Natural Variability in Nitrogen Export from Headwater Catchments:
Snow-Cover Controls on Ecosystem Nitrogen Retention
Paul D. Brooks1, Don H. Campbell2, Kathy A. Tonnessen3, and Kristi Heuer3
The growth of urban communities in the western United States has resulted in an increase in atmo-
spheric N deposition to adjacent headwater catchments. Before the effects of this N deposition can
be evaluated, the causes of natural variability in catchment scale N export need to be understood
and quantified. Previous research has demonstrated that as much as 80% of the annual water input
and 50% or more annual N deposition is stored in the snowpack and released to the catchment
during snowmelt. The majority of this N enters an actively cycling soil N pool, and the snowmelt
flush of both soil and snowpack N is responsible for the increase in the concentration of both N and
other solutes during the first portion of snowmelt. The snowmelt period accounts for 60% to 90% of
annual N export, but exhibits significant interannual variability suggesting that changes in N
source/sink relationships in soil during melt have a large impact on N export. This study evaluates
controls on the strength of the N sink and the size of the leachable N pool concurrent with the spring
hydrologic flush that is responsible for the transport of N to surface water.
Controls on the variable source/sink relationship for N during the snowmelt period were evaluated
in alpine meadow, subalpine meadow, and subalpine forest ecosystems in four catchments in Colo-
rado. The study sites included Loch Vale in Rocky Mountain National Park (RMNP), Niwot Ridge/
Green Lakes Valley in the Front Range, and the Snake River and Deer Creek catchments in Summit
County. At all sites and in all vegetation types measurements of N leachate during melt were
inversely related (r2 > 0.9) to over-winter heterotrophic activity measured as CO2 flux, consistent
with previous work that identified soil microbial biomass as the primary sink for N during melt.
Because over-winter soil heterotrophic activity and microbial biomass are inversely related to snow
depth, catchment-scale N export should be higher in low snow years if these plot-scale results are
important to catchment N export. This hypothesis was evaluated using a long-term record of winter
precipitation, N deposition, and N export from Loch Vale in RMNP. Data from a nine-year record
identified a strong, linear relationship (r2 = 0.68) between catchment-scale N retention and winter
snow cover, consistent with the subnivean, soil-based controls on the mobile N pool identified at
the plot scale. These results indicate that while spring snowmelt is the major control on the timing
of hydrologic N export, the size of the mobile soil N pool during melt is controlled by the timing
and depth of winter snowfall. This natural variability in N export controlled by winter snow cover
needs to be considered when evaluating the potential effects of increased N deposition on either
terrestrial or aquatic ecosystems in seasonally snow-covered watersheds. For management pur-
poses, the effects of increased N deposition are likely to appear in the aquatic ecosystem during low
snow years before the terrestrial vegetation is affected.
1
U.S. Geological SurveyWater Resources Division, 3215 Marine Street, Boulder, Colorado 80303, USA
2
U.S. Geological SurveyWater Resources Division, Denver Federal Center, Lakewood, Colorado 80226,
USA
3 National Park Service, Air Resources Division, Lakewood, Colorado 80226, USA
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