Atkins, TyÌý1Ìý;ÌýWilliams, Mark WÌý2Ìý;ÌýHill, Kenneth R.Ìý3
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Previous studies have shown that nitrogen deposition in the Front Range of Colorado has steadily increased due to anthropogenic influences and led to nitrogen saturation in alpine areas (Williams et al, 1996). In addition, several studies have characterized nutrient transport and hypothesized shallow subsurface controls for the subalpine Como Creek in the Colorado Front Range (Lewis and Grant, 1979, 1980). Here we evaluate changes in the catchment over a thirty-year time period and trace source waters to determine where nutrient processing occurs. Comparing NH4+, NO3-, DON, and DOC yields from Como Creek from the last 30 years, show that alpine areas subsidize transport of inorganic nitrogen down gradient into forested areas. Surprisingly, the subalpine Como Creek catchment shows a decoupled response to increased nitrogen deposition from alpine systems. C:N ratios suggest that Como Creek is heading towards nitrogen limitation rather than saturation. Additionally, nitrate loss occurs prior to snowmelt and mimics the behavior of conservative groundwater tracers. Three-component hydrologic mixing models using O18 and Na as tracers from groundwater and surface water sources show that discharge at Como Creek is characterized by a well-mixed deep groundwater reservoir with a residence time of years to decades. This distinct flow path suggests that nutrient processing occurs in a deep groundwater reservoir across the alpine – subalpine ecotone.
Lewis, W.M., and M.C. Grant, 1979, Relationships Between Stream Discharge and Yield of Dissolved Substances from a Colorado Mountain Watershed, Soil Science, v. 128, No 6, p 353-363.
Lewis, W.M., and M.C. Grant, 1980, Relationships between Snow Cover and Winter Losses of Dissolved Substances from a Mountain Watershed, Arctic and Alpine Research, v. 12, No. 1, pp. 11-17.
Williams, M. W., J. Baron, N. Caine, R. Sommerfeld, and R. Sanford, 1996, Nitrogen saturation in the Colorado Front Range, Environmental Science and Technology, v.30, p. 640-646.