Influence of logging traffic on the hydromorphic degradation of acid forest soils developed on loessic loam in middle Belgium

Modifications of soil physical parameters (bulk density, pore space pattern and clay dispersibility) induced by compaction in wheel-rutted areas due to mechanized forest exploitation was studied in loamy soils developed under beech stands in the loessic belt of middle Belgium. The consequences of surface waterlogging on pedological processes (especially the geochemical behaviour of iron and aluminium) were also investigated. The results show that rutted areas have: (1) a significant decrease of total porosity in the eluvial upper layers of the soil (0-30 cm depth), associated with an important lowering of the transmission pore volume and a decrease in macroporosity to about 6% of the total soil volume, i.e. to less than the current threshold value for root viability (10%); (2) a reduction of macropore space to about 9.5% in the lower illuvial horizons (30-50 cm depth); (3) a decrease of redox potential due to temporary waterlogging, inducing (a) the occurrence of reduced forms of iron, (b) a strong leaching of iron oxyhydroxides from the E(g) to the B(1g) horizon, (c) an increase of organically complexed iron forms in both the Es and B(tg) horizons, and (d) high solubility of Goethite and even of ferromagnesium minerals in the eluvial horizons; (4) an increase of clay dispersibility in both the E(g) and B(tg) horizons. These results provide clear evidence that on loessic materials soil compaction due to logging operations leads to rapid soil degradation through active hydromorphic processes.