Soil Microbial Respiration at Different Water Potentials and Temperatures: Theory and Mathematical Modeling

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 (θ) 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 θ. Access to O₂ by obligately aerobic or facultatively anaerobic microbial populations was represented from O₂ diffusion gradients and active uptake rates controlled by θ 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 θ and Tₛ. Simulated CO₂ fluxes were largest when θ = 0.6 to 0.7 of total porosity and declined to <0.2 of their largest values when θ declined to 0.2 or rose above 0.9 of total porosity. The sensitivity of simulated CO₂ fluxes to θ 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 θ was >0.8 of total porosity, simulated respiratory quotients rose above 1.0 to values consistent with those recorded elsewhere at high θ. 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°C and ψₛ from −0.033 to −5.0 MPa. These hypotheses also allowed simulated changes in CO₂ evolution rates attributed to changes in Tₛ and θ to reproduce those measured in the field during 60 d under barley.