WRR, V 32, p 3175-3185, 1996

Mineral nitrogen transformations in and under seasonal snow in a high-elevation catchment, Rocky Mountains, USA

Mark W. Williams
Department of Geography and Institute of Arctic and Alpine Research University of Colorado Boulder, Colorado 80309

Paul D. Brooks
US Geological Survey Boulder, CO

Arvin Mosier
USDA-ARS Fort Collins, CO

Kathy A. Tonnessen
National Park Service-Air Quality Division Denver, CO


In an effort to understand sources of nitrate (NO 3-) in surface waters of high elevation catchments, nitrogen (N) transformations in and under seasonal snow were investigated from 1993 to 1995 on Niwot Ridge, an alpine ecosystem at 3,500m located in the Colorado Front Range of the Rocky Mountains. Ammonium (NH4+) and NO3- labelled with 15N applied as non-conservative tracers to the snow showed no evidence of nitrification in the snowpack. Furthermore, NH4+ movement through the amended snowpack was highly correlated with a conservative chloride tracer (r2 = 0.99). In an unamended snowpack, NH4+ concentrations in meltwater before contact with the ground were highly correlated with NO3- concentrations (r2 = 0.98), consistent with no nitrification in the snowpack. The isotopically-labelled 15NH415NO3 applied to the snowpack was found in underlying soils, showing that NH4+ released from snow can be rapidly immobilized. Resin bag (mixed-bed ion-exchange resins) measurements (N = 22) showed that 80% of the mobile inorganic N in unamended subnivial soils was NO3-. Measurements of KCl-extractable inorganic N from surface soils showed that highest values were prior to the initiation of snowmelt and lowest values were during the growing season. The natural @delta@15N abundance of unamended soils was negative and ranged from -12 to -2, suggesting that atmospheric deposition of @delta@15N-depleted N is an important component of N cycling in these alpine soils. These results suggest that soil mineralization under seasonal snow, rather than snowmelt release of NO3-, may control NO3- concentrations in surface waters of high-elevation catchments.