Impact of heavy traffic on soil macroporosity of two silty forest soils: Initial effect and short-term recovery

• The impact of heavy traffic is more detrimental in upper soil layers than lower ones.
• Natural regeneration of macroporosity occurred only in the upper 7 cm of soil.
• Small and medium macropores regenerated fully within 2–3 years.
• Recovery of large macropores takes more than 2–3 years.
• Macroporosity recovery is due mainly to plant-root penetration and physical processes.

Processes and rate of macroporosity changes following heavy traffic in forest ecosystems are seldom studied. The aim of this study was to determine the ability of forest soils to regenerate their macroporosity naturally. The study was performed on 2 silty temperate-forest soils classified as sensitive to compaction located in north-eastern France. Macroporosity was measured in control and trafficked plots at 3 depths (0–7, 15–30 and 30–45 cm) over 2–3 years. Soil macroporosity characteristics (shape, size and orientation) were assessed on polished sections through 2D-image analysis and micromorphic observations. Immediately after heavy traffic, macroporosity decreased by 96 to 49% from 0 to 45 cm in depth. Natural regeneration of macroporosity occurred in the upper 7 cm of soil, while the soil below remained compacted. Small and medium macropores (0.05–0.8 mm2) dominated by rounded and irregular pores regenerated completely. Large macropores (> 0.8 mm2) originally dominated by vughs, mammilated vughs and channels rarely regenerated and were gradually replaced by horizontally oriented planar pores. Our results suggest that initial stages of natural macroporosity recovery are likely due to plant-root penetration and physical processes (shrink–swell, freeze–thaw), whereas recovery due to fauna activities appears later.