Bostic, JoelDramatic increases in anthropogenic nitrogen inputs to watersheds over the past century have elevated riverine nitrate (NO3¯) loads, impairing downstream ecosystems. Impacts to receiving waters are largely determined by the amount and timing of streamwater NO3¯ export, and knowledge of the watershed-scale controls on spatiotemporal patterns of NO3¯ export is thus critical for effective mitigation. Land-use activities produce generalizable patterns of streamwater NO3¯ in specific watersheds but it remains unclear how land use might modulate more widespread nitrogen inputs, such as atmospheric deposition, and regulate temporal dynamics of streamwater NO3¯ export. To address these questions, I quantified nitrogen sources and inferred watershed-scale nitrogen cycling processes using stable nitrogen and oxygen isotopes and concentrations of NO3¯ in Chesapeake Bay watersheds. In my first chapter, I quantified streamwater export of atmospheric NO3¯ using triple oxygen isotopes (Δ17O) of NO3¯ in 832 streamwater samples collected from 14 sub-watersheds of diverse land use, nitrogen input rates, size, and lithology across two years during a range of hydrologic conditions. Results indicate that watersheds with either greater impervious surface areas or higher terrestrial nitrogen input rates associated with agricultural practices retain less unprocessed atmospheric NO3¯. I use these results to extend the kinetic nitrogen saturation conceptual model to atmospheric NO3¯ streamwater export and from forested to non-forested systems. In my second chapter, I used seasonal patterns of, and relationships between, NO3¯ concentrations, δ15N of NO3¯, and discharge in the same 832 samples to assess the relative importance of watershed-scale controls on spatiotemporal patterns of streamwater NO3¯ export. Surprisingly, similar seasonal patterns of δ15N-NO3¯ were measured across all watersheds. Similar seasonality of δ15N-NO3¯ suggests consistent temporal variation in biological processes, such as denitrification and/or assimilation, across diverse watersheds. In my third chapter, I used δ15N and Δ17O of NO3¯, as well as isotopes of water, to investigate NO3¯ source export in storm events relative to baseflow in two Baltimore County, Maryland, watersheds with contrasting land use. In the more developed watershed I found that storms had a disproportionate impact on atmospheric NO3¯ export, and the amount of NO3¯ deposited on impervious surfaces was approximately equivalent to the amount of atmospheric NO3¯ streamwater export during storms, while atmospheric NO3¯ exhibited approximately chemostatic behavior in the less developed watershed. These results highlight the importance of reducing hydrologic effects of impervious surfaces to limit atmospheric NO3¯ export, especially given predictions that increasing precipitation intensity will be associated with future climate change. In conclusion, my results demonstrate that land use modulates the retention of atmospheric NO3¯, but biological processes impart a consistent seasonal signal on streamwater NO3¯ irrespective of land use.enDissertationEnvironmental scienceHydrologic sciencesBiogeochemistry