Soil humic compounds and microbial communities in six spruce forests as function of parent material, slope aspect and stand age

The influences on soil chemical and microbial properties of parent material, north south aspect and time measured as stand age were investigated in six spruce (Picea abies (L.) Karst.) forests located in the alpine range of Northern Italy. Soil samples from A horizons were analysed for humic substances and in parallel Amplified Ribosomal DNA Restriction Analysis (ARDRA) community profiles and microbial biomass carbon and nitrogen content were determined. Chemical data were analyzed by canonical discriminant analysis while the ARDRA fingerprints were ordered in clusters using image analysis software. The geologic parent material was the most determining factor and the aspect-dependent microclimate features also played a distinct role in defining both soil chemistry and microbial community composition; in contrast the composition of the deeper humus layers (OH, A) was stable and similar within a spruce forest cycle time. Most important variables in the construction of the discriminating models resulted soil pH, Dissolved Organic Carbon content and Dissolved Organic Matter phenolic compounds. Bacterial communities appeared to be shaped first and foremost by the substratum, secondly by mountain slope orientation, and thirdly by forest stage, thus confirming the CDA model.