Atmospheric nitrate export in streams along a montane to urban gradient

Nitrogen (N) emissions associated with urbanization exacerbate the atmospheric N influx to remote ecosystems – like mountains –, leading to well-documented detrimental effects on ecosystems (e.g., soil acidification, pollution of freshwaters). Here, the importance and fate of N deposition in a watershed was evaluated along a montane to urban gradient, using a multi-isotopic tracers approach (Δ17O, δ15N, δ18O of nitrate, δ2H and δ18O of water). In this setting, the montane streams had higher proportions of atmospheric nitrate compared to urban streams, and exported more atmospheric nitrate on a yearly basis (0.35 vs 0.10 kg-Nha−1yr−1). In urban areas, nitrate exports were driven by groundwater, whereas in the catchment head nitrate exports were dominated by surface runoff. The main sources of nitrate to the montane streams were microbial nitrification and atmospheric deposition, whereas microbial nitrification and sewage leakage contributed most to urban streams. Based on the measurement of δ15N and δ18O-NO3−, biological processes such as denitrification or N assimilation were not predominant in any streams in this study. The observed low δ15N and δ18O range of terrestrial nitrate (i.e., nitrate not coming from atmospheric deposition) in surface water compared to literature suggests that atmospheric deposition may be underestimated as a direct source of N. [Display omitted] •How is nitrogen deposition reflected in streams NO3− exports in the French Alps?•NO3− exports in 7 streams along an altitude gradient were monitored for two years.•NO3− isotopic composition (Δ17O, δ15N, δ18O) was analyzed to apportion the sources.•≤21% of NO3− in montane streams and 5% in urban streams was unprocessed NO3−atm.•Nitrification of atmospheric and microbial NH4+ was the main NO3− source in streams.