ABSTRACT
Introduction The quantity of living matter present in the soil is the biomass. The relative stability of this biomass is the result of dynamic equilibria: at any given instant organisms are born, multiply and die. Microorganisms degrade substrates, synthesise new organic matter and then are subsequently lysed. Their total number varies more or less within wide limits, depending on environmental conditions and all the external interventions. The qualitative and quantitative composition of the biomass is one of the characteristics that contribute to defining a soil but, on the other hand, this composition is partly the result of the physicochemical nature and structure of that soil. Maintenance of the biomass is ensured by the nutrients supplied by plants. It depends to a great extent on numerous metabolic reactions effected by enzymes. Some of these processes, such as redox, are common to all organisms. Other enzymatic systems come into play only in specific groupscellulolytic organisms, nitrogen-fixators for example.
