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.