| Summary: | Understanding the processes, controls, and mechanisms controlling the release of soil organic carbon as CO2 and CH4 is vital in order to better predict how sensitive Arctic soils will respond to and influence global climate change in the present and into the future. Sizeable uncertainties remain as to the functioning and processes governing the carbon cycle in the Arctic during the non-growing seasons. In particular, shoulder season dynamics bear a large amount of empirical uncertainty, owing to a dearth of literature encompassing these important periods. Consequently, three associated studies have been proposed to enhance our understanding. In the first part of the research presented here, a long-term chamber flux data was marshaled to broaden our understanding of the patterns and controls of CO2 efflux and to investigate the influence of landscape heterogeneity on biogeochemical cycling during the growing season in the Arctic. It was found that estimates of the temperature response of respiration could be simplified across heterogeneous landscapes. Seasonal Q10 was seen to vary across the growing season, a valuable result that will help constrain future climate models. Finally, chamber respiration measurements at the plot level agreed well with tower scale measurements, an outcome that will further simplify future growing season modeling efforts that integrate tundra microtopography and landscape type. In the second part of the research, nearly continuous non-growing season measurements of soil CO2 were used to investigate changes in below ground concentration and the environmental controls on Arctic soil CO2 dynamics. Soil concentration was seen to double through the fall shoulder season, suggesting continued soil microbial activity through freeze-up. Upscaling soil concentration data to landscape level fluxes suggested complex and spatially heterogeneous processes in the fall shoulder season. Concentration was seen to be seasonally controlled by physical, hydrological, and biotic activity that varied across years. The importance of multi-year operations and high-temporal coverage datasets should not be overlooked, as spatially and temporally dissimilar processes govern the non-growing season carbon cycle. In the last part of the research, temperature-controlled soil incubations enhanced our understanding of CH4 consumption dynamics across the zero curtain and shoulder season. The wetland soils of Utqiaġvik and Ivotuk kept a substantial methanol oxidation potential even under anaerobic/frozen conditions. While Q10 values differed markedly in subzero and nonfreezing temperature, methylotrophy continued down to -2°C. The difference in temperature dependence on methanol oxidation at the lower temperatures between these wetland soils could give insight towards predicting methane dynamics in the Arctic under continued warming. As microbial communities are geographically and potentially temporally heterogeneous the role of methane oxidizing bacteria on CH4 cycling during the non-growing seasons should be investigated more fully in the future.
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