Dynamic winter microbial communities shape nitrogen cycling potential in Arctic tundra soils

Arctic winters are long and cold and have traditionally been considered a period of limited biological activity. However, the seasonal dynamics of microbial community composition and functional potential during winter remain poorly understood. Here, we investigated taxonomic (bacteria, fungi, archaea) and functional (fungal guilds and nitrogen cycling genes) dynamics throughout a full year at two Arctic tundra heath sites with contrasting snow regimes. A steep drop in microbial abundances in early to mid-winter, likely linked to freeze-thaw events, coincided with shifts in soil pH and elevated community turnover. Saprotrophic and root-associated fungi were more abundant in the cold-season, while inorganic nitrogen cycling groups were more abundant in summer and declined toward winter despite high bacterial abundance. This indicates sustained organic matter cycling during the winter and expanded inorganic nitrogen cycling in the summer. Functional gene ratios further suggested a higher early-winter nitrogen loss potential via nitrous oxide and greater late-winter nitrogen retention. Site-specific differences in snow regime altered the timing and magnitude of these dynamics. Together, our results demonstrate that winter represents a critical and dynamic period for microbial community restructuring with important implications for nitrogen turnover in Arctic tundra soils.