Evaluating Variation in Sources of Atmospheric Nitrogen Deposition in the Rocky Mountains using d15N−NO3 and d15N−NH4+

Leora Nanus1, Christopher M.B. Lehmann2 and M. Alisa Mast3

Spatial and temporal variation in sources of nitrogen (N) deposition to high-elevation ecosystems in the Rocky Mountains were evaluated using N isotope data for water years 1995-2016. This unique dataset links N in wet deposition and snowpack to emissions sources, and improves understanding of the impacts of anthropogenic activities and environmental policies that mitigate effects of accelerated N cycling. d15N−NO3 at 50 US Geological Survey Rocky Mountain Snowpack sites ranged from −3.3 ‰ to +6.5 ‰, with a mean value of +1.4 ‰. At 15 National Atmospheric Deposition Program (NADP)/National Trends Network wet deposition sites,  summer d15N−NO3 is significantly lower ranging from −7.6 ‰ to −1.3 ‰ while winter d15N−NO3 ranges from −2.6 ‰ to +5.5 ‰, with a mean value of +0.7 ‰ during the cool season. Winter NADP wet deposition d15N−NO3is lower than snowpack, possibly due to the influence of dry deposition in the snowpack. Spatial trends in wet deposition are similar to Snowpack, with higher NO3 concentrations and d15N−NO3 in the Southern Rockies located near larger anthropogenic N emission sources compared to the Northern Rockies. Wet deposition d15N−NH4+ ranged from −10 ‰ to 0 ‰, with no observed spatial pattern. However, the lowest d15N−NH4+(−9 ‰), and the highest NH4+ concentration (35 meq/L) were observed at a Utah site dominated by local agricultural activities, whereas the higher d15N−NH4+ observed in Colorado and Wyoming are likely due to mixed sources, including fossil fuel combustion. These findings show spatial and seasonal variation in N isotope data that reflect differences in sources of anthropogenic N deposition to high-elevation ecosystems and have important implications for environmental policy.


1San Francisco State University, lnanus@sfsu.edu
2NADP, Illinois State Water Survey, clehmann@illinois.edu
3US Geological Survey, mamast@usgs.gov