Ham, J.M., C.E. Owensby, and P.I. Coyne. 1993. Technique for measuring air flow and CO₂ flux in large open-top chambers. J. Environ. Qual. 22:759-766 PDF

Open-top chambers are an economical means of testing ecosystem-level responses to elevated CO₂. They provide the only feasible method of measuring canopy level trace gas fluxes on a continual basis. The effects of the chamber on micrometeorology are a serious drawback to their use (Owensby et al. 1993: Ham et al. 1993). While chamber effects are unavoidable, knowing their impact allows for meaningful interpretation of the results. Our experience with open-top chambers indicates that certain precautions are necessary for their effective use, the most important of which are the use of a barrier in the soil and matching chamber pressure with atmospheric pressure.

Soil Barrier

Chamber design is particularly important when measuring the impact of elevated CO₂ on water relations. A watertight barrier must be placed beneath the chamber to prevent water exchange with the surrounding soil. Any difference in soil water status induced by different atmospheric CO₂ concentrations may be severely altered due to lateral movement of soil water in or out of the soil volume that is associated with the chamber (Quadri et al. 1994; Clothier et al. 1996). The chamber environment reduces water use and creates a lateral gradient in ambient chambers as well. Thus, ozone and other trace gas studies should also use a barrier. The smaller the chamber area, the greater is the impact of lateral soil water movement in the soil. The magnitude of the soil water movement is dependent on the difference in soil water status between the chamber and the surrounding area and soil physical properties. Barriers should be as deep as the effective rooting zone of the major dominants in the plant community. Failure to incorporate a soil barrier partially negates the water savings induced by elevated CO₂, because water can move to the drier soil adjacent to the chamber. In ecosystems in which water is a primary limiting resource, the true impact of elevated CO₂ on ecosystem processes is greatly altered. For the chambers we used, the barrier was placed to a 1-m depth using a trenching machine and there was no disturbance to the chamber soil. The soil barrier was sealed to the chamber.

Pressure Adjustment

In order to accurately measure gas fluxes in open-top chambers, the atmospheric pressure inside the chamber must equal that outside. Relatively small increases in pressure inside the chamber can result in substantial movement of chamber air into the soil (Kanemasu et al. 1974; Nakayama and Kimball 1988). Using a two-dimensional advection diffusion model, we simulated the impact of 0, 1, and 2 Pa pressure inside an open-top chamber on CO₂ from the soil surface and the volume flow of air into the soil (Table 1). The results indicate that soil-surface CO₂ flux was substantially reduced and that a large amount of air was forced into the soil. The average CO₂ flux from the soil is several times greater than that which is fixed by photosynthesis, and the pressure developed by the flow of air into the chamber greatly reduces the soil CO₂ flux. When chamber CO₂ flux under elevated CO₂ is measured, that air movement into the soil and the subsequent reduced soil CO₂ flux is interpreted as a sequestration by the plant canopy. In the absence of a soil barrier, there is a flow of air through the chamber soil to the outside soil and to the soil surface, where it is lost to the atmosphere. Indeed, the soil CO₂ flux adjacent to the chamber would be much greater than normal. Pressure inside the chamber can be adjusted using a variable speed fan (0.6 m dia.) in the top chamber orifice and a differential pressure transducer to measure pressure differences. During most of the day the shading from the fan falls outside the plot area, but does shade a small area during midday. When we used fans in the top of the OTCs to equalize pressure, we found no statistical difference in soil CO₂ flux between the ambient OTCs and control plots (Ham et al., 1995). We measured flux on a weekly basis over the entire growing season using a soil-surface chamber.

Table 1. Simulated values of CO2 flux from the soil surface and the volume flow of air into the soil within an open-top chamber. Pressureinside the chamber was simulated at 0, 1, and 2 Pa above atmospheric pressure. The CO2 concentrations at the upper boundary (aerial CO2 was held constant at 14.5 umol m-3 (350 ppm) both inside and outside the chamber.
Pressure Differential P chamber-P atmosphere (Pa)	Soil Surface CO2 Flux (umol m-2 s-1)	Volume Air Flow into the Soil within the Chamber (m3 air m-2 area day -1)
0	4.2	0
1	3.6	1.03
2	3.2	2.81

Literature Cited

Bremer, D.J., J.M. Ham, and C.E.Owensby. 1996. Effect of elevated atmospheric carbon dioxide and open-top chambers on transpiration in a tallgrass prairie. J. Environ. Qual. 25:691-701

Clothier, B.E., S.R. Green, and H. Katou. 1996. Multidimensional infiltration: points, furrows, basins, well, and disks. Soil Sci. Soc. Am. J. 59:286-292.

Ham, J.M., C.E. Owensby, and P.I. Coyne. 1993. Technique for measuring air flow and CO₂ flux in large open-top chambers. J. Environ. Qual. 22:759-766

Ham, J.M., Owensby, C.E., Coyne, P.I., and D.J. Bremer. 1995. Fluxes of CO₂ and water vapor from a prairie ecosystem exposed to ambient and elevated atmospheric CO₂. Agric. Forest Meteorol. 77:73-93.

Kanemasu, E.T., W. L. Powers, and J.W. Sij. 1974. Field chamber measurements of CO₂ flux from soil surfaces. Soil Sci. 118(4):233-237.

Nakayama, F.S, and B.A. Kimball. 1988. Soil carbon dioxide distribution and flux within the open-top chamber. Agron. J. 80:394-398.

Owensby, C.E., P.I. Coyne, J.M. Ham, L.M. Auen, and A.K. Knapp. 1993. Biomass production in a tallgrass prairie ecosystem exposed to ambient and elevated levels of CO₂. Ecological Appl. 3:644-653.

Owensby, C.E., J.M. Ham, A.K. Knapp, C.W. Rice, P.I. Coyne, and L.M. Auen. 1996. Ecosystem-Level responses of Tallgrass Prairie to elevated CO₂. In Carbon Dioxide and Terrestrial Ecosystems. Koch, G., and H. Mooney, eds. Physiological Ecology Series. pp.175-193. Academic Press. New York.

Quadri, M.B., B.E. Clothier, R. Angulo-Jaramillo, M. Vauclin, and S.R. Green. 1994. Axisymmetric transport of water and solute underneath a disk permeameter: experiments and numerical model. Soil Sci. Soc. Am. J. 58:696-703.

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