Compaction effect on the gas diffusion coefficient in soils

Gas diffusion is important in determining the aeration status of soils for crop production and in providing estimates of transport of volatile chemicals at waste disposal or chemical spill sites. This study investigated the effects of compaction on the gas diffusion coefficient in four soils. The experiment involved equilibrating bulk loose soil to a known matric potential in a pressure chamber, compressing the equilibrated moist soil in metal cores at a given applied stress, and measuring the concentration of N2 diffusing through these cores into a diffusion chamber. The diffusion coefficient was calculated by fitting an analytical solution of the transient diffusion equation to the measured N, concentration in the diffusion chamber as a function of time. The diffusion coefficient of N2 in four soils at four water contents and four applied loads varied exponentially as a function of air-filled porosity and was nearly the same function for all four soils. At an airfilled porosity of -10%, the diffusion coefficient was close to zero, reflecting a discontinuity in the pathways at an air-filled pore space of 10% or lower. A model for predicting the diffusion coefficient of gases in soils based on the diffusion coefficient of individual soil constituents was tested with the data set. For known shape factors for soil solid and soil water, the model overpredicted the diffusion coefficient for all four soils. A reduction factor suggested in the literature to account for blocked air slightly improved predictions. The ratio of the measured to predicted diffusion coefficient vs. degree of air-filled saturation suggests a second-degree correction factor to account for blocked air in soils. T EXCHANGE of gases between the soil and the atmosphere can occur by both convection and diffusion. Diffusion has been recognized as the most important process determining not only the aeration of soil for crop production (Glinski and Stepniewski, 1985; Hillel, 1980) but also the transport of volatile chemicals at waste disposal or chemical spill sites (Baehr, 1987; Thorstenson and Pollock, 1989) and the transport of naturally occuring radioactive gases, such as radon, from mine tailing sites and basements (Nielsen et al., 1984). Because of the difficulty of measuring the gas diffusion coefficient of soil, many studies have been undertaken to develop relationships between the gas diffusion coefficient and some easily measured soil properties like air-filled porosity. A review of these studies has been given by Glinski and Stepniewski (1985). Table 1 summarizes some of the relationships in the literature between the gas diffusion coefficient and easily measurable soil properties. Except for studies by Currie (1983) and Stepniewski X. Xu and J.L. Nieber, Dep. of Agricultural Engineering, and S.C. Gupta, Dep. of Soil Science, Univ. of Minnesota, St. Paul, MN 55108. Contribution from the Minnesota Agric. Exp. Stn. , Scientific Journal Series, Paper no. 19516. Received 16 Oct. 1991. * Corresponding author. Published in Soil Sci. Soc. Am. J. 56:1743-1750 (1992). (1981), there is limited information on the effects of compaction on the gas diffusion coefficient of soils. Currie (1983) proposed that, for a unimodal poresize distribution, the gas diffusion coefficient (D) is related to the fourth power of the air-filled porosity