Assessment of nitrogen deposition effects and empirical critical loads of nitrogen for ecoregions of the United States

This report synthesizes current research relating atmospheric nitrogen (N) deposition to effects on terrestrial and aquatic ecosystems in the United States and to identify empirical critical loads for atmospheric N deposition. The report evaluates the following receptors: 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 presented. This 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 four different life forms: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants, shrubs, trees.