Impact of compaction on two sensitive forest soils in Lorraine (France) assessed by the changes occurring in the perched water table

The mechanisation of forestry operations is too recent in France to have enough perspective and scientific knowledge of the effects on extensively managed soils. Two experimental sites on soils sensitive to compaction (silty acidic soils laying on top of a clayey subsoil) were set in Lorraine (France). A gentle and controlled compaction of these physically very analogous soils was carried out during spring 2007 at the Azerailles site (AZ) and during spring 2008 at the Clermont-en-Argonne site (CA). Immediate changes were observed for all ecosystem physical, chemical and biological functioning indicators. Soil restoration dynamics were followed using continuous monitoring of numerous parameters, including soil moisture and occurrence of a perched water table (PWT) (monitored at a daily time step) and PWT chemistry (monthly time step). Mid-term monitoring (7–8 years) results showed that both soils shifted towards a hydromorphic soil type attested by the increase in PWT frequency and duration, which invaded the soil upper layers, leading to characteristic Fe, Mn and S mobility and a change in organic compounds stability. Though soil types were very similar, the PWT characteristics varied notably between both sites. The mean residence time of the PWT was shown to be a major driver of its geochemistry, but with strong interaction with soil characteristics. A previous study made on both sites suggested that soil clay content and clay mineralogy controls the PWT dynamics through effects on aggregation. From a PWT perspective, seven years of monitoring were insufficient to observe any soil compaction recovery in the richer soil of AZ but a partial recovery was observed for the chemically poorer soil (CA). Compaction durably impacted the two studied soils and the probability that a new compaction event may occur before the complete recovery from the first disturbance must be considered in forest management.