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.

► Agriculture is an important source of nitrogen in rivers and coastal waters. ► Nitrogen in waters is a major source of the greenhouse gas nitrous oxide (N2O). ► N2O emission factors and global N2O budget are still highly uncertain. ► N2O emission uncertainties make it hard to focus climate change mitigation strategies. ► Experimental and process-based model studies on aquatic N2O emissions are needed, especially in the tropics.