Effects of lengthening growing season and increasing temperature on soil carbon fluxes and stocks in Arctic tundra

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Abstract Global warming is lengthening the growing season and increasing temperature in the arctic ecosystem. The lengthened growing season is featured by earlier snow melt and later fall freeze-up. Photosynthesis and respiration may respond differently to these changes, resulting in changes in the seasonal pattern, the annual carbon budget, whole-system carbon stocks, and the nutrient supply in the Arctic. Given the large amount of carbon stored in the arctic system and high sensitivity of carbon to temperature, the changing seasonality will influence the global carbon cycle and the climate system.The objectives of this project are to examine if and how microbial respiration, root respiration, and photosynthesis respond differently to earlier snow melt in spring and later soil freeze-up in fall, if and how these processes respond differently to enhanced temperature in the growing season, how the annual carbon budget changes, and what the implication is for the future arctic tundra ecosystem and the global carbon cycle.The study site will be in a tussock tundra ecosystem at the Toolik Field Station on the North Slope of Alaska. Open-top chambers will be used to manipulate temperature and the growing season. A mini-trenching approach will be used to partition soil respiration and separate the different response between root respiration and microbial respiration to the manipulated growing season and soil temperature. Combining two treatments (warming and trenching), 9 warming, 9 warming + trenching, 9 trenching, and 9 control plots will be established.Microbial and root respiration will be continuously, automatically measured with a belowground CO2 profile system, which is a better approach than the chamber measurement method in the tundra ecosystem. This continuous system allows examination of the mismatch of photosynthesis and respiration in response to the change in temperature and the timing of snowmelt and refreezing, while causing negligible disturbance to soils and aboveground plants in the tundra. Corresponding measurement of soil temperature, water content, canopy air pressure, air temperature and water vapor pressure will also be conducted. A portable photosynthesis measurement system with imbedded light sources will be used to measure leaf photosynthesis and its response to light. Periodic measurement of photosynthesis and leaf respiration will be extrapolated with a photosynthesis model. Continuous time-series data of photosynthesis, leaf respiration, root respiration, and microbial respiration with its correlation with temperature will be analyzed to examine the response to changing seasonality. Net ecosystem production will be calculated to examine changes in the annual carbon budget, its coupling with nitrogen availability, and implication for the carbon stock in the arctic and global system.