Nitrous oxide production in soil isolates of nitrate-ammonifying bacteria

Here we provide the first demonstration of the potential for N2 O production by soil-isolated nitrate-ammonifying bacteria under different C and N availabilities, building on characterizations informed from model strains. The potential for soil-isolated Bacillus sp. and Citrobacter sp. to reduce NO3 (-) , and produce NH4 (+) , NO2 (-) and N2 O was examined in batch and continuous (chemostat) cultures under different C-to-NO3 (-) ratios, NO3 (-) -limiting (5 mM) and NO3 (-) -sufficient (22 mM) conditions. C-to-NO3 (-) ratio had a major influence on the products of nitrate ammonification, with NO2 (-) , rather than NH4 (+) , being the major product at low C-to-NO3 (-) ratios in batch cultures. N2 O production was maximum and accompanied by high NO2 (-) production under C-limitation/NO3 -sufficiency conditions in chemostat cultures. In media with lower C-to-NO3 -N ratios (5- and 10-to-1) up to 2.7% or 5.0% of NO3 (-) was reduced to N2 O by Bacillus sp. and Citrobacter sp., respectively, but these reduction efficiencies were only 0.1% or 0.7% at higher C-to-NO3 (-) ratios (25- and 50-to-1). As the highest N2 O production did not occur under the same C-to-NO3 (-) conditions as highest NH4 (+) production we suggest that a re-evaluation may be necessary of the environmental conditions under which nitrate ammonification contributes to N2 O emission from soil.