Long-term analysis of ecosystem processes involves, to a great extent, inventories of element and energy transfers between biotic and abiotic compartments. In terrestrial ecosystems, the heterotrophic activity of soil micro-organisms plays a central role in this element and energy transfer as it involves the release of organic nutrients and mineral elements during microbial mineralisation activities of organic residues from the primary producers. Chronic detrimental impacts would directly affect this vital microbial compartment with subsequent changes in the element and energy budget of a system. For that reason, it is an obligatory necessity to develop a sound knowledge about the size of this microbial pool and the microbial biomass of total fungal and bacterial cells, together with an understanding of its controlling mechanisms. It is most likely that adverse changes in an ecosystem will more easily and at an early stage be detectable at the microbial community level. The objectives of this study were (1) to survey a large number of different forest sites in Lower Saxony for microbial biomass content, microbial growth or activity indices, in order to (2) provide the underlying principles of microbial development in forest soils. All the methods used are well described in the pertaining literature.