Freeze-thaw responses of soil microbial communities from contrasting elevations

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Melissa A. Pastore, Marie E. English, Karin Rand and E. Carol Adair, Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, VT, Melissa A. Pastore, Aimée T. Classen and E. Carol Adair, Gund Institute for Environment, University of Vermont, Burlington, VT, Aimée T. Classen, Ecology and Evolutionary Biology Department, University of Michigan, Ann Arbor, MI, Aimée T. Classen, University of Michigan Biological Station, Pellston, MI, Serita D. Frey, Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH

Background/Question/Methods
As climate warms, winters in temperate ecosystems of the northeastern United States will experience reductions in snow cover depth and duration, increasing the frequency of soil freeze-thaw cycles. These changes could affect microbial community composition and activity, with important effects on soil carbon storage and nutrient cycling. Using a laboratory incubation, we investigated how historical snow conditions influence CO2 fluxes, soil carbon and nitrogen dynamics, and microbial community biomass during freeze-thaw cycles. We inoculated sterile soil with microbial communities collected from low elevation (thin snowpack) or high elevation (thick snowpack) spruce-fir forests in Vermont and exposed them to either diurnal freeze-thaw cycles (-7 and 4°C) or constant temperature (4°C). After 9 freeze-thaw cycles, all samples were kept at constant 4°C during a 7-day recovery period to assess lagged effects and the ability of the differently adapted microbial communities to recover from repeated freeze-thaw disturbance. CO2 fluxes were measured daily, and microbial community composition/biomass, soil inorganic nitrogen, and dissolved organic carbon were measured after the treatment period and again after the recovery period.

Results/Conclusions
We found that freeze-thaw cycles decreased CO2 fluxes by 37 and 54% in the high and low elevation microbial communities, respectively, compared to their unfrozen controls. However, during recovery CO2 fluxes eventually met those of the controls in both microbial communities, indicating that reduced microbial respiration was due to reversibly impaired microbial activity rather than microbial death. Interestingly, CO2 fluxes from the high elevation microbial community surpassed control levels during the first two recovery days, revealing a short-term pulse in microbial respiration during the extended thaw period after successive freeze-thaw cycles for that community. Thus, the microbial community with a history of thicker snowpack (i.e., from high elevation) exhibited less impairment of activity during freeze-thaw cycles and a pulse in respiration during the extended thaw compared to the community with a history of thinner snowpack, indicating greater soil carbon losses. Freeze-thaw cycles also increased soil ammonium by 10 and 11% in the high and low elevation microbial communities, respectively, compared to their controls. This indicates that freeze-thaw cycles may increase soil nitrogen availability earlier in the season before plants are active, potentially enhancing ecosystem nitrogen losses. Overall, our results suggest that as the frequency of soil freeze-thaw cycles increases in the future, forests that have historically experienced persistently frozen soils may exhibit greater microbially-induced soil carbon losses compared to forests that have historically experienced repeated freeze-thaw cycles.