Nitrification in Marine Systems

Methods for measuring nitrification rates in water and sediments 216 

Environmental Variables Affecting Nitrification Rates and Distributions 234 

Nitrification is the process whereby ammonium (NH4+) is oxidized to nitrite (N02 ) and then to nitrate (N03 ). It thus links the most oxidized and most reduced components of the nitrogen (N) redox cycle and helps determine the overall distributions of these important nutrients. Ammonium rarely occurs at significant concentrations in oxygenated habitats. It is recycled rapidly between heterotrophic and N2 fixing organisms (which excrete NH4+ directly or release organic N that is microbiaUy degraded to NH4+) and many heterotrophic and photosynthetic plankton (which utilize NH4+ as a N source) in the surface ocean. 

Similarly, N02 rarely accumulates in oxygenated habitats (see below for exceptions), although N02 is an essential intermediate in several oxidation and reduction processes in the N cycle. Nitrate, the end product of nitrification, however, accumulates in the deep ocean, and seasonally in the deep water of lakes, where there is no demand for inorganic N by phytoplankton. 

Several other useful reviews are available (Hagopian and Riley, 1998 Kaplan, 1983 Ward, 1986, 2000, 2002 Ward and O MuUan, 2005). The 2002 review contains expanded information on the molecular ecology of nitrifiers, the 2005 chapter contains detailed information on methods for both rate measurement and molecular ecological investigations of AOB, and the present chapter contains new information on the NH3-oxidizing Archaea, which were unknown at the times of the earher reviews. 

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