Ice-on-snow and compacted and absent snowpack exert contrasting effects on soil carbon cycling in a northern boreal forest

In boreal forests, snow typically covers the forest floor and insulates soil from low and fluctuating air temperature for a large part of the year. Snowpack regimes are currently changing in response to climate warming, and it is uncertain how changing subnivean conditions (i.e. those under the snow cover) will impact soil processes. Here, we experimentally tested in a northern boreal forest how three various types of snow cover changes 1) ice encasement, 2) the absence of snow, and 3) snow compaction influence wintertime soil gas concentrations, growing season microbial respiration and biomass, extracellular enzyme activities (EEAs), and dissolved organic carbon (DOC) and nitrogen (N). The consequences varied drastically among experimental treatments. An initially strong increase in wintertime soil CO2 accumulation in response to ice encasement weakened during the course of the 3-year experiment, when at the same time, growing season phenol oxidase decreased by 13% and β-glucosidase activity increased by 25%. The absence of snow enhanced β-glucosidase activity by 47% and, by the third year of experiment, increased inorganic soil N concentration. Snow compaction, in turn, enhanced phenol oxidase activity by 27% together with soil microbial respiration and biomass carbon. These findings suggest that consequences of snowpack regime change on the boreal forest soil carbon and nutrient cycling may vary from positive to negative depending on the type of change in the snow cover that becomes dominant.