Soil biogeochemistry during the early spring in low arctic mesic tundra and the impacts of deepened snow and enhanced nitrogen availability

Air temperature freeze-thaw cycles often occur during the early spring period directly after snowmelt and before budbreak in low arctic tundra. This early spring period may be associated with nitrogen (N) and carbon (C) loss from soils as leachate or as trace gases, due to the detrimental impact of soil freeze-thaw cycles and a developing active layer on soil microorganisms. We measured soil and microbial pools of C and N in early spring during a period of fluctuating air temperature (ranging from −4 to +10°C) and in midsummer, in low arctic birch hummock tundra. In addition we measured N₂O, CH₄ and CO₂ production in the early spring. All of these biogeochemical variables were also measured in long-term snowfence (deepened snow) and N-addition plots to characterize climate-change related controls on these variables. Microbial and soil solution pools of C and N, and trace gas production varied among the five early spring sample dates, but only marginally and no more than among sample dates in midsummer. N-addition greatly elevated N₂O fluxes, indicating that although denitrifiers were present their activity during early spring was strongly limited by N-availability, but otherwise trace gas production was very low in early spring. The later thaw, warmer winter and colder spring soil temperatures resulting from deepened snow did not significantly alter N pools or rates in early spring. Together, our results indicate strong stability in microbial and soil solution C and N pool sizes in the early spring period just after snowmelt when soil temperatures are close to 0°C (−1.5 to +5°C). A review of annual temperature records from this and other sites suggests that soil freeze-thaw cycles are probably infrequent in mesic tundra in early spring. We suggest that future studies concerned with temperature controls on soil and microbial biogeochemistry should focus not on soil freeze-thaw cycles per se, but on the rapid and often stepped increases in soil temperature that occur under the thawing snowpack.