Biogeochemistry, 1998 (in press)

Inorganic N and microbial biomass dynamics before and during spring snowmelt

Paul D. Brooks
US Geological Survey
Boulder, CO

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

Steven Schmidt
EPO Biology
University of Colorado
Boulder, Colorado 80309


Recent work in seasonally snow covered ecosystems has identified thawed soil and high levels of heterotrophic activity throughout the winter under consistent snow cover. We performed measurements during the winter of 1994 to determine how the depth and timing of seasonal snow cover affect soil microbial populations, surface water NO3- loss during snowmelt, and plant N availability early in the growing season. Soil under early accumulating, consistent snow cover remained thawed during most of the winter and both microbial biomass and soil inorganic N pools gradually increased under the snowpack. At the initiation of snowmelt, microbial biomass N pools increased from 3.0 to 5.9 g N m-2, concurrent with a decrease in soil inorganic N pools. During the latter stages of snowmelt, microbial biomass N pools decreased sharply without a concurrent increase in inorganic N pools or significant leaching losses. In contrast, soil under inconsistent snow cover remained frozen during most of the winter. During snowmelt, microbial biomass initially increased from 1.7 to 3.1 g N m-2 and then decreased as sites became snow-free. In contrast to smaller pool sizes, NO3- export during snowmelt from the inconsistent snow cover sites of 1.14 (+/- 0.511) g N m-2 was significantly greater (p<0.001) than the 0.27 (+/- 0.16) g N m-2 exported from sites with consistent snow cover. These data suggest that microbial biomass in consistently snow-covered soil provides a significant buffer limiting the export of inorganic N to surface water during snowmelt. However, this buffer is very sensitive to changes in snowpack regime. Therefore, interannual variability in the timing and depth of snowpack accumulation may explain the year to year variability in inorganic N concentrations in surface water these ecosystems.