Unusual seasonal patterns and inferred processes of nitrogen retention in forested headwaters of the Upper Susquehanna River |
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Authors: | Christine L Goodale Steven A Thomas Guinevere Fredriksen Emily M Elliott Kathryn M Flinn Thomas J Butler M Todd Walter |
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Institution: | 1. Department of Ecology and Evolutionary Biology, Cornell University, E215 Corson Hall, Ithaca, NY, 14853, USA 2. School of Natural Resources, University of Nebraska-Lincoln, Lincoln, NE, USA 3. Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA, USA 4. Department of Biology, McGill University, Montreal, QC, Canada 5. Cary Institute of Ecosystem Studies, Millbrook, NY, USA 6. Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY, USA
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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 Susquehanna’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 (<1 μmol 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 snowmelt 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 soil-, 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. |
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Keywords: | 15N 18O Geologic nitrogen In-stream uptake Nitrogen retention Nitrate seasonality |
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