The oceanic source

Globally, oceans add ~3 Tg N2O/year to the atmosphere. Like CH4, much of the N2O in the surface waters of the oceans arises from micro-bial activity in and around sinking particles, such as faecal pellets. These particles provide the anaerobic conditions necessary for deni-trification. Denitrification is a predominantly

21%

31%

6%

42%

□ Oceans

□ Tropical soils

■ Atmosphere

□ Temperate soils

Fig. 13.1. Natural sources of nitrous oxide (N2O).

Fig. 13.1. Natural sources of nitrous oxide (N2O).

microbially mediated process where bacteria use nitrate, rather than oxygen, as a terminal electron acceptor to respire organic matter and hence gain energy. In doing so, they reduce nitrate to nitrogen gas (N2). N2O is one of the intermediate steps in the process and, where the reduction of nitrate to N2 is incomplete (as is invariably the case), significant production of N2O can result.

In some areas of the ocean, surface waters can become oxygen-depleted allowing active denitrification and N2O production in open water. Large amounts of oceanic N2O can also arise from denitrification in marine sediments, particularly in nutrient-rich areas such as those of estuaries. Some N2O is also produced as a by-product of nitrification in the world's oceans. The rate of emission of this dissolved N2O from ocean waters is dependent on its outgassing from the surface waters to the atmosphere, which can be very rapid when wind speeds are high and the surface of the ocean is well mixed.

As with CH4, there can be significant human impacts on 'natural' oceanic N2O emissions. These lie primarily in our effect on oceanic nutrient inputs through rivers and estuaries. The nitrogen-rich waters of many rivers, produced by sewage input and agricultural runoff, can lead to eutrophic conditions in estuaries and coastal waters. Such nutrient-rich waters and sediments are ideal for denitrifica-

tion and hence N2O production, with oxygen levels in the water often being very low, and with plenty of nitrogen and organic carbon on which the denitrifying bacteria grow. Increased atmospheric deposition of reactive nitrogen to the surface of the world's oceans, again due to the activities of humans, may also result in increased oceanic N2O emissions.

Elevated nitrogen concentrations in estuaries and coastal waters, through sewage and agricultural runoff, have been the subject of increasing concern in recent years. Steps have been taken in some countries to reduce key nitrogenous pollutants, such as nitrate fertilizers, through better land management practices. Similarly, efficient sewage treatment has also led to reductions in levels of coastal eutrophication in some areas. However, in many parts of the world such coastal nitrogen pollution continues to increase in line with a growing human population and the need for an ever more intensive agriculture.

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