A model of the production and transport of CO2 in soil: predicting soil CO2 concentrations and CO2 efflux from a forest floor

A process-based model of the transport of heat and water in the soil and surface energy balance components is extended to include production of CO2 from heterotrophic (microbial decomposition) and autotrophic (root) respiration, and transport of the resulting CO2 in both the gaseous and liquid phases. The production of CO2 is determined by the amount and type as well as distribution in the soil profile of organic matter and roots, and their respective first-order rate constants. Influences of soil water and temperature are considered through their effects on respiration, CO2 diffusivities and chemical equilibria among various C species in the soil–solution–air continuum. Also included is CO2 uptake by plant roots associated with root water uptake. Model calculations, using independently determined parameters, are compared with measurements of soil CO2 concentrations and chamber-measured forest floor fluxes in a second-growth 54-year-old Douglas-fir forest located on the east coast of Vancouver Island, Canada. Measured and modeled forest floor CO2 effluxes and soil CO2 concentration profiles, as well as soil water contents and temperatures showed good agreement. The efflux was most sensitive to soil temperature, and the influence of soil water content was relatively small. In this rapidly draining soil, the CO2 efflux, at time scales as small as half-hour, was very well approximated by the total production of CO2 in the soil profile, even during and after rainfall, which significantly increases soil water content. This is because CO2 diffusion in this forest soil is relatively rapid compared to changes in the rates of CO2 production. In this podzolic soil ecosystem with low soil pH, liquid-phase diffusion appears to play a minor role in CO2 transport. High soil CO2 concentrations, up to 10,000 μmol mol−1 in summer and 6000 μmol mol−1 in winter, and a positive downward gradient at the 1 m depth indicate some CO2 sources associated with very low CO2 diffusivity at deeper depths.