Persistence of subsoil compaction and potential for natural recovery in a sandy loam soil

<p>Mechanization operations in agriculture have, for the last two decades, involved consistently higher wheel loads with an increased risk of soil compaction, particularly in the subsoil. Subsoil compaction is more persistent compared to the topsoil due to limited mitigation options. There is, however, a potential for the natural recovery of compacted subsoils through processes such as freeze-thaw and wet-dry cycles, and biological activity. The objectives of the present study were to (i) quantify the persistent effects of subsoil compaction on subsoil pore characteristics and water flow, based on visual and quantitative methods, and (b) investigate the potential for natural recovery nine years after the compaction event.</p><p>Soil compaction was achieved by tractor-trailer combinations for slurry application with a maximum wheel load of 8 Mg on a loamy Luvisol. The compacted plots were trafficked annually for four years (2010-2013). Nine years later (2022), field measurements (saturated hydraulic conductivity, visual evaluation of subsoil structure [SubVESS], and penetration resistance) were conducted at a depth of 0.3-0.4 m. Additionally, undisturbed samples of 100 cm³ were taken for measurements of gas flow (air permeability [<em>k</em>_a] gas diffusivity [<em>D</em>_p/<em>D</em>_o], and air-filled porosity[&#949;]) after equilibration at a matric potential of &#8722;100 hPa. The negative impact of compaction on the measured variables was compared to previous measurements conducted four years after (2017) the compaction event.</p><p>Nine years after compaction, there was still a marked negative effect of compaction on the soil structure assessed by the SubVESS method, with the largest impact observed on soil strength, root growth restriction, and aggregate friability. Saturated hydraulic conductivity was 63% lower in the compacted treatments compared to the control, while penetration resistance increased from 1.91 to 2.65 MPa after compaction. We also observed a strong negative effect of compaction on soil air permeability (&#8722;54%), gas diffusion (&#8722;30%) and the effective air-filled porosity (&#8722;24%). These changes were reflected in decreased pore organization [<em>PO</em>] and tortuosity in the compacted plots. Compared to five years earlier, there was a potential for natural recovery of the gas transport and pore structure variables (<em>k</em>_a, <em>D</em>_p/<em>D</em>_o, &#949;, <em>PO</em>). In 2017, there was an average compaction-induced reduction of 55% in the mentioned variables, and this changed to 38% in 2022, suggesting an increased recovery with time. Thus, although the effect of compaction on the subsoil was persistent after several years, there is a possibility that natural processes may play a significant role in recovering critical soil functions after compaction in the upper subsoil.</p>