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Research Project: Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Improving the accuracy of the gradient method for determining soil carbon dioxide efflux

Author
item Sanchez-cañete, E.p. - University Of Arizona
item Scott, Russell - Russ
item Van Haren, J. - University Of Arizona
item Barron-gafford, G.a. - University Of Arizona

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/17/2016
Publication Date: 1/5/2017
Citation: Sanchez-Cañete, E., Scott, R.L., Van Haren, J., Barron-Gafford, G. 2017. Improving the accuracy of the gradient method for determining soil carbon dioxide efflux. Journal of Geophysical Research-Biogeosciences. 122:50-64. https://doi.org/10.1002/2016JG003530.
DOI: https://doi.org/10.1002/2016JG003530

Interpretive Summary: Soils emit significant amounts of carbon dioxide (CO2) gas to the atmosphere, but continuous measurements of this important greenhouse gas are only sparsely available. For the past decade, the gradient method has been proposed as an inexpensive and relatively simple technique to measure soil CO2 emissions, but wide adoption of the method has been hindered due to uncertainties in determining the diffusion coefficient, a measure of the ease at which CO2 moves through the soil. In this paper, we describe a method to determine the diffusion coefficient in the field, and we find that this approach is shown to greatly improve the accuracy of gradient method. Our approach should help to improve the availability and accuracy of this important gas emission.

Technical Abstract: Continuous soil CO2 efflux (Fsoil) estimates can be obtained by the gradient method (GM), but the utility of the method is hindered by uncertainties in the application of published models for the diffusion coefficient (Ds). We compared two in-situ methods for determining Ds, one based calibrating the GM with chamber measurements and another using a conservative gas tracer with 14 published diffusion models. Published models using the GM underestimated cumulative Fsoil by 55% to 361%. Calibrating the GM most closely approximated cumulative chamber Fsoil (0.6% error). Surprisingly, the tracer diffusivity model combined with the GM underestimated Fsoil by 32%. Differences between in-situ models could stem from the Chamber model implicitly accounting for production and consumption of soil CO2, while the tracer approach does not. We recommend using the GM only after calibration with chamber measurements to generate reliable long-term ecosystem Fsoil measurements.