Detection of negative consequences of traffic on subsoil properties depends on measurement type and scale: The case of a Calcaric Chernozem

Larger, heavier agricultural machinery compacts subsoil, degrading structure, impairing hydraulic properties, and increasing greenhouse gas (GHG) emissions. The impacts of traffic-induced compaction can be assessed in the laboratory or the field, though results from these methods often differ. This study aimed to quantify changes in subsoil properties caused by field traffic using lab and field methods, compare traffic-induced pore structure alterations at two sample size scales, and explore links between pore structure and greenhouse gas emissions. A compaction experiment was conducted on a Calcaric Chernozem loam soil, comparing a trafficked treatment (TF) with a 3 Mg wheel load to a non-trafficked (NT) reference. Six months later, we evaluated the impact of the traffic event on the subsoil (30–35 cm depth) by conducting field measurements [saturated hydraulic conductivity (K_s), visual evaluation of subsoil structure (SubVESS), and penetration resistance (PR)]. Different laboratory measurements conducted on intact soil cores (100 cm³ and 580 cm³) included soil water retention curve (SWC), air-filled porosity (ε_a), Darcy air permeability (k_a-Darcy), gas diffusivity (D_p/D₀), and bulk density (BD), X-ray computed tomography (CT) analysis of soil pore structure, and potential N₂O and CO₂ emissions. Both field and laboratory measurements indicated negative impacts of traffic. In the field, traffic reduced K_s by 31 % (p = 0.45), increased PR by 22 % (p = 0.2), and lowered SubVESS structural quality (p = 0.001). Laboratory measurements showed that traffic significantly (p < 0.05) increased BD by 4 %, and at pF 2, it decreased ε_a, k_a-Darcy, and D_p/D₀ by 23 %, 71 %, and 48 %, respectively, alongside a decrease (p > 0.05) in CT-derived macroporosity volume. Potential greenhouse gas emissions were higher in the trafficked (TF) treatment. The effect of compaction on BD, ε_a, and k_a-Darcy was similar regardless of sample size, though, for a given treatment, the 100 cm³ samples were denser and had lower ε_a, and slower airflow (k_a-Darcy) compared to 580 cm³samples. The effect of traffic on soil properties, relative to established thresholds indicating harmful deterioration of soil functions, varied depending on whether the variable was measured in the field or laboratory, and whether small or large rings were used. Further research is needed to establish critical limits for soil properties and functions using different sample scales.