Effects of compaction by heavy machine traffic on soil fluxes of methane and carbon dioxide in a temperate broadleaved forest

Soil compaction decreases soil aeration and water infiltration, lowering air-filled porosity, which may impact biological processes involved in soil fluxes of carbon dioxide (FCO2) and methane (FCH4), and decrease the greenhouse gas emissions mitigation potential offered by the forestry sector. We recorded FCO2 and FCH4 continuously for two years using automated chambers connected to a laser-based gas analyser in an experimental forest site on an acidic ruptic Luvisol that was established to assess the long-term impact of a loaded forwarder. Heavy machine traffic had considerably increased soil surface roughness. Air-filled porosity (AFP) in the first 0.1 m was lower in the trafficked plot - especially in hollows - than in the control almost all year long. The temperature sensitivity of FCO2 was higher for the control plot than for both mounds and hollows in the trafficked plot. Cumulative FCO2 was much higher in the control than in hollows and mounds of the trafficked plot. In contrast, annual FCH4 did not significantly differ between the control plot and either the mounds or the hollows in the trafficked plot, but was significantly higher in mounds than in hollows. FCH4 was negative all year round indicating a net uptake of CH4, except during winter when a net emission of CH4 was occasionally observed in the hollows on the trafficked plot. While seasonal variations of FCH4 were well related to variations in AFP, the potential rate of methane uptake at optimal air-filled porosity was higher in the trafficked plot than in the control.