This study quantified the immediate impact of soil deformation caused by agricultural vehicle traffic on the anisotropy of the topsoil pore system and gas transport properties (air permeability, gas diffusivity). A field experiment was conducted with five repeated passes of a two-axle self-propelled agricultural vehicle (wheel load 8 Mg, tyre size: 1050/50 R32, tyre inflation pressure: 100 kPa) on an arable clay soil (crop at the time of the experiment: grass ley) in north-western Switzerland. Undisturbed cylindrical soil cores were collected in non-wheeled areas, at the edge of the wheel rut (i.e., at 0.5 m lateral distance from the centre of the wheel track), and at the centre line of the wheel track. The soil cores (0.1 m diameter, 0.06 m in height) were taken in two directions (vertical and horizontal) in the topsoil (0.1 m depth). Air-filled porosity (εa), air permeability (ka) and relative gas diffusivity (Dp/D0) were measured at three matric potentials (corresponding to pF 1.5, 2.0, and 2.5, respectively). The vehicle-induced deformation resulted in significantly reduced εa, ka, Dp/D0 in the topsoil. Air permeability was highly anisotropic in non-wheeled soil, with higher ka in vertical direction. Compaction mainly affected macropores and hence ka at the wet end (pF 1.5), decreased the vertical ka more than the horizontal ka, and consequently, ka became less anisotropic due to compaction. This effect was stronger under the edge of the wheel rut than in the centre of the wheel rut. The anisotropy of Dp/D0 was little affected by the vehicle-induced soil deformation. Our results show that soil deformation due to vehicle traffic not only decreases the gas transport capacity of soil but also changes the anisotropy of air permeability, with consequences on soil aeration and soil-atmosphere gas exchange.
