Modeling global N₂O emissions from aquatic systems

Human activities on land have increased the N inputs to rivers and coastal waters worldwide. This increased aquatic emissions of nitrous oxide (N₂O). Global, spatially explicit modeling of N flows from land to sea and associated N₂O emissions have been developed for a number of decades. During the 1990s, global N₂O studies focused to a large extent on closing the global budget. Since then, aquatic emissions of N₂O have been subject of scientific discussions. Although it is widely recognized that human activities on land increase aquatic N₂O emissions, quantification is difficult because of lack of experimental data. In order to reduce uncertainties, additional long-term studies are required measuring N and N₂O concentrations in aquatic systems. More explicit modeling of N₂O formation and the underlying biogeochemical cycling in aquatic systems would improve our understanding of aquatic N₂O emissions. Global models preferably include both N cycling, N₂O production, and river transport in a spatially explicit way, as well as biogeochemical cycling in coastal seas and oceans. Integrative studies are needed that account for the interactions between different impacts of increased levels of reactive N in the environment. We argue that it is still difficult to close the global N₂O budget.