Terrestrial Nitrogen Inputs Affect the Export of Unprocessed Atmospheric Nitrate to Surface Waters: Insights from Triple Oxygen Isotopes of Nitrate

Atmospheric nitrate (NO3−Atm) deposition has increased dramatically during the past ~ 150 years and contributes to ecosystem eutrophication. NO3−Atm deposition is widespread, but the role of different landscapes in modulating watershed-scale processing and export of NO3−Atm remains unclear. We measured triple oxygen isotopes (a tracer of NO3−Atm) of NO3− for 832 stream samples collected during baseflow and stormflow from 14 watersheds of varied land use throughout two years in the Chesapeake Bay watershed, and we used these data to assess the influence of land use on NO3−Atm dynamics. Watersheds with more agricultural (> 35%) and developed (> 70%) land exported more NO3−Atm than predominantly forested (> 75%) watersheds. Agricultural lands likely facilitate greater NO3−Atm export because of elevated rates of terrestrial N addition relative to rates of NO3− consumption. In contrast, developed lands likely have limited biotic processing of NO3−Atm because of greater hydrologic connectivity of overland flow pathways to channels. Our results, along with data from prior studies, can be interpreted by extending the conceptual model of kinetic N saturation to NO3−Atm streamwater export across varied land use watersheds. In this framework, elevated rates of terrestrial N inputs overwhelm NO3− sinks, allowing proportionally more NO3−Atm to leak from watersheds. Changes in watershed-scale N inputs that increase stream NO3− concentrations additively affect NO3−Atm, with agricultural watersheds, and their associated large terrestrial N inputs, increasing NO3−Atm export.