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.
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.