Microbial activity and nitrogen cycling under seasonal snowpacks, Niwot Ridge, Colorado

Process level controls on microbial activity and nitrogen cycling during the snow covered season were identified in an alpine ecosystem in the Colorado Front Range. Experiments were conducted to identify the controls on, and the spatial distribution of, microbial respiration under seasonal snowpacks, to quantify atmospheric N inputs, N mineralization, nitrification, denitrification, N immobilization, and hydrologic export of N. Continuous snow cover was found to insulate soils from extreme air temperatures allowing soils to thaw well before the initiation of snowmelt. Soils thawed from the surface resulting in a clearly defined zone of microbial activity. Microbial respiration increased as soils warmed and the active microbial population grew under the snowpack reaching maximum values of 500 to 600 mg CO{dollar}sb2{dollar}-C m{dollar}sp{lcub}-2{rcub}{dollar} d{dollar}sp{lcub}-1{rcub}{dollar}. Nitrogen mineralization was highest in soils which froze early in the year and then exhibited high levels of heterotrophic microbial activity under a continuous snow pack. Nitrification was significantly higher under shallow snowpacks characterized by lower levels of heterotrophic respiration. Microbial biomass increased throughout the spring in thawed soils reaching seasonal peaks shortly before snowmelt and decreased rapidly as sites became snow free. The increase in microbial biomass N was more than sufficient to immobilize inorganic N inputs from mineralization of soil organic matter. This activity is apparently responsible for the observation of no net export of N in surface water. Frozen soil below the active surface layer limited export of N in subsurface flowpaths resulting in only gaseous losses of N from the system. Denitrification N loss ranged from trace amounts to 250 ug N{dollar}sb2{dollar}O-N m{dollar}sp{lcub}-2{rcub}{dollar} d{dollar}sp{lcub}-1{rcub}{dollar} depending on the depth and timing of snowpack accumulation. Together, these data suggest that microbial activity in snow covered soils is important in controlling the export of N in surface water, as well as potentially providing an N source for vegetation early in the growing season.