Organic and inorganic nitrogen pools in talus fields and subtalus water, Green Lakes Valley, Colorado front range

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 soil samples were divided into four classes: subtalus dry, subtalus wet, surface vegetated and surface bare. The size of the individual talus soil patches ranged from 0.5 to 12.0 m² in area, with bulk density ranging from 0-98 to 1-71 kg m^-3 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 mg N kg^-1, and NO₃^-, of 1.0 mg N kg^-1, were comparable with developed alpine soils on Niwot Ridge. Average microbial biomass in subtalus soils of 5.4 mg N kg^-1 and total N of 1000 mg N kg^-1 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 with 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 sufficient biotically conditioned 'soil' within talus fields to influence the solute content of interstitial waters. Mean NO₃^- concentrations of 20 μeq 1^-1 from 29 samples of subtalus water were significantly higher than the 6-7 μeq 1^-1 in snow, while NH₄⁺ concentrations in subtalus water of 0·7 μeq 1^-1 was significantly lower than in snow at 5·2 μeq 1^-1 (p = 0·001). Nitrate concentrations in subtalus water were significantly (p < 0·0001) correlated with concentrations of geochemicàl weathering products such as Ca²⁺ (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 increased with increased residence time, consistent with a biological source for this subtalus water NO₃^-. The high NO₃^- concentrations in subtalus water compared with atmospheric deposition of NO₃^- suggests that NO₃^- in talus fields may contribute to NO₃^- in stream waters of high-elevation catchments.