A Calibration System for Soil Carbon Dioxide‐Efflux Measurement Chambers

Comparisons have revealed large discrepancies among the many methods for measuring soil CO 2 efflux indicating the need for an absolute calibration of methods. This study presents a calibration system, constructed to imitate an area of soil, and its application to two different chamber systems for the measurement of soil CO 2 efflux: one open and one closed dynamic. Air rich in CO 2 was allowed to diffuse through a layer of sand on top of a box of known volume. By measuring the decrease in CO 2 concentration inside the box, the exact CO 2 efflux could be calculated. The CO 2 –efflux rates measured by the chambers could then be compared with the efflux rates calculated from the box. The error of the closed‐chamber system ranged from an underestimate of 19% to an overestimate of 21%. The errors were most likely caused by a combination of underestimated chamber volume, causing an underestimation of CO 2 efflux, and turbulence within the chamber, which increased the flux by disturbing the boundary layer above the surface. The open‐chamber system always overestimated the CO 2 efflux. Disturbing the boundary layer alone was believed to cause a 17% increase in efflux. Increasing negative pressure difference caused a mass flow of CO 2 –rich air into the chamber. At a pressure difference of −0.15 Pa, the error was 11 to 40%, depending on air‐filled soil volume. Accordingly, soil‐water content, a parameter to which soil CO 2 efflux is often related, was found to substantially affect the measurements made by both tested systems. These results point to the need of calibrating systems used for measuring soil CO 2 efflux is measured against a known flux, to elucidate the limits and applicability of each system.