Quantifying Seasonal Contributions to the Total Annual Carbon Budget of an Alaskan Arctic Tundra Ecosystem

The Arctic is a critical area for understanding climate change and controls on carbon exchange. While the summer seasons in Arctic are relatively well studied, insufficient information on the role of non-summer seasons on trace gas feedbacks to the atmosphere remains. Here, a yearlong study of carbon dioxide fluxes in an Alaskan Arctic tundra ecosystem is presented. In addition, impacts of likely changes in season timing and duration on the CO2 budget of the Arctic are examined, alongside changes in environmental controls on CO2 fluxes. A new protocol to quantitatively define seasons in the Arctic based on available energy is developed, rather than the traditional date-based method. Eddy covariance data were corrected using the standard density (WPL) term, adjusted for recently developed open-path surface heating correction. The result was a significant apparent reduction in the magnitude of sink terms when compared to the WPL correction alone. The non-summer season was a significant source of carbon to the atmosphere that overwhelmed the net CO2 uptake of the summer period. Summer was a net sink of 11.4 g C m-2 yr-1, while the non-summer seasons released more than four times the CO2 uptake observed in the summer, resulting in a net annual source of 37.6 g C m-2 yr-1 to the atmosphere. Results of regression analyses to determine the most significant controls on CO2 flux significantly improved when seasons were added to the analyses (P < 0.01). Significant relationships, such as that of soil temperature on carbon exchange, only became apparent when seasonality was included, because the relationship changes from positive to negative by season. These results serve as evidence of the significance of all seasons on the carbon metabolism in the Arctic, and of the changing functional relationships with carbon exchange throughout the year.