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

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.