POTENTIAL DENITRIFICATION ACTIVITY ASSAY IN SOIL - WITH OR WITHOUT CHLORAMPHENICOL

A common way to characterize denitrification in soil is to determine t he potential denitrifying activity (PDA). Our objectives were to compa re different techniques of mathematically describing experimental data obtained in the PDA assay, both with and without use of chloramphenic ol (CAP), and to quantify the effect of CAP on the process. The PDA as say was carried out in the presence of acetylene in slurries of three agricultural soils containing 1 mM glucose and 1 mM KNO3. When CAP was not used in the assay, growth related curves of N2O-formation were ob tained for all three soils. These data were used to calculate the init ial rate by: (1) assuming the initial phase to be linear and using the four first data points for linear regression; and by (2) using a grow th-associated product formation equation. The good fit to the data tha t was obtained with the latter method suggests that PDA is a continuou s process without distinct phases. Moreover, our results clearly show that denitrifying activity is inhibited by CAP even at the lowest conc entration tested, 20 mg 1(-1). The inhibiting effect increased with in creasing concentrations of CAP. The PDA was 17-42% lower at 1 g CAP 1( -1) compared with assays without CAP. This shows that not only synthes is of new enzymes is affected but also that the activity of already ex isting enzymes is decreased. Results from our study strongly suggest t hat single concentrations of CAP must not be used in PDA assays. An al ternative strategy could be to use multiple CAP concentrations and the n extrapolate to the rate at 0 g CAP 1(-1). However, we recommend assa ys without CAP and that data should be fitted to the growth-associated product formation equation. By using this method, values of the PDA a nd the growth rate of the denitrifying bacterial population are object ively obtained.