Nitrous oxide production and mRNA expression analysis of nitrifying and denitrifying bacterial genes under floodwater disappearance and fertilizer application.

A pulse of nitrous oxide (N2O) emission has been observed following the disappearance of floodwater by drainage. However, its mechanism is not well understood. We conducted a column study to clarify the mechanism for N2O production during floodwater disappearance by using a microsensor and determining the bacterial gene expression. An increase in N2O flux was observed following floodwater disappearance after the addition of NH4+, with a corresponding increase in the concentrations of NO3- and dissolved N2O in the oxic and anoxic soil layers, respectively. The transcription level of the bacterial amoA mRNA did not change, while that of nirK mRNA increased sharply after an hour of floodwater disappearance. An additional anoxic soil slurry experiment demonstrated that the addition of NO3- induced the expression of nirK gene and caused a concomitant increase in N2O production. These findings suggest that NO3- production in the oxic layers is important as it provides a substrate and induces the synthesis of denitrification enzymes in the anoxic layer during N2O production.