EVALUATION OF THE MOST PROBABLE NUMBER METHOD FOR ENUMERATING DENITRIFYING BACTERIA

Most probable number (MPN) enumerations of denitrifying bacteria were conducted on an agricultural soil (pH 6. 6) and a forest soil (pH 5. 6), in both neutral (pH 6. 8) and acid (pH 5. 6) media. The neutral medium yielded higher enumerations for both soils and, therefore, appears to be optimal, even for denitrifiers present in acid soils. Enzyme activity assays and MPN enumerations of several soil samples were weakly correlated (r equals 0. 71; p equals 0. 07). One soil exhibited a high MPN enumeration, but had no detectable denitrifying enzyme activity. While the enzyme assay indexes denitrification potential of soils under the environmental conditions at the time of sampling, the MPN enumeration indexes the denitrifying capability of the populations inhabiting soils. A population's denitrifying capability is related to its size, but other factors which might also influence MPN enumerations are discussed. Refs.