Nitrate Isotopic Composition, Hubbard Brook Experimental Forest, Watershed 3, 2011

Despite decades of measurements, the nitrogen balance
of temperate forest catchments remains poorly understood.
Atmospheric nitrogen deposition often greatly exceeds
streamwater nitrogen losses; the fate of the remaining
nitrogen is highly uncertain. Gaseous losses of nitrogen to
denitrification are especially poorly documented and are
often ignored. Here, we provide isotopic evidence (δ15NNO3
and δ18ONO3) from shallow groundwater at the Hubbard Brook
Experimental Forest indicating extensive denitrification
during midsummer, when transient, perched patches of
saturation developed in hillslopes, with poor hydrological
connectivity to the stream, while streamwater showed no
isotopic evidence of denitrification. During small rain
events, precipitation directly contributed up to 34% of
streamwater nitrate, which was otherwise produced by
nitrification. Together, these measurements reveal the
importance of denitrification in hydrologically disconnected
patches of shallow groundwater during midsummer as largely
overlooked control points for nitrogen loss from temperate
forest catchments. These data were gathered as part of the Hubbard Brook
Ecosystem Study (HBES). The HBES is a collaborative effort at
the Hubbard Brook Experimental Forest, which is operated and
maintained by the USDA Forest Service, Northern Research
Station.