Nitrogen Cycling Processes

The biogeochemical cycling of nitrogen has been extensively studied in different ecosystems and the main processes are listed below. 

Generally we consider the 0.781 atm of nitrogen in the atmosphere almost as inert filler, because elemental nitrogen is rather unreactive. The known aspects of N biogeochemistry lead us to the conclusion that the atmospheric content of nitrogen, like carbon, is regulated principally by biological processes. 

The atmosphere contains about 3.9 x lO tons of elemental nitrogen. Biological nitrogen fixation and production of NO in combustion and thunderstorms are the major natural sinks. Finally, NO deposits as HNO3 in rainwater. In the Haber process N2 fixes industrially. 

Terrestrial and aquatic nitrogen in the form of NH4+and NOr are cycled through the biosphere to make proteins and nucleic acids. The processes of decay return the nitrogen to the atmosphere as N2 and as N2O by the action of denitrifying bacteria. Almost all the nitrogen fixed by the Haber process is used as fertilizer so that increased 

Most N2O is formed through biological denitrification in aerobic soils with anaerobic zones or in systems that are alternating between the anaerobic and anaerobic state (see below). 

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The influence of benthic macrophytes on nitrogen cycle processes... 

Methodologies Used for Measurements of Nitrogen Cycle Process Rates... 

Table 19.4 Impact of Various Physical Parameters and Chemical Species on Rates of Nitrogen Cycle Processes...

Figure 21.1 Microbial nitrogen cycling processes in sedimentary environments on a coral reef (A) nitrogen fixation (B) ammonification (C) nitrification (D) dissimilatory nitrate reduction and denitrification (E) assimilatory nitrite/nitrate reduction (F) ammonium immobilization and assimilation. Adapted from D Elia and Wiebe (1990). Anammox (the anaerobic oxidation of NH4" with NO2 yielding N2 ) is not represented, as it has not yet been shown to occur on coral reefs, but may be found to be important in reef sediments.

Figure 5.4 Schematic showing the key nitrogen cycle processes in intertidal sediments.

Clark, F. E., and T. Rosswall, eds. (1981). Terrestrial nitrogen cycles processes, ecosystem, strategies and management impacts. Ecol. Bull. (Stockholm) 33. 

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