Plant and Soil 214: 125–132, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 125 Soil CO 2 efflux in a beech forest: comparison of two closed dynamic systems Val´ erie Le Dantec 1 , Daniel Epron 2,∗ and Eric Dufrêne 1 1 Laboratoire d’Ecophysiologie V´ eg´ etale, Bâtiment 362, CNRS-U.P.R.E.S.A. 8079, Universit´ e de Paris XI, 91405 Orsay cedex, France and 2 Laboratoire de Biologie et Ecophysiologie, Institut des Sciences et des Techniques de l’Environnement, Universit´ e de Franche-Comt´ e, BP 427, 25211 Montb´ eliard cedex, France Received 15 December 1998. Accepted in revised form 6 July 1999 Key words: boundary layer, dynamic closed system, Infra-Red Gas Analysis (IRGA), soil CO 2 efflux, soil respiration, wind speed Abstract The aim of this study was to understand why two closed dynamic systems with a very similar design gave large differences in soil CO 2 efflux measurements (PP systems and LI-COR). Both in the field (forest beech stand) and in the laboratory, the PPsystems gave higher estimations of soil CO 2 efflux than the LI-COR system (ranging from 30% to 50%). The difference in wind speed occurring within the soil respiration chambers (0.9 m s −1 within the SRC-1 and 0.4 m s −1 within the LI-6000-09 chambers) may account for the discrepancy between the two systems. An excessive air movement inside the respiration chamber is thought to disrupt the high laminar boundary layer over the forest floor. This would promote an exhaust of the CO 2 accumulated into the upper soil layers into the chamber and a lateral diffusion of CO 2 in the soil towards the respiration chamber. The discrepancy between the two systems was reduced (i) by decreasing fan speed within the SRC-1, (ii) by increasing wind speed over the soil surface outside the respiration chamber, or (iii) by using an artificial soil design without high CO 2 concentration in soil pores. We show that wind speed is an important component of soil CO 2 diffusion which must be taken into account when measuring soil CO 2 efflux, even on very fine textured soil like silt-loam soil. Proper measurement can be achieved by maintaining wind speed inside the chamber below 0.4 m s −1 since low wind speed conditions predominate under forest canopies. However, more accurate measurements will be obtained by regulating wind speeds within the chamber at a velocity representative of the wind speed recorded simultaneously at the floor surface. Introduction Even though soil CO 2 efflux is one of the key com- ponents of the ecosystem carbon balance (Raich and Schlesinger, 1992), reliable estimations are relatively scarce. Soil CO 2 efflux results from the combina- tion of biological (i.e. the respiration of roots, soil micro-organisms and soil macro fauna) and physical processes (i.e. the CO 2 diffusion from sources to soil surface) which are both highly variable in space (decimetre) and time (day to season). ∗ FAX No: +33 381994685. E-mail: daniel.epron@pv_pm.univ-fcomte.fr To deal with the difficulties of soil CO 2 efflux measurements, several different methods have been employed, each with associated strengths and weak- nesses. Until now, no single method is recognised as ‘the standard’ methodology. In recent years, non intrusive micrometeorological techniques have been used to measure soil CO 2 efflux spatially integrated over few square meters (Baldocchi et al., 1986, Bal- docchi and Meyers, 1991, Dugas, 1993). To be applied successfully, this technique requires a number of spe- cific conditions and assumptions, such as no source or sink of CO 2 between the soil surface and the sensor level, an homogeneous upwind fetch and atmospheric