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Nitrogen Saturation in the Rocky Mountains: Linking Emissions, Deposition, and Ecosystem
Effects using Stable Isotopes of Nitrogen Compounds

Donald H. Campbell*, Leora Nanus
Corresponding author - U.S. Geological Survey, MS 415 Federal Center, Lakewood CO

JK Bohlke
U.S. Geological Survey, Reston, VA

Karen Harlin
National Atmospheric Deposition Program, Champaign, IL

Jeff Collett
Colorado State University, Ft. Collins, CO


Elevated levels of atmospheric nitrogen (N) deposition are affecting terrestrial and aquatic ecosystems at high elevations in Rocky Mountain National Park and adjacent areas of the Front Range of Colorado. Federal and state agencies are now working together to develop cost-effective means for reducing atmospheric N deposition. To do this, there is a critical need for better understanding of N emission source areas and source types that contribute to N deposition in the Rocky Mountains Front Range of Colorado. In this study, isotopic signatures of N species in atmospheric deposition were related to source areas and source types of emissions such as energy generation, energy resource development, transportation, and agriculture.

Snow, wetfall, and bulk precipitation samples were collected from sites on the eastern slope of the Continental Divide within the Park, and from a site approximately 150 km away on the western slope of the Colorado Rocky Mountains, where deposition rates are lower. There was a seasonal pattern with lighter d15N(NO3) during the warmer months at all sites, which may indicate a shift in emissions types, or a change in atmospheric transport and transformations of nitrogen species. During the winter and spring months, the d15N values of nitrate in deposition at eastern slope sites were lower than those measured on the western slope, possibly because of greater influence of vehicle emissions and other sources of NOx on the eastern slope. During summer, the west-east differences were not evident, but individual events indicated large differences in isotopic signature. These results indicate that incorporating local source characterization and finer spatial and temporal sampling into future studies could provide additional insight into nitrogen deposition source attribution.