Triple Nitrate Isotopes Indicate Differing Nitrate Source Contributions to Streams Across a Nitrogen Saturation Gradient

Lucy A. Rose, Emily M. Elliott, Mary Beth Adams

Research output: Contribution to journalArticlepeer-review

35 Scopus citations


Nitrogen (N) deposition affects forest biogeochemical cycles worldwide, often contributing to N saturation. Using long-term (>30-year) records of stream nitrate (NO3 ) concentrations at Fernow Experimental Forest (West Virginia, USA), we classified four watersheds into N saturation stages ranging from Stage 0 (N-limited) to Stage 3 (N-saturated). We quantified NO3 contributions from atmospheric and microbial sources using δ15N, δ18O, and Δ17O of NO3 and characterized the concentrations and isotopes of NO3 in precipitation. Despite receiving identical atmospheric inputs, the proportions of atmospheric NO3 in streams averaged from 7 to 10% in the hardwood watersheds (stages 1, 2, and 3) and 54% in the conifer watershed (Stage 0). This suggests that the hardwood watersheds may be less responsive to future reductions in N deposition than the conifer watershed, at least in the short term. As shown in other studies, atmospheric NO3 proportions were higher during stormflow. Despite large proportions of atmospheric NO3 in the Stage 0 stream, total atmospheric NO3 –N flux from this watershed (2.9 g ha−1) was lower than fluxes in the other watersheds (range = 117.8–338.5 g ha−1). Seasonal patterns of δ15N–NO3 in the hardwood watersheds suggest enrichment of the soil NO3 pool during the growing season due to plant uptake. In all watersheds, δ18O-based mixing models over-estimated atmospheric NO3 contributions to streams by up to 12% compared to Δ17O-based estimates. Our results highlight the importance of atmospheric deposition as a NO3 source in low-concentration streams and demonstrate the advantage of using Δ17O–NO3 over δ18O–NO3 for NO3 source apportionment.

Original languageEnglish (US)
Pages (from-to)1209-1223
Number of pages15
Issue number7
StatePublished - Nov 1 2015

Bibliographical note

Funding Information:
This work was supported by NSF Grant #0910521 (to EME) and Agriculture and Food Research Initiative Grant #2012-67011-19663 (to LAR) from the USDA National Institute of Food and Agriculture. We thank Christopher Cassidy for assistance with sample collection and processing, Katherine Redling for isotopic analysis, and the helpful comments of three anonymous reviewers during the preparation of this manuscript.

Publisher Copyright:
© 2015, Springer Science+Business Media New York.


  • Fernow Experimental Forest
  • nitrate isotopes
  • nitrogen deposition
  • nitrogen saturation
  • ΔO–NO
  • δN–NO
  • δO–NO


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