Hydrologic Processes, V 11, N 13, 1997 (in press).

Organic and inorganic nitrogen pools in talus soils and water, Green Lakes Valley, Colorado Front Range

Mark W. Williams
INSTAAR and Department of Geography University of Colorado Boulder, CO

Thomas Davinroy
INSTAAR and Department of Geography University of Colorado Boulder, CO

Paul Brooks
US Geological Survey Boulder, CO


Organic and inorganic pools of nitrogen (N) were measured in talus fines or "soils" and subtalus water during the summer of 1995 in the alpine Green Lakes Valley catchment of the Colorado Front Range. Nineteen talus samples were divided into four classes: subtalus dry, subtalus wet, surface vegetated and surface bare. The size of individual talus soil patches ranged from 0.5 to 12.0\|@m2@ in area, with bulk density ranging from 0.98 to 1.71\|@kgm3@ and soil texture ranging from sandy gravel in the subsurface talus to a loam in the vegetated surface. All samples contained KCl-extractable @nh4@ and @no3@, organic N and carbon (C), and 17 of 19 samples contained microbial biomass. The mean subtalus values for KCl-extractable @nh4@ of 3.2\|@mgNkg@ and @no3@ of 1.0\|@mgNkg@ were comparable to developed alpine soils on Niwot Ridge. Average subtalus microbial biomass of 5.4\|@mgNkg@ and total N of 1,000\|@mgNkg@ were about an order of magnitude lower than alpine tundra soils, reflecting the reduced amount of vegetation in talus areas. However, these measurements in surface vegetated patches of talus were comparable to the well-developed soils on Niwot Ridge. These measurements in talus of microbial biomass, total N, and KCl-extractable @nh4@ and @no3@, show that there is a viable organic system in talus. Mean @no3@ concentrations of 20\|@mueq@ from 29 samples of subtalus water were significantly higher than the 6.7\|@mueq@ in snow, while @nh4@ concentrations in subtalus water of 0.7\|@mueq@ was significantly lower than in snow at 5.2\|@mueq@ (@p@\|=\|0.001). Nitrate concentrations in subtalus water were significantly (@p@\|<\|0.0001) correlated with concentrations of geochemical weathering products such as @ca@ (@r2@\|=\|0.84) and silica (@r2@\|=\|0.49). The correlation of @no3@ in subtalus water with geochemical weathering products suggests that @no3@ concentrations in subtalus water increase with increased residence time, consistent with a biological source for this subtalus water @no3@. The high @no3@ concentrations in subtalus water compared to atmospheric deposition of @no3@ indicates that @no3@ in talus is mobile and may contribute to @no3@ in stream waters of high-elevation catchments.