Microbial Community Response to Freezing in Two Contrasting Arctic Tundra Soil Types

Vegetation change in arctic ecosystems is a well-documented, widespread phenomenon. This change is partly driven by belowground feedbacks between plants and microbes. Microbes can regulate plant community composition and productivity by controlling nutrient supply, which also regulates decomposition rates and carbon balance. We investigated what happens to microbes as soils freeze and how growing season biogeochemical cycling may be affected by the antecedent conditions that develop over the long, cold winters.

Organic soils from moist acidic tussock (Eriophorum vaginatum) and shrub (Betula nana, Salix spp.) tundra were selected for analysis. Soil cores were subjected to a simulated freezing event (to -2°C) approximating the natural pattern observed as these soils freeze, including an acclimation phase. Additional treatments removed any acclimation phase with immediate freezing to temperatures of either -2 or -20°C. Respired CO₂, active microbial biomass and dissolved inorganic nitrogen (DIN, NH₄+ and NO₃^-) were measured, and DNA extracted for analysis of community structure.

In shrub tundra soils, active microbial biomass was insensitive to acclimation or freezing, but more DIN was released in soil solution during the acclimation and frozen phases. Tussock soils appeared to be more sensitive to freezing than shrub soils; active microbial biomass declined and DIN accumulated in solution. However, if tussock soils had undergone acclimation before freezing, less DIN was released into solution. When combined with information on basal respiration rates, these data suggest that organic substrates can accumulate in soils as they freeze, but this accumulation is slowed if microbial communities are allowed to acclimate to low temperatures prior to freezing. Therefore, the speed and intensity of freezing in winter, as well as the plant community, may exert a significant control on plant nutrient cycling and decomposition upon soil thaw in the spring.