『Abstract
　Atmospheric deposition contributes a large fraction of the annual nitrogen (N) input to the basin of the Susquehanna River, a river that provides two-thirds of the annual N load to the Chesapeake Bay. Yet, there are few measurements of the retention of atmospheric N in the Upper Susquehannna's forested headwaters. We characterized the amount, form (nitrate, ammonium, and dissolved organic nitrogen), isotopic composition (δ¹⁵N- and δ¹⁸O-nitrate), and seasonality of stream N over 2 years for 7-13 catchments. We expected high rates of N retention and seasonal nitrate patterns typical of other seasonally snow-covered catchments: dormant season maxima and growing season minima. Coarse estimates of N export indicated high rates of inorganic N retention (＞95％), yet streams had unexpected seasonal nitrate patterns, with summer peaks (14-96 μmol L^-1), October crashes (＜1μmol L^-1), and modest rebounds during the dormant season (＜1-20 μmol L^-1). Stream δ¹⁸O-nitrate values indicated microbial nitrification as the primary source of stream nitrate, although snow-melt or other atmospheric source contributed up to 47％ of stream nitrate in some March samples. The autumn nitrate crash coincided with leaffall, likely due to in-stream heterotrophic uptake of N. Hypothesized sources of the summer nitrate peaks include: delayed release of nitrate previously flushed to groundwater, weathering of geologic N, and summer increases in net nitrate production. Measurements of shale δ¹⁵N and sol-, well-, and streamwater nitrate within one catchment point toward a summer increase in soil net nitrification as the driver of this pattern. Rather than seasonal plant demand, processes governing the seasonal production, retention, and transport of nitrate in soils may drive nitrate seasonality in this and many other systems.

Keywords: ¹⁵N; ¹⁸O; Geologic nitrogen; In-stream uptake; Nitrogen retention; Nitrate seasonality』

Introduction
Methods
　Site description
　Atmospheric deposition
　Catchment water sampling and analysis
　Isotopic separation of nitrate sources
　Geologic nitrogen
Results
　Weather and streamflow
　Atmospheric deposition
　Stream and soil water chemistry
　Isotopic separation of nitrate sources
　Geologic nitrogen
Discussion
　Annual N loss and retention
　Summer nitrate peaks
　Nitrate crash at leaffall and in-stream uptake
　Nitrate loss during the dormant season
　Common drivers of unusual nitrate seasonality
Conclusions
Acknowledgments
References