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Nitrous oxide Sources - Oceans

a sunlit view of the southern ocean on a calm dayGlobally, oceans are thought to add around 3 million tonnes of nitrous oxide-N to the atmosphere each year. Like methane, much of the nitrous oxide in the surface waters of the oceans arises from microbial activity in and around sinking particles, such as fecal pellets. These particles provide the anaerobic conditions necessary for denitrification, a process which produces nitrous oxide as a by-product.

In some areas of the ocean, large areas of surface water can become oxygen depleted, allowing active denitrification in open water. Large amounts of oceanic nitrous oxide can also arise from denitrification in marine sediments, particularly in nutrient rich areas such as those of estuaries. Some nitrous oxide is also produced as a byproduct of nitrification in the world's oceans.

Human Impact

As with methane, man's impact on oceanic nitrous oxide emission lies 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 run-off, lead to eutrophic conditions in estuaries and coastal waters.

Such nutrient rich waters and sediments are ideal for denitrification and nitrous oxide production, with oxygen levels in the water often being very low and with plenty of nitrogen and organic carbon on which the denitrifying bacteria are able to grow. Increased atmospheric deposition of nitrogen on the surface of the world's oceans, again due to the activities of man, may also result in elevated oceanic nitrous oxide emissions.

Potential for control

Elevated nitrogen concentrations in estuaries and coastal waters, through sewage and agricultural run-off, has been the subject of increasing concern in recent years. Steps have been taken in some countries to reduce key nitrogenous pollutants, such as nitrate, 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 ever more intensive agriculture.

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