In aquatic environments, the requirements for denitrifica-tion are the same as in terrestrial systems: a nitrogen and carbon source and low O2 concentrations. The major end product of denitrification in aquatic systems is N2, but surface layers of the water columns are also often enriched in N2O in relation to the atmosphere, thus leading to the emission of N2O from the water. Denitrification occurs predominately in the top few mm of the sediments and rates of denitrification depend on the concentrations of organic C and N. Largest denitrification rates can be expected from the most polluted rivers and lakes and estuaries followed by coastal shelves and lowest rates from the open oceans. The main sources of NO3 are drainage and runoff from agriculturally managed fields and effluents from sewage farms and industries. For rivers, denitrification rates were shown to be inversely related to channel size, which would regulate the residence time of the NO3.
Estuaries have a relatively high organic carbon content compared to many other aquatic systems, due to high rates of sedimentation and upwelling. They can denitrify between 20% and 50% (and more in isolated cases) of the NO3 added by the rivers and thus stop the export of terrestrial N pollution to open oceans. Denitrification appears to be independent of salinity over the range of 1-13 parts per trillion (ppt), but directly dependent on nitrate concentration.
In the interior of oceans, away from the euphotic surface layer and coastal shelves, nitrification and denitrification rates are primarily dependent on the mineralization of nitrogen-rich organic matter originating from N2 fixing organisms. The demand for anaerobic conditions is required in the same way as for terrestrial systems. Denitrification rates in benthic sediments are most sensitive to concentrations of labile organic carbon at the sediment-water interface and have been shown to increase with nitrate concentrations. The major areas of low O2 are the thermocline of the Arabian Sea and the eastern tropical South and North Pacific. Many regions of the world's oceans do not denitrify or fix nitrogen.
Measurements ofthe isotopic ratios 14/15N and 16/18O of N2O originating from surface and from deep waters in the subtropical North Pacific Ocean have revealed large differences. Surface water N2O was depleted in 15N and 18O compared to atmospheric and deep water N2O. This implies that nitrification, rather than denitrification, is probably the main source of surface water N2O in open oceans.
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