Biogeochemical dynamics during snowmelt and in summer in the Alps

In alpine zones, soil microbial biomass and activity are strongly dependent on the seasonal snow cover. Current models assume that microbial biomass reaches an annual peak in winter under the insulating snowpack with a subsequent sharp decline during snowmelt. In this study, we investigated the seasonal dynamics of the soil microbial biomass in the Central Alps, where usually early snowfall buffers winter soil temperatures. We conducted a large-scale survey in three mountains around Davos (Switzerland) along altitudinal gradients from approximately 1900 to 2800 m above sea level. Using a space-for-time approach during snowmelt, soil samples were taken (1) under, (2) at the edge of, and (3) one meter away from remaining snow patches. One additional sample per site was taken in summer to further evaluate the seasonal dynamics. In total, 184 soil samples from 46 different sites were analyzed. We measured microbial biomass C and N, enzymatic activity and dissolved C and N. We observed an increase of microbial biomass and dissolved C during and immediately after snowmelt, as well as an increase from spring to summer. We suggest that the absence of soil freezing in winter and the growing amounts of dissolved C supported a continued growth, without a sudden collapse of the microbial biomass. Our results underline the importance of the insulating effect of the seasonal snow cover for the microbial dynamics. Global warming is modifying the timing and abundance of the seasonal snow cover, and our results will help to refine models for the dynamics of soil microbes in alpine ecosystems.