Soil microbial respiration at different water potentials and temperatures: theory and mathematical modeling.
Grant R.F. | Rochette P.
If ecosystem simulation models are to be used to study changes in C distribution under proposed changes in climate, then they must represent the effects of soil physical conditions upon microbial activity. Hypotheses for the effects of soil water content and temperature (Ts) on microbial oxidation rates were formulated into mathematical algorithms as part of the ecosys modelling program. Access to organic substrates by heterotrophic microbial populations was represented from competitive enzyme kinetics, which were presumed to be sensitive to soil water content. Access to O2 by obligately aerobic or facultatively anaerobic microbial populations was represented from O2 diffusion gradients and active uptake rates controlled by soil water content and Ts. Sensitivity to Ts of substrate hydrolysis and oxidation by heterotrophic microbial populations was modelled from an Arrhenius function. Rates of simulated respiration were tested against rates measured under laboratory and field conditions at different soil water content and Ts. Simulated CO2 fluxes were largest when soil water content 0.6 to 0.7 of total porosity and declined to < 0.2 of their largest values when soil water content declined to 0.2 or rose above 0.9 of total porosity. The sensitivity of simulated CO2 fluxes to soil water content was consistent with that measured during laboratory incubations, except in the range of 0.65 to 0.80 of total porosity, where sensitivity of measured fluxes was greater than that simulated. When soil water content was > 0.8 of total porosity, simulated respiratory quotients rose above 1.0 to values consistent with those recorded elsewhere at high soil water content Model hypotheses allowed simulated CO2 evolution rates to reproduce those reported from wheat residue during a 30-d incubation at Ts from 0 to 20 degrees C and water potentials from -0.033 to -5.0 MPa.
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