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| Titel |
An inverse analysis reveals limitations of the soil-CO2 profile method to calculate CO2 production and efflux for well-structured soils |
| VerfasserIn |
B. Koehler, E. Zehe, M. D. Corre, E. Veldkamp |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 7, no. 8 ; Nr. 7, no. 8 (2010-08-04), S.2311-2325 |
| Datensatznummer |
250004923
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| Publikation (Nr.) |
 copernicus.org/bg-7-2311-2010.pdf |
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| Zusammenfassung |
| 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 (D) calculated based on relationships with soil
porosity and moisture ("physically modeled" D). 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 D. 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 D would be required to explain
the observed CO2 concentrations, assuming the model perception is
valid. In the top soil, this inversely modeled D closely resembled the
physically modeled D. In the deep soil, however, the inversely modeled D
increased sharply while the physically modeled D 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 D
disappeared. A radon (Rn) mass balance model, in which diffusion was
calculated based on the physically modeled or constrained inversely modeled
D, simulated observed Rn profiles reasonably well. However, the CO2
concentrations which corresponded to the constrained inversely modeled D 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. |
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