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 pa ttern and clay dispersibility) induced by compaction in wheel-rutted a reas due to mechanized forest exploitation was studied in loamy soils developed under beech stands in the loessic belt of middle Belgium. Th e consequences of surface waterlogging on pedological processes (espec ially the geochemical behaviour of iron and aluminium) were also inves tigated. The results show that rutted areas have: (1) a significant de crease 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 s oil volume, i.e. to less than the current threshold value for root via bility (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 pote ntial due to temporary waterlogging, inducing (a) the occurrence of re duced forms of iron, (b) a strong leaching of iron oxyhydroxides from the E(g) to the B-tg horizon, (c) an increase of organically complexed iron forms in both the E(g) and B-tg horizons, and (d) high solubilit y of Goethite and even of ferromagnesium minerals in the eluvial horiz ons; (4) an increase of clay dispersibility in both the E(g) and B-tg horizons. These results provide clear evidence that on loessic materia ls soil compaction due to logging operations leads to rapid soil degra dation through active hydromorphic processes.