Spatial and temporal variation in sources of atmospheric nitrogen deposition in the Rocky Mountains using Nitrogen isotopes

Nanus, Leora; Campbell, Donald; Lehmann, Christopher; Mast, M

Variation in source areas and source types of atmospheric nitrogen (N) deposition to high elevation ecosystems in the Rocky Mountains were evaluated using spatially and temporally distributed N isotope data from atmospheric deposition networks for 1995-2016. This unique dataset links N in wet deposition and snowpack to mobile and stationary emissions sources, and enhances understanding of the impacts of anthropogenic activities and environmental policies that mitigate effects of accelerated N cycling across the Rocky Mountain region. d15N−NO3 − at 50 U.S. Geological Survey Rocky Mountain Snowpack (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 (NADP Wetfall) 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. The strong seasonal difference in NADP Wetfall d15N−NO3 − is due in part to variation in the proportion of N originating from source regions at different times of the year due to seasonal changes in weather patterns. Snowpack NO3 − and d15N−NO3 − are significantly related to NADP Wetfall (fall and winter) suggesting that bulk snowpack samples provide a reliable estimate at high elevations. Spatial trends show higher NO3 − concentrations and d15N−NO3 − in the Southern Rocky Mountains located near larger anthropogenic N emission sources compared to the Northern Rocky Mountains. NADP Wetfall 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 and agricultural sources. 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 across the Rocky Mountain region.