Tortuosity phenomena of pore space influence the transport of water, solutes, and gases in soil. This study presents three analyses linking tortuosity and transport in unsaturated soil. The first is a diffusion‐based analysis of tortuosity in the soil water and soil air phases, related to soil surface area (SA) and pore‐size distribution (PSD) (characterized by Campbell b and content of pores >30 μm). The analysis is based on recent models to predict the diffusion coefficients, Dp, of (i) a solute in soil, (ii) a gas in repacked soil, and (iii) a gas in undisturbed soil, each as a function of fluid‐phase (soil water or soil air) content, α. For use in the analysis, the relation between SA and the threshold water content where solute diffusion ceases due to disconnected water films was measured for eight soils (5–46% clay). The tortuosity analysis supported by measured Dp(α) data shows that SA governs and has a larger impact on liquid‐phase tortuosity than PSD has on gaseous‐phase tortuosity. At the same value of α, the tortuosity is typically larger in the soil water than in the soil air phase, and the difference becomes more pronounced with increasing SA and at low α. In the second analysis air permeability, ka, and gas diffusivity, DP,g, are linked in the Millington and Quirk fluid flow model to describe soil structure‐forming potential and to establish a model platform to describe ka as a function of DP,g and α. Measurements on repacked, nonaggregated soil support the ka(DP,g;α) model platform, while measurements on repacked, aggregated soils and on undisturbed soils show that ka is greatly affected by soil aggregation and structure and DP,g is not. In the third analysis, a constitutive parameter model is applied to gas and solute diffusivities and air and water permeabilities in six soils along a soil texture gradient. This illustrates the different behavior of the four transport parameters with PSD and α. The liquid‐phase transport parameters show a steeper decrease with α compared with the gaseous‐phase parameters, in part due to the higher tortuosity in the liquid phase. Also, ka in undisturbed soil exhibited a less steep decrease with α compared with DP,g, probably due to preferential air flow in larger pores during convective transport. Any attempt to develop a unifying and PSD‐dependent model for transport parameters in the soil liquid and gaseous phases will require careful distinction between repacked and undisturbed soils.

Investigations of gas transport and fate processes in packed soil systems require knowledge of the gas diffusion coefficient, DP, as a function of air‐filled porosity, ϵ. On the basis of the literature, data from six studies over the porosity range of 0.1 to nearly 1.0, it is reconfirmed that the Marshall (1959) model better predicts DP(ϵ) in completely dry, repacked porous media than do the Penman (1940) and Millington (1959) models. The smaller DP value in wet soil, as compared with dry soil at the same air‐filled porosity, is accounted for by introducing a water‐induced linear reduction (WLR) term, equal to the ratio of air‐filled porosity to total porosity, in the DP(ϵ) model. By adding the WLR term in each of the three DP(ϵ) models for dry porous media, the so‐called WLR(Marshall), WLR(Penman), and WLR(Millington) DP(ϵ) models for wet soil are developed. To test the three WLR models, DP was measured at different soil‐water contents in six differently textured (6–38% clay) repacked soils. The WLR (Marshall) model accurately and best described DP(ϵ) for all six soils and additional soils from the literature. All three WLR models performed better than previous DP(ϵ) models. This study implies that the smaller DP in a wet soil, which is due to water‐induced changes in air‐filled pore shape and pore connectivity, can be described by a simple, linear function of relative air‐filled porosity. The WLR(Marshall) model represents a conceptual and accurate model to predict DP(ϵ) in sieved, repacked soil.

The gas diffusion coefficient in soil (DP), 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 DP as a function of air‐filled porosity (ϵ) 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 (r2 = 0.97) for a simple, nonlinear expression describing DP at −100 cm H2O of soil water potential (DP,100) as a function of the corresponding air‐filled porosity (ϵ100), equal to the volume of soil pores with an equivalent pore diameter >30 μm. A new DP(ϵ) model was developed by combining the DP,100(ϵ100) expression with the Burdine relative hydraulic conductivity model, the latter modified to predict relative gas diffusivity in unsaturated soil. The DP,100 and Burdine terms in the DP(ϵ) 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 DP(ϵ) 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 DP(ϵ) 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 DP(ϵ) 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.

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