Inter-annual and seasonal dynamics of soil microbial biomass and nutrients in wet and dry low-Arctic sedge meadows

Biogeochemical activity occurs year-round in arctic soils, and previous studies from both alpine and arctic tundra sites have revealed that soil microbial biomass (MB) in the active layer reaches an annual peak in winter, and decreases during or shortly after soil thaw. This decline occurs concurrently with, or is followed by, a peak in soil nutrients that provide an important nutritional resource for plant growth. We documented both intra- and inter-annual MB and nutrient patterns in wet and dry sedge meadows near Churchill, Manitoba, Canada between June 2004 and June 2008. Intensive sampling occurred during the winter-spring transition and soils were analyzed for MB, microbial nitrogen, dissolved organic carbon and nitrogen, and inorganic nitrogen. A consistent seasonal pattern was observed wherein large winter MB pools responded negatively to winter warming event, and decreased steeply during soil thaw, suggesting that soil physical factors drive the observed microbial declines. Nutrient pools showed similar seasonal fluctuations, however a post-thaw nitrogen increase was observed in 2006, but not in other years. Inter-annual patterns were similar between wet and dry sedge meadows, including relatively low peak values of all variables by 2008 in both ecotypes, which could be related to observed hydrological changes. As northern climates continue to change, seasonal biogeochemical events that affect the timing and magnitude of nutrient pulses will be altered, with important implications for primary productivity and ecosystem functioning.