Indirect agricultural N2O emissions via the aquatic pathway occur while reactive nitrogen leached from agricultural fields is transported downstream through aquifers, riparian areas, wetlands, rivers and estuaries. Estimates of indirect N2O fluxes from leached N are derived from direct or indirect measurements, process-based models or empirical EFs. EFs for each system are based on a relatively large database for rivers, estuaries and aquifers. Few data are available for land drainage, constructed wetlands or riparian areas. There are different concepts of EFs and various methods have been used to estimate them. Despite the resulting difficulty in comparing the contribution of each system to total indirect N2O fluxes, some clear conclusions can be drawn:

• The highest EFs occur in more open systems such as riparian areas and constructed wetlands, where a relatively large fraction of N2O production can rapidly be released to the atmosphere.

• The lowest EFs occur in deeper aquifers, where produced N2O is almost trapped until groundwater is discharged to wells springs or streams.

• Current estimates of the total N2O emission from this pathway are lower than previously assumed, with a global total of 0.4Tg N per year or 11 per cent of total agricultural N2O fluxes.

• In view of the large variation within the limited data sets, these estimates are still uncertain.

• Mitigating indirect N2O fluxes by reducing emissions of reactive N to the environment is thus most effective in systems with the highest EFs, and vice versa. Principally there are some possible end-of-the-pipe measures to reduce indirect fluxes, but their effectiveness is probably relatively low.

Agricultural activities affect N2O emissions from natural and semi-natural ecosystems via volatilization of reactive nitrogen (NH3, NO/NO2) and following re-deposition. In many regions in Europe, Asia and North America, atmospheric N deposition to natural/semi-natural ecosystems is already exceeding the ecosystem net N demand. Part of the surplus nitrogen is emitted as N2O. Estimated EFs for N deposition are in a range of 1.4-5.4 per cent, significantly higher than suggested by IPCC (1 per cent). Published reports suggest that N2O emissions due to atmospheric N deposition are strongly affected by vegetation type, for example indirect N2O fluxes due to N deposition tend to be higher for deciduous forests as compared to coniferous forests. Existing estimates of nitrogen deposition effects on N2O emissions from natural and semi-natural terrestrial ecosystems are still uncertain and more long-term studies for contrasting environmental settings are needed to improve our understanding of mechanisms and environmental consequences of nitrogen deposition with regard to N2O, but also with regard to nitrate leaching, acidification or ecosystem carbon storage. The only promising mitigation strategy is a strategy to reduce nitrogen deposition. Finally, this means that losses of reactive nitrogen from agriculture, being one of the main sources of atmospheric NH3 and NOx, need to be minimized as far as possible.

To improve and validate the current knowledge of indirect agricultural N2O fluxes, it would be useful to combine process-based models with direct flux measurements. Moreover, to optimize mitigation strategies, there is a need to validate systems-specific EFs. This is necessary to better identify the stages of downstream flow where the retention of each unit of reactive N causes highest N2O fluxes.

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