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 (theta) and temperature (T-s) 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 theta. Access to O-2 by obligately aerobic or facultatively anaerobic microbial populations was represented from O-2 diffusion gradients and active uptake rates controlled by theta and T-s. Sensitivity to T-s 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 held conditions at different theta and T-s. Simulated CO2 fluxes were largest when theta = 0.6 to 0.7 of total porosity and declined to <0.2 of their largest values when theta declined to 0.2 or rose above 0.9 of total porosity. The sensitivity of simulated CO2 fluxes to theta 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 theta was >0.8 of total porosity, simulated respiratory quotients rose above 1.0 to values consistent with those recorded elsewhere at high theta. Model hypotheses allowed simulated CO2 evolution rates to reproduce those reported from wheat residue during a 30-d incubation at T-s from 0 to 20 degrees C and psi(s), from -0.033 to -5.0 MPa. These hypotheses also allowed simulated changes in CO2 evolution rates attributed to changes in T-g and theta to reproduce those measured in the field during 60 d under barley.