Subsoil compaction impacts soil quality indicators in a Calcaric Chernozem

Soil compaction caused by modern mechanized agriculture has severe impacts on soil functioning and negative consequences on crop production. In the subsoil, these effects are persistent and difficult to ameliorate. To further clarify the compaction effect on the subsoil, we imposed compaction on 21^st April 2022 on a moist Calcaric Chernozem (silty clay loam) by pulling a full 2-axle slurry tanker (10 m³, pendulum tandem axle, max. wheel load 3 Mg) with a tractor (19.5 Mg total load) through the field. Six months after the compaction event (in autumn (18^th October 2022)), we evaluated the effects on field-measured soil structural quality indicators (SubVESS, bulk density, penetration resistance) and saturated hydraulic conductivity, then compared them to laboratory-measured gas flow-related parameters on intact 100 cm³ soil cores at -100 hPa (air-filled porosity, gas diffusion, and air permeability) at one depth in the subsoil (30-40 cm) and quantified pore geometry and tortuosity and geometry.

Simulation of traffic using TerranimoÒ indicated a risk of compaction down to 30 cm depth. The compacted treatment did not noticeably affect the soil bulk density. However, based on visual evaluation by SubVESS, compaction decreased porosity and aggregate friability. The overall soil structural quality (Ssq) scores were 2.1 and 1.4 for the compacted and control subsoil layers, respectively. Further, compared to the control treatment, a higher penetration resistance for the compacted treatment was observed from 5 cm to 35 cm. Field-measured saturated hydraulic conductivity decreased by 42% after compaction. Soil gas transport by convection (air permeability) and diffusion (gas diffusivity) decreased by 67% and 48%, respectively, after compaction. Furthermore, compaction decreased air-filled porosity and pore organization by 28 and 60%, respectively, and increased pore tortuosity. It can be concluded that although compaction did not increase bulk density in the subsoil, the negative effects of traffic was detectable by SubVESS, and the quantitative parameters related to air and water flow.