
Lovett, G. M. 2013. Critical issues for critical loads. Proceedings of the National Academy of Sciences of the United States of America 110:808-809.