Ratio of Non‐Darcian to Darcian Air Permeability as a Marker of Soil Pore Organization

Abstract

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. Core Ideas Darcian air permeability is underestimated if measured at large pressure gradients. The bias is stronger for compacted than for noncompacted subsoil. The bias in air permeability can be used to evaluate the soil pore system. Compaction is likely to increase the risk of preferential water flow.

Soil pore system evaluated from gas measurements and CT images: A conceptual study using artificial, natural and 3D‐printed soil cores

“…An increase of the inertial forces will reduce the air flow velocity and consequently the air permeability. For a given increase in pressure gradient, the level of inertial forces will be related to the morphology of the active air‐filled pore space (Schjønning et al, 2013; Schjønning, Pulido‐Moncada, Munkholm, & Iversen, 2019). Surface roughness, but also the amount of convolution of the air‐filled pore space may induce larger differences in acceleration during the flow.…”

A Note on the Forchheimer Approach for Estimating Soil Air Permeability

“…3a). We have previously observed a gradual decrease in R with an increase in soil air velocity, v pore-5hPa (Schjønning and Koppelgaard, 2017;Schjønning et al, 2013bSchjønning et al, , 2019. The reduction in the apparent permeability, k a-5hPa , relative to the Darcian permeability, k a-Darcy , with increasing velocity of the flowing air has to do with inertial forces in some way.…”

“…Our results revealed that serious errors may occur in measurements of soil air permeability even at a 1-hPa pressure difference. Furthermore, the bias may seriously distort the evaluation of management effects on soil pore systems (Schjønning et al, 2019). However, a caveat may attach to the use of the Forchheimer approach.…”

“…Convective air flow through drained soil samples reveals important soil pore characteristics (e.g., Ball, 1981b; Blackwell et al, 1990; Groenevelt et al, 1984; Schjønning et al, 2013b, 2019). Air permeability in soil science is typically estimated from measurement of air flow at a specific pneumatic pressure gradient, as suggested in early studies (e.g., Tanner and Wengel, 1957).…”

Microplastic fibers affect dynamics and intensity of CO2and N2O fluxes from soil differently

“…However, these findings do not provide information exactly which parameters of soil structure caused the changed CO 2 and N 2 O fluxes. This requires further investigations in which parameters such as relative gas diffusivity, air permeability, air connectivity, air distance, air tortuosity, which allow a clear quantification of the gas movement in the soil, are systematically determined 39,60,61 .…”