Critical issues for critical loads

In the United States and most other countries, primary air quality standards are designed to protect human health and are based on concentrations of pollutants in the air. However, from the perspective of ecosystem health, a more appropriate metric for the impacts of nitrogen (N) and sulfur (S) pollutants is the total atmospheric deposition (or “load,” in kg⋅ha−1⋅y−1) of N and S, because ecosystem effects are more strongly determined by cumulative annual loading than by short-term atmospheric concentrations. The European Commission has adopted the concept of a “critical load,” defined as the load of a pollutant “below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge” (1). In PNAS, Payne et al. (2) provide new information on the impact of N deposition on European grassland ecosystems and illustrate some of the important complexities involved in quantifying and implementing critical loads. Nitrogen deposition affects ecosystems in two ways. It can be an acidifying agent if the N is nitrified in the soil or nitrate is leached from the ecosystem. Nitrogen can also cause eutrophication, meaning that the oversupply of this nutrient can stimulate productivity of certain species (3). This may sound like a good thing, but in many herbaceous plant communities, the excess N deposition can stimulate growth of N-responsive (nitrophilic) graminoids that crowd out less responsive plants, particularly forbs and bryophytes (Fig. 1). The soil acidification and increased competition can lead to the decline of sensitive species in high-deposition areas. This general pattern of species change in favor of nitrophilic or acid-tolerant species has been established by previous work in grasslands, coastal dunes, heathlands, and alpine tundras, among other ecosystem types, although the pace and outcome of the change can be altered by such factors as soil pH, …