LAUSR

Mechanical stresses from agricultural machinery affect subsoil layers, influencing pore systems and hence soil processes. The low resilience of the inflicted plastic deformation necessitates a better understanding of the impacts on soil functions and the risk of compromised soil ecosystem services. Soil samples were collected at 0.3-, 0.5-, 0.7-, and 0.9-m depths in a sandy loam subjected to repeated high wheel loads during 4 yr of slurry application at a water content close to field capacity. The 100-cm³ soil samples were drained successively to matric potentials of –30 and –100 hPa, in which air permeability was measured via the Forchheimer approach, including estimation of apparent permeability (kₐₚₚ) at four pneumatic pressure gradients. For all soil depths, the apparent permeability at 5 hPa pneumatic pressure for both control and compacted soil was significantly lower than the true Darcian permeability (kDₐᵣcy) derived from the relationship between the superficial air velocity and the pressure gradient. For high permeabilities, the ratio R (kₐₚₚ/kDₐᵣcy) was generally lower than 0.3. This ratio was lower in compacted soil than in the control soil, significantly so for the 0.3-m depth. For this depth, the decrease in R with increases in the average pore air velocity was more pronounced and a regression model explained more of the variation in data for compacted than for control soil. We consider that severe soil compaction may reduce the complexity of the subsoil pore system, closing a considerable part of the marginal pores branching from vertical (arterial) biopores.