Ratio of Non‐Darcian to Darcian Air Permeability as a Marker of Soil Pore Organization. Issue 4 (8th August 2019)

• Record Type: Journal Article
• Authors: Schjønning, Per; Pulido‐Moncada, Mansonia; Munkholm, Lars J.; Iversen, Bo V.
• Is Part Of: Soil Science Society of America Journal. Volume 83:Issue 4(2019)
• Page Start: 1024
• Page End: 1031
• Publication Date: 2019-08-08
• Subjects: kapp, apparent permeability -- kDarcy, true Darcian permeability -- R, kapp/kDarcy ratio -- vpore, average pore air velocity; Soils -- United States -- Periodicals; Soil science -- Periodicals; Periodicals; 631.4973
• DOI: 10.2136/sssaj2018.11.0452
• Languages: English
• ISSNs: 0361-5995

Abstract

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‐cm 3 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 ( k app ) 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 ( k Darcy ) derived from the relationship between the superficial air velocity and the pressure gradient. For high permeabilities, the ratio R ( k app / k Darcy ) 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.