Environmental Science and Technology, V 30, pp 640-646, 1996
Nitrogen saturation in the Colorado Front Range
Mark W. Williams
INSTAAR and Department of Geography
University of Colorado
Boulder, CO
Jill Baron
NREL, Colorado State University
Fort Collins, CO
Nel Caine
INSTAAR and Department of Geography
University of Colorado
Boulder, CO
Richard Sommerfeld
US Forest Service
Fort Collins, CO
Abstract:
Nitrogen (N) saturation is occurring throughout
high-elevation catchments of the Colorado Front Range.
NADP results show that annual inorganic N loading in
wet deposition to the Rocky Mountains
of about 4 kghayr
is about twice that of Pacific States and
similar to many sites in the northeastern US.
In the last ten years
at Niwot Ridge/Green Lakes Valley and Glacier Lakes,
annual minimum concentrations of NO3 in surface waters during
the growing season have increased from below
detection limits to about 10 ueq/L,
indicating that these two catchments are at the threshold
of N saturation.
The Loch Vale watershed is N saturated,
with annual minimum concentrations of NO3 in surface waters
generally above 10 ueq/L;
annual volume-weighted mean (VWM) concentrations of 16 ueq/L NO3 in
surface waters are greater than that of
about 11 ueq/L NO3 in wet deposition.
At these high-elevation catchments
there has been a shift in ecosystem dynamics from
an N-limited system to an N-saturated system
as a result of anthropogenically-fixed N in wetfall and dryfall.
Results from the Western Lakes Survey component
of the National Surface Water Survey show that
N saturation is a regional problem in the Colorado Front Range,
with many lakes having
(NO3) concentrations greater than 10 ueq/L.
These results
provide an early-warning
signal for disruption of N cycling in downstream forested ecosystems
of the Colorado Front Range and for high-elevation catchments in
other areas of the Rocky Mountains.
These results also suggest that critical deposition loads for
N are being exceeded in the Colorado Front Range.