Biogeochemistry, V 32, pp 93-113, 1996.

Microbial activity under alpine snowpacks, Niwot Ridge, CO

Paul D. Brooks
INSTAAR and EPO Biology University of Colorado Boulder, CO

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

Steven K. Schmidt
EPO Biology University of Colorado Boulder, CO


Experiments were conducted during 1993 at Niwot Ridge in the Colorado Front Range to determine if the insulating effect of winter snow allows soil microbial activity to significantly affect nitrogen inputs and outputs in alpine systems. Soil surface temperatures under seasonal snowpacks warmed from -14c in January to 0c by May 4th. Snowmelt began in mid-May and sites were snowfree by mid-June. Heterotrophic microbial activity in snow-covered soils, measured as CO2 production, was first identified on March 4, 1993. Net CO2 flux increased from 55 mg CO2-C m^2- day^1- in early March to greater than 824 mg CO2-C m^2- day^1- by the middle of May. Carbon dioxide production decreased in late May as soils became saturated during snowmelt. Soil inorganic N concentrations increased before snowmelt, peaking between 101 and 276 mg kg^1- soil in May, and then decreasing as soils became saturated with melt water. Net N mineralization for the period March 3 to May 4 ranged from 2.23 to 6.63 g N m^2-, and were approximately two orders of magnitude greater than snowmelt inputs of 50.4 mg N m^-2 for NH4 and 97.2 mg N m^-2 for NO3. Both NO3 and NH4 concentrations remained at or below detection limits in surface water during snowmelt, indicating the only export of inorganic N from the system was through gaseous losses. Nitrous oxide production under snow was first observed in early April. Production increased as soils warmed, peaking at 75 ug N2O-N m^-2 day^-1 in soils saturated with melt water one week before the sites were snow free. These data suggest that microbial activity in snow-covered soils may play a key role in alpine N cycling before plants become active.