Nitrous Oxide Production by Nondenitrifying Soil Nitrate Reducers

Of 214 soil bacterial isolates able to reduce NO3-, 209 produced nitrous oxide (N2O), even though only 46 were respiratory denitrifiers (competent to carry out complete reduction of NO3- to N gases). Nitrite or NH4+ was the major product of NO3- reduction by the nondenitrifying organisms, but typically about 5 to 10% and up to 34% of the NO3- reduced by them was released as N2O during a 2-week incubation. Bacillus and Enterobacter were the most commonly observed genera of nondenitrifying N2O producers. Fermentative NO2- reduction and N2O production by a Bacillus sp. and a Citrobacter sp. were characterized in pure culture studies. Dinitrogen (N2) was not produced in detectable quantities by these organisms. When added to autoclaved soil, they accumulated more N2O than two denitrifying pseudomonads, since the latter consume and produce N2O. In tryptic soy broth (TSB), which allows active fermentative growth, NH4+ was apparently the major product of NO3- reduction. In nutrient broth (NB), NO2- accumulated. Added NH4+ did not inhibit N2O production or apparent reduction to NH4+, indicating that these processes are not assimilatory. The effect of added glucose on N2O production varied with the organism and media composition. Nitrous oxide production from NO2- by these organisms was shown to be at least partially a biochemical reaction. The N2O evolved slowly in batch cultures and mostly after apparent growth ceased. This is apparently a novel mechanism of N2O generation which differs significantly from respiratory denitrification