『Abstract
Nitrogen from atmospheric deposition serves as the dominant source
of new nitrogen to forested ecosystems in the northeastern U.S.
By combining isotopic data obtained using the denitrifier method,
with chemical and hydrologic measurements we determined the relative
importance of sources and control mechanisms on nitrate (NO3-) export from five forested watersheds
in the Connecticut River watershed. Microbially produced NO3- was the dominant source (82-100%)
of NO3- to the sampled streams
as indicated by the δ15N and δ18O of NO3-. Seasonal variations in the δ18O-NO3- in streamwater are controlled by
shifting hydrologic and temperature affects on biotic processing,
resulting in a relative increase in unprocessed NO3-
export during winter months. Mass balance estimates find that
the unprocessed atmospherically derived NO3-
stream flux represents less than 3% of the atmospherically delivered
wet NO3- flux to the region. This
suggests that despite chronically elevated nitrogen deposition
these forests are not nitrogen saturated and are retaining, removing,
and reprocessing the vast majority of NO3-
delivered to them throughout the year. These results confirm previous
work within Northeastern U.S. forests and extend observations
to watersheds not dominated by a snow-melt driven hydrology. In
contrast to previous work, unprocessed atmospherically derived
NO3- export is associated with
the period of high recharge and low biotic activity as opposed
to spring snowmelt and other large runoff events.
Keywords: Atmospheric deposition; Nitrate processing; Nitrogen;
Northeastern forests; Stable isotopes』
Abbreviations
Introduction
Methods
Spatial analysis
Nitrogen deposition fluxes
Sample collection
Sample analysis
Daily flow estimation and hydrograph separation
Statistical methods
Site description
Results
Discussion
Seasonality of the isotopic composition of stream NO3-
Comparison of δ18O-NO3-
values across studies
Retention of atmospheric NO3-
Conclusions
Acknowledgements
References