CHANGES IN HUMUS MICROBIOLOGICAL ACTIVITY-INDUCED BY THE SUBSTITUTIONOF THE NATURAL BEECH FOREST BY NORWAY SPRUCE IN THE BELGIAN ARDENNES

Different microbiological methods, including determination of enzymati c activity and potential nitrogen availability, and microbial biomass estimation were used to evaluate soil modifications induced by the mas sive planting of Norway spruce on acid soils of the climatic deciduous forest of high Belgium. Investigations were made on the surface humic layers (holorganic and hemiorganic horizons) of ochreous brown earths (Dystrochrepts) in two neighbouring forest stands, a natural beech hi gh-forest (Fagus sylvatica L.) and a 92-year-old spruce plantation (Pi cea abies (L.) Karst.). Results show that spruce monoculture has a sig nificant depressive effect on soil microbiological activity, clearly s hown by the lowering of microbial biomass and the decrease of both FDA hydrolytic activity and nitrogen potential availability.