Seasonality of microbial functional potentials and community diversity in high-Arctic tundra soils

High-Arctic tundra soils are highly sensitive to climate change, which alters snow cover, light, water availability, and growing season length, impacting ecosystem functioning. Despite the key role of microorganisms in carbon, nitrogen, and phosphorus cycling, their seasonal variations and effects on greenhouse gas (GHG) fluxes remain poorly understood in high-Arctic tundra soils. Here, we measured potential microbial functions in biocrusts and their underlying mineral soils along toposequences near Ny-Ålesund (Svalbard) during each season, and explored their relationship with prokaryotic and fungal diversity and community structure. A peak in soil multifunctionality (i.e. an index representing multiple ecosystem functions), as well as in most microbial extracellular enzyme activities involved in acquiring C, N and P was observed in July-August. GHG fluxes were also highly variable across sampling times. While CO 2 fluxes were clearly higher in July-August, CH 4 emissions were slightly higher in September, especially in the upslope and hence drier soils. N 2 O fluxes were substantially higher in both shoulder seasons (i.e. June and September). Prokaryotic and fungal community structure and diversity, however, depended more on topography and soil layers than on seasonal dynamic. Our study shows that seasonal dynamics and topography jointly shape microbial diversity and functions in High-Arctic soils, emphasizing their role as key drivers under rapidly changing environmental conditions.