Biogeochemical cycle of nitrogen

Nitrogen is an essential element for all forms of life and its biogeochemical cycle is one of the most important in the modem biosphere. It is a structural component of amino acids from which proteins are synthesized. Animal and human tissue (muscle, skin, hair, etc.), enzymes and many hormones are composed mainly of proteins. 

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

The atmosphere contains about 3.9 x 10 tons of elemental nitrogen. Biological nitrogen fixation and production of NO in combustion and thunderstorms are the major natural sinks (reaction (1)). Finally, NO deposits as HNO3 in rainwater. In the Haber process N2 fixes industrially (reaction (2)) 

31 ppmv over the past 25 years. Nitrous oxide, N2O, is rather non-reactive and has a residence time of 20 yr. This species contributes to ozone depletion potential (see Chapter 6). 

Species Valence Boiling point, °C kJ/mol, 298K AQ (f),kJ/mol, 298K 

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The biogeochemical cycling of nitrogen is very much controlled by redox reactions. This perspective is presented in Figure 24.3 for the redox reactions that take place in the water column and sediments. The major pathways of reduction are nitrogen fixation, assimilatory nitrogen reduction, and denitrification. The major oxidation processes are nitrification and anaerobic ammonium oxidation (anammox). Each of these is described next in further detail. 

Table 4.6. Estimates of some parameters of the global biogeochemical cycle of nitrogen in the biosphere.

Savchuk, O.P., and Wulff, F. (2001) A model of the biogeochemical cycles of nitrogen and phosphorus in the Baltic. In Ecological Studies, A System Analysis of the Baltic Sea (Wulff, F., ed.), pp. 374-415, Springer-Verlag, Berlin. 

Figure 11 Schematic representation of the biogeochemical cycle of nitrogen, indicating the approximate magnitude of fluxes and reservoirs (After O Neill. )...

Table 17 depicts data on the global biogeochemical cycle of nitrogen calculated by various authors. Describe these results and explain the possible reasons of data deviations. 

In the major extent the natural biogeochemical cycle of nitrogen has been changed in agroecosystems. Discuss this problem by taking into acconnt the small catchment approach. 

Consider the disturbance of biogeochemical cycle of nitrogen and explain why leaching plays the most importance role in environmental biogeochemistry of this element. 

Discnss the biogeochemical cycle of nitrogen in mixed ecosystems. Stress your attention on wetlands and upland landscapes. 

Discuss the accumulation of nitrate nitrogen in ground waters of different regions. Why may these waters be considered as biogeochemical barrier in environmental biogeochemical cycle of nitrogen ... 

Bernhard-Reversat, F. (1982). Biogeochemical cycle of nitrogen in a semi-arid. savanna. Oikos 38, 321 -332. 

Nitroxyl (HNO/NO ) heme-model complexes ( FeNO , according to the Enemark-Feltham notation) have received special attention due to the intermediacy of nitroxyl-heme adducts in a variety of catalytic processes related to the biogeochemical cycle of nitrogen (104). For example, for the six-electron reduction of nitrite to ammonia that is catalvzed by cytochrome c nitrite reductase (ccNir), a heme FeNO complex is proposed as an intermediate (Scheme 5) (105,106). This intermediate has also been suggested for the reduction of NO to N2O by P450nor (Scheme 6) (107). Then, the isolation of a suitable FeNO heme complex that allows structural and functional characterizations will help to imderstand the reaction mechanism of ccNir and other enz5mies. 

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