Soil carbon dioxide fluxes with time and depth in a bare field

Authors: XIAO, XINHUA - North Carolina State University; Sauer, Thomas; HEITMAN, JOSHUA - North Carolina State University; HORTON, ROBERT - Iowa State University

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/4/2015
Publication Date: 7/24/2015
Citation: Xiao, X., Sauer, T.J., Heitman, J.L., Horton, R. 2015. Soil carbon dioxide fluxes with time and depth in a bare field. Soil Science Society of America Journal. 79:1073-1083. doi: 10.2136/sssaj2015.02.0079.

Interpretive Summary: The decomposition of organic matter in the soil is of interest for several reasons. Crop residues, manures, composts, or any other organic material added to the soil will decompose with the release of carbon dioxide but with the potential for some of the organic residues to be incorporated into the soil for years or even centuries. The release of carbon dioxide, a greenhouse gas, affects atmospheric carbon dioxide concentrations. Agricultural practices are often evaluated for how much carbon dioxide they produce. This release of carbon dioxide is often measured by putting a chamber on the soil surface and measuring the change in carbon dioxide concentration with time. Another method is to measure the carbon dioxide concentration in the pore space at different depths in the soil, estimate or measure how fast air moves through the soil layers, and then calculate how fast the carbon dioxide can diffuse out of the soil. This is called the gradient method. These methods were compared for a bare soil in Iowa. The results indicate that the two methods produce nearly the same results so the choice of which method to choose depends on the available resources and goals of the research. The automated chamber system is more expensive, may introduce some changes in soil surface properties, but is more direct. The gradient method is less expensive, does not affect soil surface properties, but requires several other measurements in addition to soil carbon dioxide concentration and some additional data processing. This research is of interest to scientists and land managers interested in options for continuous measurement of carbon dioxide production in soils.

Technical Abstract: Soil carbon dioxide (CO2) efflux is an important component of the terrestrial carbon cycle. The amount of CO2 emitted from soil to the atmosphere has significant effects on the soil-atmosphere system. The objectives of this study are 1) to determine bare soil CO2 fluxes continuously with time and depth with a concentration gradient method that uses measured soil CO2 concentrations and estimated gas diffusion coefficients, 2) to estimate CO2 production with time and depth in a bare soil, 3) to compare surface CO2 effluxes determined by the gradient method and by a closed-chamber method. Soil CO2 concentrations were measured by solid-state sensors. Observed CO2 concentrations were used with a gradient method to calculate soil CO2 fluxes with time and depth. Surface CO2 fluxes were also measured hourly with long-term dynamic chambers. Results showed that soil CO2 concentrations increased with soil depth, while soil CO2 fluxes decreased with soil depth. For a 12-day period, 8% of the cumulative soil CO2 was produced below a depth of 175 mm, 2% was produced in the 100-175 mm soil layer, and 90% was produced in the 0-100 mm soil layer. The CO2 concentration gradient effluxes and the closed-chamber CO2 effluxes agreed well with the gradient daily mean CO2 fluxes within the range of the closed-chamber measurements on 10 out 12 days. Thus, there is evidence that the concentration gradient method was able to accurately measure bare field soil CO2 fluxes and soil production rates with time and depth.