Gas diffusion, fluid flow and derived pore continuity indices in relation to vehicle traffic and tillage

Abstract

Relative gas diffusivity, air permeability and hydraulic conductivity were measured in undisturbed soil cores from tillage and traffic experiments. Continuity indices were taken as the quotient of relative diffusivity and air-filled porosity, and of air permeability and air-filled porosity (and the square of air-filled porosity). These were applied to individual measurements or to treatment means. More general continuity indices were derived from the changes in flow or diffusion with porosity, where the variations in porosity were due to both field variability and applied changes of water potential. These indices were the exponent in the relationship between relative diffusivity and air-filled porosity and the slope of log–log plots of air permeability and air-filled porosity or hydraulic conductivity and degree of saturation. Some physical significance was attached to the exponents by comparison with models of soil porosity. Positive intercepts of the relative diffusivity or air permeability plots on the air-filled porosity axes were taken as porosities blocked to gas movement. Continuity indices and flow measurements showed differences between tillage and traffic treatments which did not necessarily reflect differences in bulk density. Intrinsic permeability was better estimated from air permeability than from unsaturated hydraulic conductivity.