Abstract: Farmland drainage changes the hydrological process in the river-ditch-paddy field water networks and rebuilds the spatial distribution of nitrogen, with consequence on N₂O production. It is of great significance to explore the production and emission of N₂O in water from agricultural activities for improving the global N₂O budget and indirect emission factors. In this study, we explored the production mechanism of N₂O and source and sink effects of surface waters during the drainage period of farmlands in the Nongjiang River Basin in Sanjiang Plain. Headspace equilibrium gas chromatography was used to determine the concentration of N₂O in the water body. The boundary layer model was used to calculate the N₂O flux, and the physical and chemical indices in the water body were measured simultaneously. The results showed that surface waters were the source of atmospheric N₂O. The average N₂O fluxes in the Yalu River, the Nongjiang River, and ditches were 0.72, 0.20, and 0.34 μmol·m^-2·h^-1, respectively. The average emission factor (EF_5r) for rivers and ditches was 0.85% and 0.32%, respectively, which were higher than the recommended EF_5r values by the International Intergovernmental Panel on Climate Change (IPCC) and the average EF_5r values for world rivers. The significant positive correlation (P<0.05) between inorganic nitrogen (DIN) and N₂O concentration indicated that high levels of inorganic nitrogen can increase nitrification and denitrification rates, and thus stimulate the production of N₂O. The significant positive correlation (P<0.05) between dissolved organic nitrogen (DON) and N₂O concentration in Yalu River indicated that high concentration of DON will promote mineralization and affect the production of N₂O. The sink of N₂O in the mixed urban and agricultural watershed indicated that the lower denitrification rate may cause the consumption of N₂O.

Key words: agricultural watershed, drainage period, denitrification, N₂O flux