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

摘要: 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 saturation stages ranging from Stage 0 (N-limited) 3 (N-saturated). We quantified NO3 − contributions atmospheric and microbial sources using δ15N, δ18O, Δ17O characterized the isotopes in precipitation. Despite receiving identical inputs, proportions streams averaged 7 10% hardwood (stages 1, 2, 3) 54% conifer watershed (Stage 0). This suggests that may be less responsive future reductions than watershed, least short term. As shown other studies, were higher during stormflow. large stream, total −–N flux this (2.9 g ha−1) was lower fluxes (range = 117.8–338.5 ha−1). Seasonal patterns δ15N–NO3 suggest enrichment soil pool growing season due plant uptake. In all watersheds, δ18O-based mixing models over-estimated by up 12% compared Δ17O-based estimates. Our results highlight importance as a source low-concentration demonstrate advantage Δ17O–NO3 over δ18O–NO3 for apportionment.