Nitrous oxide production and nitrification kinetics by natural bacterial communities of the lower Seine river (France)

Nitrous oxide (N2O) is produced in the lower Seine river at 40 to 60 kg N-N2O d–1, in a section greatly affected by effluents from the Achères Wastewater treatment plant (WWTP). Because water column denitrification is weak in this sector, we hypothesized that N2O could be produced by the nitrifier-denitrification process. To understand the controls of N2O emission, nitrifying bacterial cultures were grown from Seine river water (in batches and continuous flow experiments). The population diversity of ammonia oxidizing bacteria (AOB) in these experiments was determined by denaturing gradient gel electrophoresis (DGGE) and found to be similar to those naturally present in the Seine. We determined nitrification kinetics of the 2 functional bacteria populations (AOB and nitrite oxidizing bacteria [NOB]). During nitrifying batch and continuous flow experiments, the N2O production kinetics were examined under contrasted conditions. We tested the effect of dissolved oxygen, ammonia and nitrite concentrations on the N2O production rate. To our knowledge, this is the first study determining the optimal concentrations of O2, NH4 and NO2 that will lead to maximum N2O production by nitrifier-denitrification of mixed nitrifying bacteria populations from natural freshwater. We tested a range of oxygen concentrations and observed a peak in N2O emission within the narrow range from 1.1 to 1.5 mg O2 l–1. We plotted the N2O production as a function of ammonium and nitrite concentrations under optimal dissolved oxygen conditions. The results followed the hyperbolic Michaëlis-Menten type curves, and kinetics parameters (Vmax and Ks) were determined. The maximum N2O production rate (Vmax) was estimated at 8 to 9 μg N-N2O mg C biomass–1 h–1. The halfsaturation constants of nitrifier-denitrification were Ks = 1.5 to 3 mg N-NH4 l–1 for ammonium, and Ks = 1 to 4 mg N-NO2 l–1 for nitrite.