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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D08S10, doi:10.1029/2005JD006435, 2006

On maintaining pressure equilibrium between a soil CO₂ flux chamber and the ambient air

Liukang Xu

LI-COR Biosciences, Lincoln, Nebraska, USA

Michael D. Furtaw

LI-COR Biosciences, Lincoln, Nebraska, USA

Rodney A. Madsen

LI-COR Biosciences, Lincoln, Nebraska, USA

Richard L. Garcia

LI-COR Biosciences, Lincoln, Nebraska, USA

Daniel J. Anderson

LI-COR Biosciences, Lincoln, Nebraska, USA

Dayle K. McDermitt

LI-COR Biosciences, Lincoln, Nebraska, USA

Abstract

Pressure equilibrium between inside a soil CO₂ flux chamber and the surrounding air outside the chamber must be maintained during a measurement if measured soil CO₂ flux (F _CO2) is to accurately represent the rate occurring naturally outside the chamber. In previous studies a simple vent tube connecting to the chamber has often been used to maintain pressure equilibrium. This approach, however, can be effective only under calm conditions. Under windy conditions, negative pressure excursions will occur inside the chamber that are artifacts resulting from wind passing over the vent tube's external open end, a phenomenon known as the Venturi effect. This causes anomalous mass flow of CO₂-rich air from the soil into the chamber, leading to a significant overestimation of F _CO2. In this present study, we found that negative chamber pressure excursions due to the Venturi effect cannot be observed unless the differential pressure measurement is made with the chamber resting on an impermeable base. Making pressure measurements with a chamber resting on porous soil can lead to the erroneous conclusion that an anomalous mass flow is not a problem precisely when it is causing serious artifacts. We also present a new vent design for a soil CO₂ flux chamber capable of maintaining pressure equilibrium between inside the chamber and the ambient air outside the chamber under both calm and windy conditions. Differential pressure measurements from field experiments show that the pressures inside our newly designed vented chamber equal those outside the chamber when wind speed at a height of 0.5 m is up to 7 m s⁻¹, thus virtually eliminating artifacts due to the Venturi effect. Our field data show that the problem of overestimation in measured F _CO2 by a chamber with older vent designs under windy conditions can be avoided with our newly designed vented chamber.

Received 29 June 2005; accepted 28 October 2005; published 11 March 2006.

Keywords: soil CO₂ efflux measurement; chamber-based method; dynamic pressure; static pressure; vent design; chamber pressure equilibrium.

Index Terms: 0428 Biogeosciences: Carbon cycling (4806); 0452 Biogeosciences: Instruments and techniques; 0490 Biogeosciences: Trace gases.

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Citation: Xu, L., M. D. Furtaw, R. A. Madsen, R. L. Garcia, D. J. Anderson, and D. K. McDermitt (2006), On maintaining pressure equilibrium between a soil CO₂ flux chamber and the ambient air, J. Geophys. Res., 111, D08S10, doi:10.1029/2005JD006435.

Copyright 2006 by the American Geophysical Union.