Nitrogen mobilization in early spring in the Arctic tundra

A spring flush of nutrients has rarely been recorded in arctic birch hummock tundra. After spring soil thaw but before the growing season begins, there is a period of microbial activity in arctic tundra with less competition for nutrients from plants. This period may represent a time when winter pools of mineralized and organic-nitrogen (N) and dissolved organic-carbon (C) are lost from the system as leachate or as trace gases, in particular during the phase-change from frozen to thawed soil. Because we assumed that low N-availability may limit N₂O production, we added N to replicate tundra plots the previous summer (10 g m^-2yr^-1). Over a ten-day period in early spring we measured soil and microbial pools of labile C and N, as well as trace gas production (CO₂, CH₄ and N₂O), from nitrogen-fertilized and control plots. We also compared frozen and thawed soil dynamics within the control plots. N₂O flux was very low from these soils at this time, although this increased when N was not limiting. Soil and microbial C pools did not differ between frozen and thawed soils, but there were larger soil N pools and less N stored in the microbial biomass in frozen soils, relative to thawed soils. In summary, our results suggest that there was an efficient transfer of N from the soil solution to the microbial biomass at thaw, and that N cycling is tightly constrained at this time of year, even without plant uptake.