Technical Report
Assessment of nitrogen deposition effects and empirical critical loads of nitrogen for ecoregions of the United States
Pardo, LH; Robin-Abbott, MJ; Driscoll, CT
U.S. Department of Agriculture, Forest Service, Northern Research Station
Newtown Square, PA
Assessment of nitrogen deposition effects and empirical critical loads of Nitrogen for ecoregions of the United States
has individual parts
2259905 Wetlands
has individual parts
2260000 Synthesis
has individual parts
3123044 North American deserts
has individual parts
3123047 Inland surface water
has individual parts
3123050 Northwestern forested mountains
has individual parts
3123056 Eastern temperate forests
has individual parts
3449415 Mediterranean California
has individual parts
4088641 Temperate sierras
Human activity in the last century has led to a substantial increase in nitrogen (N) emissions and deposition. This N deposition has reached a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. One approach for quantifying the level of pollution that would be harmful to ecosystems is the critical loads approach. The critical load is defi ned as the level of a pollutant below which no detrimental ecological effect occurs over the long term according to present knowledge.
The objective of this project was to synthesize current research relating atmospheric N deposition to effects on terrestrial and aquatic ecosystems in the United States and to identify empirical critical loads for atmospheric N deposition. The receptors that we evaluated included freshwater diatoms, mycorrhizal fungi and other soil microbes, lichens, herbaceous plants, shrubs, and trees. The main responses reported fell into two categories: (1) biogeochemical, and (2) individual species, population, and community responses.
The range of critical loads for nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is 1 to 39 kg N ha-1 y-1. This broad range spans the range of N deposition observed over most of the country. The empirical critical loads for N tend to increase in the following sequence for different life forms: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, trees.
The critical loads approach is an ecosystem assessment tool with great potential to simplify complex scientifi c information and effectively communicate with the policy community and the public. This synthesis represents the fi rst comprehensive assessment of empirical critical loads of N for ecoregions across the United States.
air pollution, atmospheric N deposition, critical nitrogen loads, biodiversity, nitrate leaching, nitrogen saturation, plant nitrogen concentration, natural resource protection, vegetation type conversion