Simulating N₂O Emissions From Inland Waters: Model Development and Site-Level Study

Inland waters are significant sources of atmospheric nitrous oxide (N₂O), and their emissions are expected to rise rapidly due to human activities. However, current estimates remain highly uncertain, partly because sparse field measurements and existing models fail to capture fine-scale variations in N₂O emissions, especially due to the interactions between fluid dynamics and biogeochemical drivers. This study developed an N₂O module in the Environmental Fluid Dynamics Code (EFDC), a comprehensive numerical model capable of simulating hydrodynamics and water quality in different inland aquatic systems, by incorporating key nitrogen (N) processes that influence N₂O production and emissions. The developed model was applied to a typical mountainous reservoir in China, and reliably reproduced the temporal and spatial variations in N₂O concentrations and fluxes. Seasonal patterns of N₂O fluxes were primarily driven by temperature, stratification, and nutrient availability, with autumn exhibiting the peak values. In contrast, spatial variations were mainly regulated by fluid dynamics and nutrient availability, with the highest N₂O fluxes observed in the main part of the reservoir. Furthermore, the whole-reservoir N₂O budget indicated that sediments were the primary source of N₂O, which is mainly produced via denitrification under low dissolved oxygen conditions, underscoring the importance of understanding the sedimentary N cycle. Overall, our model enables the quantification of significant spatiotemporal variations in N₂O emissions from inland waters and unravels the underlying mechanisms, thereby enhancing our mechanistic understanding of N₂O dynamics and improving the quantification accuracy of N₂O emissions at broader scales.