A subsurface, closed-loop system for soil carbon dioxide and its application to the gradient efflux approach

Carbon dioxide concentrations in the soil can vary both temporally and spatially. Methodology was developed to semicontinuously measure subsurface concentrations of CO2 using expanded, porous Teflon (ePTFE) tubing. Lengths of ePTFE tubing (7.6 m) were buried at 0.02, 0.1, and 0.18 in below the soil surface in a Harps loam soil (fine-loamy, mixed, superactive, mesic Typic Calciaquoll) in central Iowa, and also positioned directly on the soil surface (0 m). Soil atmospheric gases that diffused through the walls of the tubing were circulated in a closed-loop design through solid-state CO2 sensors to determine the concentration of CO2 at each depth. Independent measures of CO2 concentrations were also determined by sampling the in-line gas stream of the ePTFE system and from samples extracted from gas wells positioned near the buried tubing. Good agreement (r(2) > 0.95) was observed between the ePTFE system and the independent measures, with the ePTFE having biases of 1.2 and 1.37 times greater than the in-line and gas well samples, respectively. The soil-gas diffusion coefficient of CO2 (D-s) was determined using intact soil cores and values were about 2.5 times less than two popular models used to predict D-s in soil. Estimates of CO2 flux using Fick's Law, six approaches to determine the vertical CO2 concentration gradient, and three methods to determine Ds ranged from >800 to <1 mu mol m(-2) s(-1) on Day of the Year 239.5. Although Fick's Law is commonly used to estimate CO2 flux from soil, the approach used to determine the vertical CO2 concentration gradient and method used to determine D-s can both include sources of uncertainty.