Biogeochemical activity occurs year-round in Arctic soils, and previous studies from both alpine and Arctic tundra sites has revealed that active-layer soil microbial biomass (MBC) reaches an annual peak at the end of winter, and declines during or shortly after soil thaw. This occurs concurrently with, or is followed by, a peak in soil nutrients that provide an important nutritional resource for plant growth. We did an observational study in dry sedge tundra and wet sedge tundra to monitor the annual pattern of soil microbial biomass and nutrient resources over four consecutive years, sampling at all times of year but focusing our efforts during the dynamic period of the winter-spring transition. Soils were collected on approximately 55 occasions between June 2004 and June 2008 from both wet and dry sites, and 6 replicates were usually sampled from each ecotype. Soil MBC, microbial nitrogen (MBN), and microbial phosphorus (MBP) (chloroform fumigation and extraction), extractable carbon (C), nitrogen (N), and phosphorus (P), inorganic N (ammonium and nitrate), fresh weight/dry weight, bulk density, and total soil carbon and nitrogen were quantified. Redox and pH measurements were taken for some but not all time points.

Results/Conclusions

A consistent pattern was observed in wet sedge meadows with high late winter MBC, MBN, and MBP pools that declined steeply during the period of soil thaw. Soil temperatures from nearby and comparable sites show a consistent correlation between MBC, MBN, and MBP declines and soil temperature warming, both at thaw and during early warming events in April each year, suggesting that soil physical factors drive the observed microbial declines. Nutrient pools showed similar patterns, however a post-thaw nutrient-N pulse was observed in 2006 but not in other years. Dry site results were comparable but pool dynamics were dampened relative to wet sites. An N pulse was observed in 2006 and resulted in exceptionally high MBN values during the summer of that year. This pulse was probably the result of a dry thaw period resulting in minimal leaching losses, followed by a damp spring possibly supporting above-normal algal growth. Over four years, peak winter values of all variables declined in both ecotypes. Soils appeared to become drier, suggesting that if these ecosystems experience long-term drying trends, winter decomposition could be negatively affected, and the nutrient supply to plants in spring could be jeopardized.