Triple nitrate isotopes indicate differing nitrate source contributions to streams across a nitrogen saturation gradient
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Ecosystems. doi: 10.1007/s10021-015-9891-8.
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.
Keywordsnitrogen saturation; nitrogen deposition; nitrate isotopes; Fernow Experimental Forest; δ15N-NO3-; δ18O-NO3-; Δ17O-NO3-
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. https://doi.org/10.1007/s10021-015-9891-8.