Designing and Conducting a Proper Soil Enzyme Assay.

A wide diversity of organic and inorganic materials from microorganisms, plants, and animals are continuously being degraded and/or transformed in soil. These reactions are often catalyzed by extracellular enzymes, and without their presence, the soil would no longer function properly. The discipline of soil enzymology is based upon making measurements of enzyme activity where either product appearance or substrate disappearance is measured when substrate is incubated in soil under standard conditions. Factors that must be considered and controlled during the measurement of the activity of an enzyme in soil include the extraction efficiency of product or substrate from soil, whether the soil is in an air-dried or field-moist state, buffer pH, substrate concentration, amount of soil, time span of reaction, temperature, shaking vs. not shaking during reaction, the stoichiometry of the reaction, the selection of an appropriate analytical procedure, storage or pretreatment of the soil before assay, whether there is a need for cofactors for the reaction to proceed, and the use of a proper control. These factors must all be carefully evaluated and optimized in a variety of soils to provide the most valid soil enzyme assay and to ensure that the only limitation to the reaction rate is enzyme concentration, i.e. the reaction rate that is measured is proportional to, or accurately reports, the enzyme concentration in soil. This paper will review the steps required to stringently develop and test soil enzyme assays and assess their kinetic and thermodynamic properties. The result of a properly conducted soil enzyme assay will be the enhancement of knowledge of known (and newly discovered) enzymes in soil and the processes they catalyze.