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DIVISION S-7—FOREST & RANGE SOILS

A Calibration System for Soil Carbon Dioxide-Efflux Measurement Chambers

Description and Application

^a Department for Production Ecology, Faculty of Forestry, P.O. Box 7042, Swedish University of Agricultural Sciences, 750 07, Uppsala, Sweden
^b Department of Physical Geography and Ecosystems Analysis, P.O. Box 118, Lund University, 221 00, Lund, Sweden

* Corresponding author (Anders.Lindroth{at}nateko.lu.se)

Comparisons have revealed large discrepancies among the many methods for measuring soil CO₂ 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 CO₂ efflux: one open and one closed dynamic. Air rich in CO₂ was allowed to diffuse through a layer of sand on top of a box of known volume. By measuring the decrease in CO₂ concentration inside the box, the exact CO₂ efflux could be calculated. The CO₂–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 CO₂ 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 CO₂ efflux. Disturbing the boundary layer alone was believed to cause a 17% increase in efflux. Increasing negative pressure difference caused a mass flow of CO₂–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 CO₂ 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 CO₂ efflux is measured against a known flux, to elucidate the limits and applicability of each system.

Abbreviations: DP34, dry sand of 34% porosity • DP60, dry sand of 60% porosity • IRGA, infrared-gas analyzer • F_b, CO₂ flux from the calibration box with a chamber on top • F_br, CO₂ flux from the calibration box with no chamber on top—reference flux • F_b-Co, CO₂ flux from the calibration box outside the chamber • F_Cc, CO₂ efflux measured by a closed-dynamic chamber system • F_Co, CO₂ efflux measured by an open chamber system • LOI, loss on ignition • WP34, wet sand of 34% porosity

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