Background/Question/Methods Human activity in the last century has led to an exponential increase in nitrogen (N) emissions and deposition. This N deposition has reached a level that has caused or is likely to cause alterations and damage in 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 defined 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 where possible. The receptors that we evaluated included aquatic diatoms, mycorrhizal fungi and other soil microbes, lichens, herbaceous plants (forbs, graminoids), shrubs, and trees. The main responses reported fell into two categories: (1) biogeochemical and soil microbial responses and (2) individual, population, and community plant and lichen responses. Biogeochemical and soil microbial responses included increased N mineralization and nitrification (and N availability for plant and microbial uptake), changes in microbial community structure (including shifts in the relative proportion of bacteria:fungi), increased gaseous N losses (ammonia volatilization, nitric and nitrous oxide from nitrification and denitrification), and increased N leaching. Plant and lichen responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root:shoot ratios, shifts in competitive interactions and community composition, increased susceptibility to secondary stresses, changes in species richness and other measures of biodiversity, increases in invasive species, and altered fire regime.
Results/Conclusions The range of critical loads for nutrient N reported for U.S. ecoregions, inland surface waters, and wetlands is 1-39 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 scientific information and effectively communicate with the policy community and the public. This synthesis represents the first comprehensive assessment of empirical critical loads of N for ecoregions across the United States.