Sinks for nitrous oxide at the earth’s surface

Controls of N₂O uptake in soils

We consider a system a sink when the net N₂O uptake occurs over a relatively large area and prolonged period of time. 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.

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. 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.

Changing sink activity

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.

Sink areas

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.