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Harvard Forest Symposium Abstract 2006

  • Title: Three years of high frequency measurements of soil surface CO2 efflux at Harvard Forest
  • Primary Author: Ruth Varner (University of New Hampshire - Main Campus)
  • Additional Authors: Patrick Crill (University of New Hampshire - Main Campus)
  • Abstract:

    Three years of high frequency measurements of soil surface CO2 efflux at Harvard Forest


    Ruth K. Varner and Patrick M. Crill


    Climate Change Research Center, Institute for the Study of Earth Oceans and Space, University of New Hampshire, Durham, NH


    Department of Geology and Geochemistry, University of Stockholm,





    Defining the seasonal and spatial differences in emission rates from natural environments is important as we try to quantitatively understand the causes and controls on the rapid accumulation of CO2 in our atmosphere. Tower based measurements of net CO2 exchange assess the flux of CO2 between the atmosphere and natural ecosystems over large areas. Eddy correlation integrates a mosaic of CO2 sources and sinks therefore in order to understand the processes we need to measure the components of the net flux. Automated sampling of CO2 exchange with chambers can provide repetitive measures at specific locations within subhabitats of a landscape to help address these issues. Our chambers have begun to quantify the magnitude and the temporal dynamics of an important component, soil emissions, of the net flux measured by the tower at Harvard Forest, MA.





    We have completed three years of autochamber measurement of CO2 efflux during snow-free seasons from a site located northwest of the Environmental Monitoring Station (EMS) tower at HF. The soil system sequentially samples 8 dark static soil chambers every 4 hours. Analysis of CO2 in the chamber headspace is made with an infrared gas analyzer (LiCor 820). The short closure times (10 minutes) allows for the smallest change of CO2 in the chamber headspace but still allows a well resolved flux calculation based on 10 to 20 observations. Soil fluxes are calculated as the change in mixing ratio of CO2 versus time during the time the chamber is closed. The 8 chambers are located along a transition from poorly drained soils at a wetland margin to well-drained upland soils. Three soil moisture and temperature profiles were installed in May of 2004. They were located to capture the upland, mid slope and wetland edge components of the site. Soil moisture and temperature data were collected every minute in 2004 and at half hourly averaged intervals in 2005.





    Soil CO2 efflux shows a significant seasonal pattern with maximum fluxes in the middle of the growing season for 2003 and 2004 and an earlier maximum in June of 2005 (Fig. 1). Even though the distribution is different, the seasonal averages for 2003, 2004 and 2005 are the same: 2.7 +/-0.4, 2.7 +/- 0.6 and 2.7 +/- 0.2 (avg +/- sd) μmoles m-2 s-1 respectively. There have been just under 30,000 fluxes made with the chamber system operational 71%, 90% and 73% of the potential time in 2003, 2004 and 2005.





    The annual trends of the chamber fluxes have been different. The highest annual average fluxes are measured at the mid slope chambers (collars 4 and 6) with generally increasing fluxes yearly (Fig.2). Chamber 5, the chamber closest to the pond edge has shown an increased flux as well. Chambers 1,2 and 8 show decreasing flux while 3 and 7 show a decrease in 2004 and a subsequent increase in 2005.





    In 2006, we plan to expand our measurement to include fluxes from the beaver pond itself. We will install two clear (Lexan) autochambers in the wetland to measure net ecosystem exchange. We will also monitor water level with an automated well.

  • Research Category: Forest-Atmosphere Exchange
    Soil Carbon and Nitrogen Dynamics