The Effects of Atmospheric Nitrogen Deposition in the
Rocky Mountains of Colorado and Southern Wyoming -- a Synthesis
and Critical Assessment of Published Results
by Douglas A. Burns
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 02-4066
ABSTRACT
The Rocky Mountain region of Colorado and southern
Wyoming receives as much as 7 kilograms
per hectare per year ((kg/ha)/yr) of
atmospheric nitrogen (N) deposition, an amount
that may have caused changes in aquatic and
terrestrial life in otherwise pristine ecosystems.
The Rocky Mountain National Park, in its role of
protecting air-quality related values under
provisions of the Clean Air Act Amendments of
1977, has provided support for this synthesis and
critical assessment of published literature on the
effects of atmospheric N deposition. Results from
published studies indicate a long-term increase in
the rate of atmospheric N deposition during the
20th century, but no region-wide increase during
the past 2 decades, although the rate of
atmospheric N deposition has increased at three
sites east of the Continental Divide in the Front
Range region since the mid-1980s. Much of the
increase in atmospheric N deposition at all three
sites has resulted from an increase in the
ammonium concentrations of wet deposition; this
suggests an increase in contributions from
agricultural areas or from vehicle traffic east of
the Rocky Mountains. Lakes at two study sites in
the Front Range (Loch Vale and Green Lakes
Valley) had NO3- concentrations of 30 to 40
micromoles per liter (μmol/L) during early spring
snowmelt and remained at 5 to 10 μmol/L during
summer. Retention of N in atmospheric wet
deposition in some sub-catchments of these lakes
was less than 50 percent, which reflects an
advanced stage of N saturation. Nitrate
concentrations in surface waters west of the
Continental Divide were lower -- often less than
10 μmol/L during snowmelt and less than 2 μmol/L
during summer -- than surface waters east of the
Divide, except in areas such as the Mt. Zirkel
Wilderness that receive elevated amounts of
atmospheric N deposition of 4 to 5 (kg/ha)/yr.
Atmospheric N deposition in the Front Range east
of the Divide may have altered the composition of
alpine tundra-plant communities and lake
diatoms, but additional studies would be needed
to definitively demonstrate the hypothesized
cause-and-effect relations. Rates of N-mineralization
and nitrification in soils of the
Front Range have increased in response to
increased atmospheric N deposition. Projected
future population growth and energy use in
Colorado and the west increase the likelihood that
the subtle effects of atmospheric N deposition
now evident in the Front Range will become more
pronounced and widespread in the future. The
likelihood of future increased N emissions along
the Front Range warrants a continuation of
existing long-term precipitation and surface-water
chemistry monitoring programs, and an expansion
of the networks into areas that receive large
amounts of atmospheric N deposition, but
currently lack adequate monitoring. Long-term
study and expanded sampling are needed to
address uncertainties about the effects of
atmospheric N deposition on terrestrial plant
communities, nutrient limitation in lake plankton,
shifts of dominant species within diatom
communities, and on amphibian response to
episodic surface-water acidification.
Citation: Burns, D.A., 2002, The Effects of Atmospheric Nitrogen Deposition in the Rocky Mountains
of Colorado and Southern Wyoming -- a Synthesis and Critical Assessment of Published Results:
U.S. Geological Survey Water-Resources Investigations Report 02-4066, 36 p.
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