What drives the soil microbial and nutrient declines at the end of the Arctic winter?

Microbial biomass (MB) peaks in Arctic and alpine soils in late winter, despite sub-zero temperatures. MB declines before spring, which is typically explained by resource exhaustion, particularly of carbon substrates. I measured MB and various soil nitrogen (N), phosphorus (P) and carbon (C) pools over two years in Arctic sedge meadows in northern Manitoba, Canada. Sampling was concentrated during the winter-spring transition, and activities of several exoenzymes also were measured. MB, nutrients, and exoenzyme activities all peaked at the same time in late winter followed by a sharp decline as soil temperatures increased towards the freezing point prior to and during spring thaw. Late-winter combined additions of glucose-C, N, and P failed to maintain MB levels above those in untreated soils, and MB and nutrient levels remained low in spring and summer. In these frozen soils that are water-saturated at thaw, a syntrophic community of fermentative microbes likely predominates at the end of winter. Increased liquid water films and physical disturbance by freeze-thaw cycles, causing fluctuations in soil oxygen levels, likely constitutes a disturbance severe enough to disrupt key anaerobic microbial functional groups, bringing about a shift in the microbial community composition as well as soil nutrient turnover rates between winter and summer. Under warmer and more variable climate conditions, the timing of these severe disturbance events could advance, altering winter microbial processes and compromising the availability of nutrients to plants in early spring.