MICROBIAL ACTIVITY UNDER ALPINE SNOWPACKS, NIWOT RIDGE, COLORADO

Experiments were conducted during 1993 at Niwot Ridge in the Colorado Front Range to determine if the insulating effect of winter snow cover allows soil microbial activity to significantly affect nitrogen input s and outputs in alpine systems. Soil surface temperatures under seaso nal snowpacks warmed from -14 degrees C in January to 0 degrees C by M ay 4th. Snowmelt began in mid-May and the sites were snow free by mid June. Heterotrophic microbial activity in snow-covered soils, measured as CO2 production, was first identified on March 4, 1993. Net CO2 flu x increased from 55 mg CO2-C m(-2) day(-1) in early March to greater t han 824 mg CO2-C m(-2) day(-1) by the middle of May. Carbon dioxide pr oduction decreased in late May as soils became saturated during snowme lt. Soil inorganic N concentrations increased before snowmelt, peaking between 101 and 276 mg kg(-1) soil in May, and then decreasing as soi ls became saturated with melt water. Net N mineralization for the peri od of March 3 to May 4 ranged from 2.23 to 6.63 g N m(-2), and were ap proximately 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 wate r 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 warm ed, peaking at 75 mu g N2O-N m(-2) day(-1) in soils saturated with mel t water one week before the sites were snow free. These data suggest t hat microbial activity in snow-covered soils may play a key role in al pine N cycling before plants become active.