Conclusion

In this study, we focus on bacterial reduction of N2O to N2 and discuss the likeliness of N2O uptake in different systems, including soils and aquatic systems. Our results indicate that soils are not only important sources of N2O, but can also act as a sink for atmospheric N2O. Global budget studies that fail to explicitly account for N2O sinks at the Earth's surface may not be a good basis for atmospheric budget studies and analyses of trends in emissions.

First, we defined when we consider a system a sink for atmospheric N2O, while taking into account the large variability in both N2O production and reduction in many systems. We consider a system a sink when the net N2O uptake occurs over a relatively large area and prolonged period of time.

Second, we described the processes involved and identified the most important factors influencing N2O uptake. Soil uptake of N2O is driven by denitrification by bacteria, converting N2O into N2. The most important factors affecting N2O uptake by soils are nitrogen availability, soil wetness and temperature. In addition, soil drainage conditions and soil pH are important. Based on the controlling factors, we argue that soils, surface waters and riparian zones may be potential sinks for atmospheric N2O, while groundwater systems are not likely to be major sinks.

Third, we presented an overview of a selection of studies reporting on observed

N2O uptake. We concluded that not many experimental studies on observed sinks exist. N2O uptake has been observed in soils, aquatic systems and riparian zones. In theory, soils are particularly powerful sinks. Whether or not a system actually acts as a sink, depends on local circumstances. From the available studies, we may conclude:

1. Although agricultural soils are usually fertilized and therefore not likely to be sinks for N2O, some studies report on N2O uptake in fertilized fields.

2. Several studies report on considerable N2O uptake in forest soils, indicating that these may potentially be important sinks for atmospheric N2O.

3. Riparian zones potentially have a large denitrification activity and depending on local conditions may be potential sinks for N2O.

4. Whether or not lakes can act as a sink is not clear from the available studies.

5. N2O uptake may occur in the open ocean.

In the future, the sink capacity of soils and aquatic systems may change. We showed that sink capacity of terrestrial and aquatic systems will be affected by trends in atmospheric deposition of nitrogen (in general less N2O uptake when nitrogen deposi tion increases), temperature (in general more N2O uptake at higher temperatures) and changes in annual net precipitation (in general more N2O uptake when the soil wetness increases). Moreover, the atmospheric concentration is important (in general more N2O uptake when the ambient concentration is higher). This indicates that the future N2O sink strength may be reduced by increasing atmospheric nitrogen deposition caused by industrial and agricultural emissions of nitrogen compounds. What the net effects of climatic changes on the N2O sink strength are, however, is not clear and depends on the net effect of local changes in temperature, net precipitation and atmospheric N2O increases.

Using the controlling factors mentioned above, we identified areas prone to high N2O sink activity in soils. Our results need to be interpreted with care because of the large uncertainties involved. Nevertheless, our analysis indicates that high sink activity may be more likely in northern regions of North America, Europe and Asia. The regions that are considered to have a 'high' or 'moderate' likeliness of N2O uptake cover ~16% of the global land area. Clearly, further studies are needed to confirm our results. Such studies could include experimental analyses as well as mechanistic models.

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