Linking Particle and Pore Size Distribution Parameters to Soil Gas Transport Properties

Accurate estimation of soil gas diffusivity (D(p)/D(o), the ratio of gas diffusion coefficients in soil and free air) and air permeability (k(a)) from basic texture and pore characteristics will be highly valuable for modeling soil gas transport and emission and their field-scale variations. From the topsoil of two Danish arable fields representing two natural clay gradients, D(p)/D(o) and k(a) were measured at soil water matric potentials between -1 and -100 kPa on undisturbed soil cores. The Rosin-Rammler particle size distribution parameters alpha and beta (characteristic particle size and degree of sorting, respectively) and the Campbell water retention parameter b were used to characterize particle and pore size distributions, respectively. Campbell b yielded a wide interval (4.6-26.2) and was highly correlated with alpha, beta, and volumetric clay content. Both D(p)/D(o) and k(a) followed simple power-law functions (PLFs) of air-filled porosity (epsilon(a)). The PLF tortuosity-connectivity factors (X*) for D(p)/D(o) and k(a) were both highly correlated with all basic soil characteristics, in the order of volumetric clay content = Campbell b > gravimetric clay content > alpha > beta. The PLF water blockage factors (H) for D(p)/D(o) and k(a) were also well (but relatively more weakly) correlated with the basic soil characteristics, again with the best correlations to volumetric clay content and b. As a first attempt at developing a simple D(p)/D(o) model useful at the field scale, we extended the classical Buckingham D(p)/D(o) model (epsilon(2)(a)) by a scaling factor based on volumetric clay content. The scaled Buckingham model provided accurate predictions of D(p)(epsilon(a))/D(o) across both natural clay gradients.