Little is known about the regional extent and variability of nitrate from atmospheric deposition that is transported to streams without biological processing in forests. We measured water chemistry and isotopic tracers (δ 18 O and δ 15 N) of nitrate sources across the Northern Forest Region of the U.S. and Canada and reanalyzed data from other studies to determine when, where, and how unprocessed atmospheric nitrate was transported in catchments. These inputs were more widespread and numerous than commonly recognized, but with high spatial and temporal variability. Only 6 of 32 streams had high fractions (>20%) of unprocessed atmospheric nitrate during baseflow. Seventeen had high fractions during stormflow or snowmelt, which corresponded to large fractions in near-surface soil waters or groundwaters, but not deep groundwater. The remaining 10 streams occasionally had some (<20%) unprocessed atmospheric nitrate during stormflow or baseflow. Large, sporadic events may continue to be cryptic due to atmospheric deposition variation among storms and a near complete lack of monitoring for these events. A general lack of observance may bias perceptions of occurrence; sustained monitoring of chronic nitrogen pollution effects on forests with nitrate source apportionments may offer insights needed to advance the science as well as assess regulatory and management schemes.
Bibliographical noteFunding Information:
Funding was provided by the Northeastern States Research Cooperative (NSRC), a partnership of Northern Forest states (New Hampshire, Vermont, Maine, and New York) and the Northern Research Station (NRS) of the USDA Forest Service. The NRS funded streamflow monitoring at the Fernow, Hubbard Brook, and Marcell Experimental Forests; sampling and analysis of some Marcell Experimental Forest (MEF) and Sleepers River waters; the contributions of S.D.S., J.L.C., and L.H.P.; and technical support of A. Gapinski (University of Minnesota) and J. Prososki. The Air Resources and Monitoring program of the Forest Service funded 2016 MEF precipitation analysis. The USGS funds research and monitoring at the Sleepers River Research Watershed, the Buck Creek watersheds, Biscuit Brook, and Dry Creek as well as the contributions of J.B.S., D.A.B., C.K., and G.B.L. The USGS funded streamflow monitoring for other sites (Table S1). Mention of commercial products and vendors is for informational purposes only and does not constitute endorsement, recommendation, or favor by the United States Government or Agencies. Other author contributions were provided in kind to this project. Nitrate concentrations were measured by J. Tilley (University of Vermont), J. Larson, D. Nelson, N. Aspelin (Forest Service), and many others. K. Redling (University of Pittsburgh), S. Wankel, C. Chang, S. Silva, D. White, M. Young (former or current USGS employees − Menlo Park, CA), K. Pecsok Ewert (University of California, Davis), and R. Heemskerk (University of Waterloo) contributed to nitrate isotopic measurements. S. Clark (USGS, retired, Montpelier, VT), G. Fredriksen (Cornell University), A. Gapinski, J. Jessup (Cary Institue, Millbrook, NY), J. Lee (University of Maine), K. Oleheiser (Xcel Engineering, Grand Rapids, MN; funded by the Department of Energy via Oak Ridge National Laboratory), and others sampled and processed waters for this study. J.-K. Böhlke (USGS, retired, Reston, VA) provided critical information on isotopic analytical methodologies. Although we only included authors of publications who provided data to this effort, we are grateful to all authors who contributed their expertise to publications from which data were contributed.
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