Porous tubing for use in monitoring soil CO2 concentrations

The composition of the soil atmosphere is an indicator of biological processes, and soil CO₂ gradients have been used to estimate CO₂ efflux from the surface. Soil atmosphere samplers, constructed with gas-permeable materials, have been used to quantify soil CO₂ concentrations. The type of material used can influence the perceived real-time concentrations of CO₂ in the soil. Previous works have not directly compared different types of materials under the same conditions. The objective of this study was to determine the diffusion coefficient (D) and time of 95% equilibrium (t_eq) of CO₂ through several materials, and to evaluate the effect of long-term soil burial (183 days) on diffusion characteristics. Materials tested included silicone, expanded Teflon (ePTFE), and ultra high molecular weight polyethylene (PE) tubing. The D of each material was determined using a closed-loop system consisting of a CO₂-enriched (7800 ppm) chamber, a CO₂ analyzer and an inner tube (experimental tubing) placed inside the chamber. Air was re-circulated through the inner tube, and as CO₂ diffused from the chamber into the tubing, the analyzer recorded the increase in concentration. The silicone tubes had values of D ranging from 8.64 to 5.80×10⁻⁶ cm² s⁻¹ with corresponding t_eq between 3.9 and 9.7 h. Diffusion coefficients of the ePTFE (1.25×10⁻⁴ cm² s⁻¹) and PE (7.70×10⁻⁴ cm² s⁻¹) materials were 2 orders of magnitude greater, with t_eq<6 min. Exposure to the soil environment for 183 days did not visibly deteriorate the materials or significantly affect the D or t_eq values. Use of the ePTFE or PE materials, over the silicone materials, may allow for better characterization of dynamic CO₂ concentrations in the soil based on the greater D and lesser t_eq values of these materials.