Soils, as we have seen, are important sources of N2O, but they can also act as a sink for atmospheric N2O. Soil uptake of N2O is driven by denitrification by bacteria, which convert N2O into nitrogen gas (N2). In many global budgets this bacterial reduction of N2O is not explicitly accounted for, but simply included in the total net flux of N2O from soils. This may, however, lead to errors in estimates of atmospheric budgets and trends in emissions.
Kroeze et al. (Chapter 15, this volume) analyse the processes underlying bacterial reduction of N2O to N2, and discuss the likeliness of N2O uptake in different systems. They focus not only on soils, but also on aquatic systems, including ground-water systems, riparian zones and surface waters. Here a given system is considered a sink when the net N2O uptake occurs over a relatively large area and prolonged period of time. Their results indicate that soils, surface waters and riparian zones may be potential sinks for atmospheric N2O, while groundwater systems are not likely to be major sinks. Whether or not a system acts as a sink depends on local circumstances. 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 these factors, Kroeze et al. hypothesize that areas prone to high N2O sink activity in soils may be located in northern regions of the Earth.
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