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

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 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.