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Harvard Forest Symposium Abstract 2005

  • Title: Two years of high temporal frequency measurements of CO2 efflux from soil
  • Primary Author: Ruth Varner (University of New Hampshire - Main Campus)
  • Additional Authors: Patrick Crill (University of New Hampshire - Main Campus)
  • Abstract:

    Understanding 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 rapidly accumulation CO2 in our atmosphere. Tower based measurements of net CO2 exchange are the best assessment of the flux of CO2 between the atmosphere and natural ecosystems over large areas. However they integrate a mosaic of different CO2 source and sink processes and, in order to understand the process, we need to measure the components of the net flux. Automated sampling of CO2 exchange with chambers can provide high temporal resolution data at specific locations within the landscape to help address these issues.


    We have completed two years (days 110-323, 2003; days 104-352, 2004) of autochamber measurement of CO2 efflux from a site located northwest of the Environmental Monitoring Station (EMS) tower at HF. The soil system sequentially samples 8 aluminum static soil chambers each closed for 9 minutes, completing a circuit every 4 hours. Analysis for CO2 from the chamber headspace is completed with an infrared gas analyzer (LiCor 820). The short closure times for the fluxes allow for the least amount of buildup of CO2 in the chamber headspace but still allowing for a flux calculation based on 10 to 20 observations. Soil fluxes are calculated as the change in mixing ratio of CO2 versus time during the chamber closed period. The 8 chambers are located along a transition from poorly drained soils at the 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.





    We measured 7437 fluxes in 2003 and 10850 fluxes in 2004. The annual average fluxes were 2.7 +/-0.4 for 2003 and 2.7 +/- 0.6 for 2004. Daily average fluxes reveal a seasonal pattern most likely related to seasonal changes in temperature (Figure 1). The daily average fluxes do not seem to indicate interannual variation in flux. Individual chamber averages do show some interannual variation in flux (Figure 2). Chambers 4, 5 and 6 (yellow shaded region) show higher fluxes on in 2004 while the other chambers show lower fluxes. These chambers are located approximately mid slope.





    Our chamber measurements have begun to quantify the magnitude and the temporal dynamics of a component of the net flux measured by the tower at Harvard Forest, MA. The automated flux measurements provide a key constraint on the carbon exchange measurements and ultimately on extrapolation across a broad range of temporal and spatial scales. These scales extend from whole ecosystem exchange measurements such as those measured by eddy correlation techniques to the molecular level such as ecophysiological studies of different fractions of soil organic matter.

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