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United States Department of Agriculture

Agricultural Research Service

Research Project: TRACE GAS EXCHANGES IN MIDWEST CROPPING SYSTEMS Title: Porous Tubing for Use in Monitoring Soil Co2 Concentrations

Authors
item Desutter, Thomas
item Sauer, Thomas
item Parkin, Timothy

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 6, 2006
Publication Date: May 16, 2006
Citation: Desutter, T.M., Sauer, T.J., Parkin, T.B. 2006. Porous tubing for use in monitoring soil CO2 concentrations. Soil Biology and Biochemistry. 38:2676-2681.

Interpretive Summary: Carbon dioxide is produced below the soil surface from both roots and microbial activity. Monitoring the changes in carbon dioxide concentrations below the soil surface helps scientists understand the complex chemical and physical changes that occur in soil. Soil carbon dioxide concentrations are commonly monitored with the aid of porous tubing that is buried at discrete depths beneath the soil surface, which are commonly called gas wells. Tubing that is commonly used is made from silicone, Teflon, and polyethylene. Diffusion of carbon dioxide was determined using three silicone, one porous Teflon, and four porous polyethylene-tubing products. Carbon dioxide moved more freely through the porous Teflon and porous polyethylene than through the silicone products. This research is important for researchers who are determining subsurface concentrations of carbon dioxide, as it will affect which kind of tubing that is used for construction of gas wells.

Technical Abstract: Monitoring CO2 concentrations below the soil surface allows one to determine areas of CO2 production and may allow for the estimation of the flux of CO2 from the soil. Use of porous tubing is one method that can aid in determining these concentrations. The objective of this study was to determine the diffusion coefficient (D) and time of 95% equilibrium (teq) of CO2 through three silicone-tubing products, one expanded Teflon (ePTFE)-tubing product, and four ultra high molecular weight polyethylene (PE)-tubing products. The D of each tubing product was evaluated using a closed-loop system where the inner tubing volume was isolated from a constant concentration of CO2. The gas inside the tubing was recirculated through a CO2 analyzer using a pump. The D and the teq of each tube were determined using the first-order rate constant. The silicone tubes had values of D ranging from 8.64 to 5.80 x 10**-6 cm**2 sec**-1 and teq values between 3.9 and 9.7 hr whereas the ePTFE tubing had a D of 1.23 x 10**-4 cm**2 sec**-1 and a teq of 5.9 min. The PE tubing had the highest D (7.70 x 10**-4 cm**2 sec**-1) and lowest teq (5.3 min) compared to the other tubing types. Exposure to the soil environment for 183 d did not deteriorate the tubing or significantly affect the D or teq values. Knowledge of the properties of porous tubing for use as a gas well should be considered when designing experiments to monitor soil CO2 concentrations.

Last Modified: 7/28/2014
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