| Summary: | Arctic warming and changes in precipitation regimes are expected to increase soil nitrogen (N) availability by stimulating microbial activity and releasing N stored in permafrost. How much contribution cold season warming is going to have, if any, on increasing available N pools and greenhouse gas fluxes is poorly understood. The cold season, a term used to describe the extended Arctic winters, is warming at twice the rate of the growing season, which can alter the soil environment and soil biogeochemistry. In this thesis, I examined changes in soil carbon (C) and N cycling processes in terrestrial Arctic ecosystems, and how they might change in response to cold season warming. I conducted a 50-day laboratory incubation study where I incubated soils mimicking the annual soil thermal regime with different cold season treatments (-10˚C, -15˚C, -20˚C, -25˚C). Throughout the study I measured soil inorganic N pools and cycling rates, and fluxes of the major greenhouse gases (CO2, CH4, and N2O). Over the cold season, both coldest and warmest treatments behaved similarly and displayed high N immobilization rates. Before thaw, extreme cold season warming lead to an increase in soil NO3- concentrations which were immediately lost at thaw. Although cold season fluxes of CH4 and CO2 differed with temperature, cold season warming did not affect growing season flux rates. The results indicate cold season is a critical period when changes in temperature, moisture, and substrate availability can lead to distinct biogeochemical outcomes at different stages of the annum. I conclude that cold season warming can play a crucial role in the seasonality of High Arctic C and N cycles. M.Sc.
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