Future climate scenarios forecast significant changes in patterns of snowfall. Snow regulates environmental stresses, which dramatically impact soil bacteria biomass while simultaneously altering soil CO2 dynamics. Despite these effects, our understanding of how snowfall variability influences soil bacterial community composition and winter CO2 production are limited, especially in temperate ecosystems. Is the addition or reduction of snow more important in structuring community composition? More snowfall may insulate soils from freezing, thus creating relatively warm conditions for bacteria. In contrast, less snowfall may induce freeze-thaw cycles, which are major stresses for bacteria. Additionally, what impacts winter CO2 production more—relatively stable winter soil temperatures or freeze-thaw cycles? To answer these questions, we created three snowfall treatments in a deciduous forest at the Kellogg Biological Station Long-Term Ecological Research site: Ambient—natural snowfall, Removal—snowfall removed, and Addition—double natural snowfall. In each of these treatments, we monitored real-time changes in soil temperature, moisture, and CO2 flux throughout the winter using in situ sensors. At the end of the three month experiment, we tested for effects of our snowfall manipulations using multivariate statistics on community composition data generated via pyrosequencing of 16S rRNA genes from surface soils (0-5 cm).
Results/Conclusions
Soils in Ambient and Removal treatments experienced multiple freeze-thaw cycles (two and five respectively) with temperatures reaching below -4°C. Alternatively, Addition soils were insulated from freezing with temperatures remaining above 0.1°C. Bacterial community composition was impacted by these different soil conditions. Specifically, bacterial composition was affected more by the addition than the removal of snow. Based on Bray-Curtis distances, communities differed between Addition and Ambient soils, while communities were more similar between Removal and Ambient soils. Major phyla and sub-phyla of bacteria demonstrated different degrees of sensitivity to the treatments. Throughout the winter, freeze-thaw events created pulses of CO2 in Ambient and Removal soils, while soil CO2 flux was relatively constant in Addition soils. Total winter CO2 production was highest in Ambient soils. These soils experienced relatively stable temperatures for most of the winter and were exposed to only two freeze-thaw cycles. Our results suggest that under snowfall scenarios predicting more snow, bacterial community composition may be altered due to warmer soil temperatures and the absence of freeze-thaw cycles, and winter CO2 dynamics and production may decline under more stable soil conditions.
