An inverse analysis reveals limitations of the soil-CO₂ profile method to calculate CO₂ production and efflux for well-structured soils

• Main Authors: M. D. Corre, E. Veldkamp, E. Zehe, B. Koehler
• Format: Article
• Language: English
• Published: Copernicus Publications 2010-08-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/7/2311/2010/bg-7-2311-2010.pdf

Soil respiration is the second largest flux in the global carbon cycle, yet the underlying below-ground process, carbon dioxide (CO₂) production, is not well understood because it can not be measured in the field. CO₂ production has frequently been calculated from the vertical CO₂ diffusive flux divergence, known as "soil-CO₂ profile method". This relatively simple model requires knowledge of soil CO₂ concentration profiles and soil diffusive properties. Application of the method for a tropical lowland forest soil in Panama gave inconsistent results when using diffusion coefficients (<I>D</I>) calculated based on relationships with soil porosity and moisture ("physically modeled" <I>D</I>). Our objective was to investigate whether these inconsistencies were related to (1) the applied interpolation and solution methods and/or (2) uncertainties in the physically modeled profile of <I>D</I>. First, we show that the calculated CO₂ production strongly depends on the function used to interpolate between measured CO₂ concentrations. Secondly, using an inverse analysis of the soil-CO₂ profile method, we deduce which <I>D</I> would be required to explain the observed CO₂ concentrations, assuming the model perception is valid. In the top soil, this inversely modeled <I>D</I> closely resembled the physically modeled <I>D</I>. In the deep soil, however, the inversely modeled <I>D</I> increased sharply while the physically modeled <I>D</I> did not. When imposing a constraint during the fit parameter optimization, a solution could be found where this deviation between the physically and inversely modeled <I>D</I> disappeared. A radon (Rn) mass balance model, in which diffusion was calculated based on the physically modeled or constrained inversely modeled <I>D</I>, simulated observed Rn profiles reasonably well. However, the CO₂ concentrations which corresponded to the constrained inversely modeled <I>D</I> were too small compared to the measurements. We suggest that, in well-structured soils, a missing description of steady state CO₂ exchange fluxes across water-filled pores causes the soil-CO₂ profile method to fail. These fluxes are driven by the different diffusivities in inter- vs. intra-aggregate pores which create permanent CO₂ gradients if separated by a "diffusive water barrier". These results corroborate other studies which have shown that the theory to treat gas diffusion as homogeneous process, a precondition for use of the soil-CO₂ profile method, is inaccurate for pore networks which exhibit spatial separation between CO₂ production and diffusion out of the soil.