Microbial community seasonal succession may have large impacts on soil biogeochemical dynamics, in particular via the release of nutrients stored in the microbial cytoplasm or changes to microbial functional groups during spring thaw. The spring flush of nutrients at thaw is important for plant growth in many ecosystems, and the availability of nitrogen (N), and in some cases phosphorus (P), is the main determinant of plant communities and thus ecosystem carbon (C) storage in arctic tundra. However, a spring nutrient flush has seldom been reported in arctic tundra. Our objective was to characterize seasonal soil biogeochemistry in arctic tundra, especially the timing, the magnitude and the drivers of the spring nutrient flush and the relationship between this flush and soil microbial community succession. We predicted that winter snow depth would be a strong control on spring soil biogeochemistry and the soil microbial community. We sampled soil biogeochemistry (inorganic and organic C, N and P) and soil microbial biomass and community structure (CFE, epifluorescent microscopy, PLFA and qPCR), from late winter until fall 2007, in a Canadian low arctic birch hummock tundra (Daring Lake, NWT).  To investigate the impact of deepened snow on winter and spring soil nutrient pools and microbial succession, we sampled under ambient (0.3 m) and experimentally deepened (1 m) snow.

Results/Conclusions

During spring thaw, two important soil temperature periods of approximately 3 weeks each were identified. When all soils hovered near -5^oC and the ambient snow pack thawed, there were large peaks and troughs in soil solution and microbial C and P under ambient and deep snow. When soil temperatures hovered near 0^oC and the deep snow was melting, ambient C, N and P pools stayed low and relatively constant, but soils under deepened snow had very large peaks and troughs in soil solution and microbial C, N and P. All of our microbial biomass and microbial community analytical methods agreed that soil bacteria and fungal biomass peaked in these two spring periods; that these soils are dominated by fungi; and that fungi declined at the end of the 2^nd period more than bacteria, and later under deeper snow. Our results support two broad conclusions: (1) that snow depth is an important control on spatial variability in soil and microbial nutrient pools, and thus possibly vegetation, across the tundra and (2) that microbial seasonal succession is an important mechanism contributing to temporal variability in tundra biogeochemistry.