Abstract
In most soils, living microorganisms (the soil microbial biomass) make up about 1-3% of total soil organic matter. We review some of the methods available to measure soil biomass. These are (1) direct microscopic examination, (2) the fumigation-incubation method, (3) the fumigation-extraction method, (4) the substrate-induced respiration method and (5) the measurement of soil ATP. In soils which do not contain actively decomposing substrates, most of the biomass is dormant or resting, having a very low respiration rate, long mean cell-division time and slow turnover rate. Yet, paradoxically, it maintains an ATP concentration and adenylate energy charge at the same level as organisms growing exponentially in vitro. Two independent pieces of new research are presented to illustrate how the new fumigation-extraction procedures can be used to measure changes in biomass following substrate addition to soils. These are the effects of straw incorporation on soil biomass and nitrogen dynamics, and the origins of "priming effects" following substrate incorporation (glucose and ryegrass) to soil.
