DYNAMIC OXYGEN AND CARBON-DIOXIDE EXCHANGE BETWEEN SOIL AND ATMOSPHERE .2. MODEL SIMULATIONS

An important general objective for modeling gaseous transport through unsaturated soils is to provide better understanding of gas exchange between the terrestrial ecosystem and the atmosphere. A more specific objective is the evaluation of problems of soil aeration. We present sensitivity analyses of a mathematical model developed for the dynamic exchange of O2 and CO2 between soil and atmosphere. We also present simulations to evaluate changes in concentrations during cycles of infiltration, evaporation, and redistribution during which simultaneous transport of water, heat, O2, and CO2 was involved. Simulation was also used to evaluate possible effects of O2 and CO2 concentrations on the rate of root growth; effects of a compacted layer on the simultaneous transport of water, O2, and CO2; effect of the presence of a crop canopy on concentrations of O2 and CO2 in the soil; and an evaluation of possible nonideal behavior of CO2 molecules in the gas phase of the soil. Results emphasize the important effects of rainfall, evaporation, and root respiration on O2 and CO2 concentrations in the soil. Simulations show that root growth below a compacted soil layer may be restricted by low O2 concentrations resulting from limited diffusion through the compacted layer. The presence of a crop canopy has a decisive effect on concentrations of O2 and CO2 at the soil surface, thereby affecting the concentrations of those gases in the root zone.