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

[摘要] Soil respiration is the second largest flux in the global carbon cycle, yetthe underlying below-ground process, carbon dioxide (CO₂) production,is not well understood because it can not be measured in the field. CO₂production has frequently been calculated from the vertical CO₂diffusive flux divergence, known as "soil-CO₂ profile method". Thisrelatively simple model requires knowledge of soil CO₂ concentrationprofiles and soil diffusive properties. Application of the method for atropical lowland forest soil in Panama gave inconsistent results when usingdiffusion coefficients (D) calculated based on relationships with soilporosity and moisture ("physically modeled" D). Our objective was toinvestigate whether these inconsistencies were related to (1) the appliedinterpolation and solution methods and/or (2) uncertainties in thephysically modeled profile of D. First, we show that the calculated CO₂production strongly depends on the function used to interpolate betweenmeasured CO₂ concentrations. Secondly, using an inverse analysis of thesoil-CO₂ profile method, we deduce which D would be required to explainthe observed CO₂ concentrations, assuming the model perception isvalid. In the top soil, this inversely modeled D closely resembled thephysically modeled D. In the deep soil, however, the inversely modeled Dincreased sharply while the physically modeled D did not. When imposing aconstraint during the fit parameter optimization, a solution could be foundwhere this deviation between the physically and inversely modeled Ddisappeared. A radon (Rn) mass balance model, in which diffusion wascalculated based on the physically modeled or constrained inversely modeledD, simulated observed Rn profiles reasonably well. However, the CO₂concentrations which corresponded to the constrained inversely modeled D weretoo small compared to the measurements. We suggest that, in well-structuredsoils, a missing description of steady state CO₂ exchange fluxes acrosswater-filled pores causes the soil-CO₂ profile method to fail. Thesefluxes are driven by the different diffusivities in inter- vs.intra-aggregate pores which create permanent CO₂ gradients if separatedby a "diffusive water barrier". These results corroborate other studieswhich have shown that the theory to treat gas diffusion as homogeneousprocess, a precondition for use of the soil-CO₂ profile method, isinaccurate for pore networks which exhibit spatial separation betweenCO₂ production and diffusion out of the soil.

[学科分类] 地球化学与岩石