Predicting the gas diffusion coefficient in undisturbed soil from soil water characteristics

The gas diffusion coefficient in soil (D-P), and its dependency on soil physical characteristics, governs the diffusive transport of oxygen, greenhouse gases, fumigants, and volatile organic pollutants in agricultural, forest, and urban soils. Accurate models for predicting D-P as a function of air-filled porosity (epsilon) in natural, undisturbed soil are needed for realistic gas transport and fate simulations. Using data from 126 undisturbed soil layers, we obtained a high correlation (r(2) = 0.97) for a simple, nonlinear expression describing D-P at -100 cm H2O of soil water potential (D-P,D-100) as a function of the corresponding air-filled porosity (epsilon(100)), equal to the volume of soil pores with an equivalent pore diameter >30 mu m, A new D-P(epsilon) model was developed by combining the D-P,D-100(epsilon(100)) expression with the Burdine relative hydraulic conductivity model, the latter modified to predict relative gas diffusivity in unsaturated soil, The D-P,D-100 and Burdine terms in the D-P(epsilon) model are both related to the soil water characteristic (SWC) curve and, thus, the actual pore-size distribution within the water content range considered. The D-P(epsilon) model requires knowledge of the soil's air-filled and total porosities and a minimum of two points on the SWC curve, including a measurement at -100 cm H2O. When tested against independent gas diffusivity data for 21 differently textured and undisturbed soils, the SWC-dependent D-P(epsilon) model accurately predicted measured data and gave a reduction in root mean square error of prediction between 58 and 83% compared to the classical, soil type-independent Penman and Millington-Quirk models. To further test the new D-P(epsilon) model, gas diffusivity and SWC measurements on undisturbed soil cores from three 0.4-m soil horizons (sandy clay loam, sandy loam, and loamy sand) within the 4 to 7 m depth below an industrially polluted soil site were carried out. For these deep subsurface soils the SWC-dependent model best predicted the measured gas diffusivities.