Effects and empirical critical loads of Nitrogen for ecoregions of the United States

By: , and 



Human activity in the last century has increased nitrogen (N) deposition to a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. We synthesized current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and estimated associated empirical critical loads of N for several receptors: freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Biogeochemical responses included increased N mineralization and nitrification, increased gaseous N losses, and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root-shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species. The range of critical loads of nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is 1–39 kg N ha−1 yr−1, spanning the range of N deposition observed over most of the country. The empirical critical loads of N tend to increase in the following sequence: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, trees.

Additional publication details

Publication type Book chapter
Publication Subtype Book Chapter
Title Effects and empirical critical loads of Nitrogen for ecoregions of the United States
DOI 10.1007/978-94-017-9508-1_5
Year Published 2015
Language English
Publisher Springer
Contributing office(s) Forest and Rangeland Ecosystem Science Center
Description 41 p.
Larger Work Type Book
Larger Work Title Critical loads and dynamic risk assessments
First page 129
Last page 169
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