Sources of soil enzymes include living and dead microbes, plant roots and residues, and soil animals. Enzymes
stabilized in the soil matrix accumulate or form complexes with organic matter (humus), clay, and humus-clay
complexes, but are no longer associated with viable cells. It is thought that 40 to 60% of enzyme activity can come from stabilized enzymes, so activity does not necessarily correlate highly with microbial biomass or respiration. Therefore, enzyme activity is the cumulative effect of long term microbial activity and activity of the viable population at sampling. However, an example of an enzyme that only reflects activity of viable cells is
dehydrogenase, which in theory can only occur in viable cells and not in stabilized soil complexes.