Isotopomer analysis of production, consumption and soil-to-atmosphere emission processes of N2O at the beginning of paddy field irrigation

In irrigated rice paddies, episodic release of nitrous oxide (N2O) from the soil to the atmosphere has been observed during the first flood irrigation, but the biogeochemical mechanisms underlying these emissions remain unclear. To elucidate both microbial pathways of N2O production, consumption and emission processes of N2O from soil surfaces, we analyzed isotopomer ratios (bulk nitrogen and oxygen isotope ratios, δ15Nbulk and δ18O, and intramolecular 15N site preference, SP) of surface-emitted N2O and N2O in soil gas from paddy fields in Japan at the beginning of irrigation. Results indicate that surface-emitted N2O is produced at shallow depths above the rising groundwater table, and that it is emitted by diffusive transport through air-filled soil pores. Immediately after soil surfaces are submerged, N2O accumulates in the soil because of the low diffusivity and high solubility of N2O in water. Isotopomer analysis revealed that high N2O emissions during the flooding process resulted mainly from N2O production by bacterial denitrification (nitrate reduction). Moreover, as soil submergence progressed, declining soil NO3− concentration promoted the use of N2O as an electron acceptor. Thereby, most of the N2O was reduced to N2. N2O emissions were increased by nitrogen fertilizer application before irrigation. The applied nitrogen fertilizer might enhance N2O production.