CO 2 flux determination by closed-chamber methods can be seriously biased by inappropriate application of linear regression

• Main Authors: Kutzbach, L., Schneider, Jodi, Sachs, T., Giebels, M., Nykänen, H., Shurpali, N. J., Martikainen, P. J., Alm, J., Wilmking, M.
• Other Authors: Institute for Botany and Landscape Ecology, Universität Greifswald - University of Greifswald, Alfred Wegener Institute Potsdam, Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung (AWI), Institute of Geoecology, Technische Universität Braunschweig = Technical University of Braunschweig Braunschweig, Department of Environmental Science, Biogeochemistry Research Group, Finnish Forest Research Inst., Joensuu Research Unit
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2007
• Subjects: [PHYS.ASTR.CO]Physics [physics]/Astrophysics [astro-ph]/Cosmology and Extra-Galactic Astrophysics [astro-ph.CO]; [SDU.ASTR]Sciences of the Universe [physics]/Astrophysics [astro-ph]; [SDU.ENVI]Sciences of the Universe [physics]/Continental interfaces; environment; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Tundra; Siberia
• Online Access: https://hal.archives-ouvertes.fr/hal-00297904; https://hal.archives-ouvertes.fr/hal-00297904/document; https://hal.archives-ouvertes.fr/hal-00297904/file/bgd-4-2279-2007.pdf

International audience Closed (non-steady state) chambers are widely used for quantifying carbon dioxide (CO 2 ) fluxes between soils or low-stature canopies and the atmosphere. It is well recognised that covering a soil or vegetation by a closed chamber inherently disturbs the natural CO 2 fluxes by altering the concentration gradients between the soil, the vegetation and the overlying air. Thus, the driving factors of CO 2 fluxes are not constant during the closed chamber experiment, and no linear increase or decrease of CO 2 concentration over time within the chamber headspace can be expected. Nevertheless, linear regression has been applied for calculating CO 2 fluxes in many recent, partly influential, studies. This approach was justified by keeping the closure time short and assuming the concentration change over time to be in the linear range. Here, we test if the application of linear regression is really appropriate for estimating CO 2 fluxes using closed chambers over short closure times and if the application of nonlinear regression is necessary. We developed a nonlinear exponential regression model from diffusion and photosynthesis theory. This exponential model was tested with four different datasets of CO 2 flux measurements (total number: 1764) conducted at three peatland sites in Finland and a tundra site in Siberia. The flux measurements were performed using transparent chambers on vegetated surfaces and opaque chambers on bare peat surfaces. Thorough analyses of residuals demonstrated that linear regression was frequently not appropriate for the determination of CO 2 fluxes by closed-chamber methods, even if closure times were kept short. The developed exponential model was well suited for nonlinear regression of the concentration over time c(t) evolution in the chamber headspace and estimation of the initial CO 2 fluxes at closure time for the majority of experiments. CO 2 flux estimates by linear regression can be as low as 40% of the flux estimates of exponential regression for closure ...