| Summary: | Nitrous oxide (N<sub>2</sub>O) is a potent greenhouse gas stemming mainly from nitrogen (N)-fertilizer application. It is challenging to quantify N<sub>2</sub>O emissions from agroecosystems because of the dearth of measured data and high spatial variability of the emissions. The eco-hydrological model SWAT (Soil and Water Assessment Tool) simulates hydrological processes and N fluxes in a catchment. However, the routine for simulating N<sub>2</sub>O emissions is still missing in the SWAT model. A submodule was developed based on the outputs of the SWAT model to partition N<sub>2</sub>O from the simulated nitrification by applying a coefficient (K<sub>2</sub>) and also to isolate N<sub>2</sub>O from the simulated denitrification (N<sub>2</sub>O + N<sub>2</sub>) with a modified semi-empirical equation. The submodule was applied to quantify N<sub>2</sub>O emissions and N<sub>2</sub>O emission factors from selected crops in two agricultural catchments by using NH<sub>4</sub>NO<sub>3</sub> fertilizer and the combination of organic N and NO<sub>3</sub><sup>−</sup> fertilizer as N input data. The setup with the combination of organic N and NO<sub>3</sub><sup>−</sup> fertilizer simulated lower N<sub>2</sub>O emissions than the setup with NH<sub>4</sub>NO<sub>3</sub> fertilizer. When the water balance was simulated well (absolute percentage error <11%), the impact of N fertilizer application on the simulated N<sub>2</sub>O emissions was captured. More research to test the submodule with measured data is needed.
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