Evaluation of antibacterial and antifungal compounds for selective inhibition of denitrification in soils.

Nitrous oxide (N2O) is an atmospheric constituent implicated in climate warming and stratospheric ozone depletion. Both bacteria and fungi participate in N2O production, but information is lacking with regard to the relative contribution of bacterial and fungal denitrifiers to the denitrification process in agricultural soils. The selective inhibition (SI) technique is widely used to assess the contribution of different groups of microbes to soil processes, but success of the technique depends on the effectiveness of the inhibitors. In this study, laboratory experiments were conducted to assess the contribution of bacteria and fungi to denitrification using soils from a woodlot, agricultural fields under conventional plowing (PT), and no-till for either 50 years (long-term) or 11 years (medium-term). A selective inhibition (SI) technique was developed using two bactericides (streptomycin and bronopol) and two fungicides (cycloheximide and captan) applied at different rates (0-32 mg per g soil). Regardless of the application rate, streptomycin and cycloheximide were not effective inhibitors of denitrification, with a degree of inhibition only between 2 and 20% relative to controls. These results are significant given the wide use of these products in SI studies. However, the bactericide bronopol and the fungicide captan effectively inhibited denitrification, with the strongest inhibition observed at an application rate of 16 mg per g soil. The ratio of fungal to bacterial denitrification activity (F : B) was generally less than 1, indicating a dominance of bacteria in denitrification activity in the soils investigated. However, an increase in the F : B ratio from 0.24 in medium-term NT to 0.87 in long-term NT soils was noted, suggesting perhaps a progressive increase in the role of fungal denitrifiers with a longer duration of NT farming.