Problems in the Estimation of Biological Activity in Soil

In recent years, soil microbiologists have tended to move from the identification of isolates and the estimation of populations to study the role of micro-organisms in soil processes. This has led to increased interest in methods for estimating the of soil microorganisms, although exactly what aspect of is meant is often undefined. For this purpose, measurement of respiration is widely used. More recently, measurement of rates of activities of certain enzymes have also been tried, in an attempt to estimate biological activity. Some difficulties associated with such attempts are reviewed. Respiration and dehydrogenase of soils are commonly used as indicators of biological activity, and some data are presented which show that in soils these two measurements are not linked in a simple way. This may be due to the fact that polyphenol oxidase, an enzyme system which transfers hydrogen from dehydrogenase to molecular oxygen, may also occur free in soils, probably participating in the formation of humic acid. It is concluded that the biochemical complexity of soil makes it unlikely that any one single measurement at present available can be used satisfactorily as a measure of biological activity in a general, undefined sense. More information and insight may be gained by postulating model systems and investigating these from two or more aspects.