Upper subsoil pore characteristics and functions as affected by field traffic and freeze–thaw and dry–wet treatments

Cultivated soils are subject to very high stresses from machinery. This may affect the soil pore system and its processes, soil functions and soil ecosystem services. Compaction experiments were performed on loamy Luvisols at three sites in Denmark: Aarslev, Flakkebjerg and Taastrup. Non-trafficked control soil was compared with soil subjected to four annual traffic events with approximately 3-, 6- or 8-Mg wheel loads from tractor–trailer combinations. A self-propelled machine with a single pass of approximately 12-Mg wheel load was tested at Aarslev. Undisturbed soil cores were sampled at 0.3m depth when the experimental plots had received either 2 years (Flakkebjerg) or 3 years (Aarslev and Taastrup) of repeated compaction treatment. The volume of air-filled pores and air permeability were quantified for soil drained to –100hPa matric potential. Freeze–thaw and dry–wet treatments were applied to soil cores in the laboratory for Aarslev and Taastrup samples. The multipass machinery significantly affected >30µm soil pores and air permeability at wheel loads of ~6 Mg or higher, whereas no or only minor effects could be detected for ~3-Mg wheel loads. Indices combining air permeabilities with air-filled porosities indicated that pore morphological features had also been affected. Estimates of hydraulic conductivity indicated critical conditions for the percolation of excess rainwater for severely compacted soil at Aarslev. Generally, the single-pass machine with a high wheel load did not affect the pores and their function. A dry–wet event was a more effective remediation of compaction than a freeze–thaw treatment. In conclusion, present-day field traffic risks creating a bottleneck soil layer for important soil functions just below the tilled topsoil.