Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/21/2007
Publication Date: 1/24/2008
Citation: De Sutter, T.M., Sauer, T.J., Parkin, T.B., Heitman, J.L. 2008. A subsurface, closed-loop system for soil carbon dioxide and its application to the gradient efflux approach. Soil Science Society of America Journal. 72:126-134. Interpretive Summary: Carbon dioxide is one the the most important greenhouse gases. When organic materials in the soil decompose, carbon dioxide is released. Measuring this release of carbon dioxide is important in determining which practices produce more greenhouse gases. The objective of this study was to test a new method that would allow continuous measurement of carbon dioxide production and release from soil. Porous tubing was buried at different depths in the soil. A pump was used to circulate air through the tubing and a sensor to measure the carbon dioxide concentration. Separate measurements of gas diffusion through the soil were made to allow calculation of how much carbon dioxide was being released. Different ways to calculate the carbon dioxide release were evaluated and compared with another independent measurement. It was determined that carbon dioxide concentrations measured very close to the soil surface were most important and that accurately measuring the rate of gas diffusion through this soil layer was very difficult. This research is important to producers who need accurate, long-term information on the effects of management practices on carbon dioxide release when choosing whether or how to change their production practices to reduce greenhouse gas emissions.
Technical Abstract: Carbon dioxide concentrations in the soil can vary both temporally and spatially. The subsurface gradient approach is commonly used to estimate CO2 efflux but spatial integration has been limited. Methodology was developed to semi-continuously measure subsurface concentrations of CO2 using porous Teflon (ePTFE) tubing. Lengths of ePTFE tubing (7.6 m) were buried at 0.02, 0.1, and 0.18 m below the soil surface in a Harps loam soil in central Iowa, USA 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 (r2 > 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 (Ds) was determined using intact soil cores and values were about 2.5 times less than two popular models used to predict Ds 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 greater than 800 to less than 1 'mol m 2 s 1 on day of 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 Ds can both include sources of uncertainty.