Determining What Levels of Air Pollution Harm U.S. Ecosystems
Nitrogen (N) deposition from air pollution has reached a level that has caused, or is likely to cause, alterations and damage in ecosystems across the United States. An international team led by a Forest Service scientist determined the critical N load, that is, the level of N deposition below which no detrimental ecological effects occur. The team synthesized current research relating atmospheric N deposition to effects in terrestrial and aquatic ecosystems. Responses to increased N deposition included changes in biological community structure, altered soil N cycling, increased N leaching to soil and surface waters, increases in susceptibility to secondary stresses, increases in invasive species, and altered fire regimes. The range of critical loads for nutrient N reported for U.S. ecoregions, inland surface waters, and wetlands is 1 to 39 kg of N/ha annually. In many regions, the critical load for sensitive receptors has been exceeded. The critical loads approach is an ecosystem assessment tool with great potential to simplify complex scientific information and provide a scientific basis for making decisions about limiting air pollution to protect vulnerable ecosystems.
Forest Service Partners