Effects of subsoil compaction on hydraulic properties and preferential flow in a Swedish clay soil

• We combined X-ray CT and solute breakthrough curves to study compaction effects.
• Preferential flow decreased due to compaction, in contrast with previous studies.
• We propose a conceptual model to explain effects of compaction on preferential flow.
• The model suggests that preferential flow is strongest at intermediate compaction levels.

Soil compaction by vehicular traffic modifies the pore structure and soil hydraulic properties. These changes potentially influence the occurrence of preferential flow, which so far has been little studied. Our aim was to study the effect of compaction on soil hydraulic and transport properties in subsoil. A randomized block design trial at two sites on a well-structured clay soil in central Sweden was established. Plots with two levels of compaction were created at both sites, in the following referred to as trafficked and control. The trafficked treatment was created by 4 passes track-by-track with a three-axle dumper with a maximum wheel load of 5.8 Mg. After one year, undisturbed soil columns (20 cm height × 20 cm diameter) from both trafficked and control plots at a depth of 30–50 cm were sampled. The columns were analyzed using X-ray CT imaging, together with measures of the degree of preferential transport derived from non-reactive tracer breakthrough curves and measurements of saturated hydraulic conductivity (Ks) and air permeability at the field moisture content (Ka).Although the traffic treatment did not cause any compaction effects at one of the two sites, it did result in significant reductions in saturated hydraulic conductivity, air permeability and number of macropores at the second site. At this site, the traffic also significantly reduced the strength of preferential flow, presumably due to compaction-induced disruption of macropore continuity. In apparent contrast, some previous studies have shown increases in the strength of preferential flow as a result of compaction. We propose a conceptual model to explain these apparently contradictory results, which suggests that preferential flow should be strongest at some intermediate level of compaction.