ABSTRACT
The soil microbial biomass (defined as the sum of the masses of all microorganisms with a diameter less than 500 µm) can be considered as the living fraction of soil organic matter. It is the agent of mineralization of all the dead plant and animal residues that enter soil; it therefore plays a vital role in the cycling of all the major nutrients on the planet (Figure 10.1). The biomass has been eloquently described as “the eye of the needle, through which all organic matter must pass as it is broken down to the simple inorganic components that plants can use again” (Jenkinson 1977).
