Microbial production of CO₂ in unsaturated geologic media in a mesoscale model

The production of microbially derived CO 2 was investigated in the 3.6 m thick unsaturated geologic media of a mesoscale model system. The mesoscale model, which was 2.4 m diameter × 4.6 m high, was filled with a medium grained lacustrine sand, capped with an A p (0.11 m) and B (0.20 m) soil profile, and maintained under steady state moisture conditions for 343 days. A near steady state soil gas CO 2 concentration profile (>100 times greater than atmospheric) developed after 130 days implying that geochemical stability was rapidly attained. The presence of metabolically active microorganisms, consumption of O 2 and dissolved organic carbon, and carbon 13 data indicated that the CO 2 was microbially produced. At steady state, the calculated and modeled CO 2 flux to the atmosphere (about 0.9 mol m −2 day −1 ) were greater than the measured CO 2 flux (0.1 mol m −2 day −1 ) and greater than the calculated CO 2 flux to the water table (0.02 mol m −2 day −1 ). The difference between the calculated and measured fluxes of CO 2 to the atmosphere was attributed to spatial variability in the gas diffusion coefficient and reflected the difficulty in estimating CO 2 production, even under steady state conditions. Laboratory estimates of microbial CO 2 fluxes (1.8 and 2.9×10 3 mol m −2 day −1 ) were much greater than those determined from measured and calculated fluxes. Measured microbial respiration rates suggested that between 50 and 70% of the respired CO 2 was produced in the unsaturated geologic medium beneath the A p and B, and that there was sufficient CO 2 production potential by bacteria to account for the respired CO 2 determined from the flux calculations. These observations indicated that long‐term experimentation in mesoscale models can overcome some of the difficulties encountered in field studies and provide insight into biogeochemical processes in natural systems.