Triple nitrate isotopes indicate differing nitrate source contributions to streams across a nitrogen saturation gradient

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

Year Published

2015

Source

Ecosystems. doi: 10.1007/s10021-015-9891-8.

Abstract

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 d¹⁵N, d¹⁸O, and ?¹⁷O 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 d¹⁵N-NO₃^- in the hardwood watersheds suggest enrichment of the soil NO₃^- pool during the growing season due to plant uptake. In all watersheds, d¹⁸O-based mixing models over-estimated atmospheric NO₃^- contributions to streams by up to 12% compared to ?¹⁷O-based estimates. Our results highlight the importance of atmospheric deposition as a NO₃^- source in low-concentration streams and demonstrate the advantage of using ?¹⁷O-NO₃^- over d¹⁸O-NO₃^- for NO₃^- source apportionment.

Keywords

• nitrogen saturation nitrogen deposition nitrate isotopes Fernow Experimental Forest δ
• 15N-NO3- δ
• 18O-NO3- Δ
• 17O-NO3-

Citation

Rose, Lucy A.; Elliott, Emily M.; Adams, Mary Beth. 2015. Triple Nitrate Isotopes Indicate Differing Nitrate Source Contributions to Streams Across a Nitrogen Saturation Gradient. Ecosystems. 18(7): 1209-1223.

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