Nitrogen cycle, major reactions

Summary of the major reactions of the nitrogen cycle. From Sprent (1987). 

Nitrogen is returned to its atmospheric form by the action of denitrifying bacteria such as Pseudomonas thiobacillus and Micrococcus denitriflcans. The process is referred to as denitrification and represents the major mechanism of nitrogen loss in the overall nitrogen cycle whereby various forms of nitrogen in the soil revert to the N2 form. The reactions and their energetics are given below ... 

The hydroxyl radical so produced is the major oxidising species in the troposphere, and a complete picture of its chemistry holds the key to furthering progress in understanding tropospheric chemistry. The chemistry discussed in detail elsewhere, is of course very complex. To take, for example, the cycle of reactions with carbon monoxide, which may be net producers or destroyers of tropospheric ozone depending upon the concentration of oxides of nitrogen present. In the presence of NO, the cycle (16)-(20) occurs, without loss of OH or NO, whereas at low NO concentrations, the cycle (17), (18) and (21), again without loss of OH. 

Figure 29.2 Schematic overview of the marine nitrogen cycle. A Important species, their oxidation state (vertical axis), and major biological transformations of nitrogen (arrows). B Typical values of the isotopic enrichment factor (e) are shown for reactions that have been characterized isotopically. Estimates of 6 were drawn from the available literature on N2-fixation and the of diazotrophs (Carpenter et al, 1997 Delwiche and Steyn, 1970 Hoering and Ford, 1960 Macko et al., 1987 Montoya et ah, 2002), denitrification (Barford et ah, 1999 Cline and Kaplan, 1975 Delwiche and Steyn, 1970 Mariotti et ah, 1981,1982 McCready et ah, 1983 Miyake and Wada, 1971 Voss et ah, 2001 Wada, 1980 ), nitrification (Delwiche and Steyn, 1970 Mariotti et ah, 1981 Miyake and Wada, 1971 Ybshida, 1988), N03 uptake (Montoya and McCarthy, 1995 Needoba et ah, 2003 Needoba and Harrison, 2004 Pennock et ah, 1996,1998 Wada and Hattori, 1978 Waser et ah, 1998a, 1998b ), NO2 uptake (Wada and Hattori, 1978 Wada, 1980), NH4 uptake (Cifuentes et ah, 1989 Montoya et ah, 1991 Pennock et ah, 1988 Wada, 1980 Wada and Hattori, 1978), and zooplankton excretion (Checkley and Miller, 1989).

In the stratosphere, the region of the atmosphere from 10 up to 50 km, ozone is synthesized from dioxygen in a complex cycle of reactions. Thanks to its strong absorption of ultraviolet radiation in the range between 232 and 290 nm, ozone provides a protective shield for the plant and animal life on the earth s surface. Because no other atmospheric species can absorb the intense radiation in this range, the ozone layer is absolutely critical to the earth s inhabitants and has been the focus of numerous investigations. A major concern is the possible depletion of the ozone layer by nitrogen oxides from automobile exhausts and by Freons from aerosal sprays. 

The extent of formation of these NOC depends upon the presence of nitrogen oxides present in the atmosphere during the manufacturing cycle. The major contaminants are NDMA, A-nitrosodiethylamine (NDEA), A-nitrosopyrrolidine (NPYR), NMOR, A-nitrosodiphenylamine (NDPhA), A-nitrosopiperidine (NPIP) and A-nitrosodibutylamine (NDBA)68. NMOR was found in the hot process areas NDMA occurred in tube production areas in which NDPhA was being used as retarder and tetramethylthiuram disulphide as an accelerator. Figure 12 shows a proposed reaction scheme of formation of NOC in the rubber industry and subsequent exposure67. 

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. 

In recent years, tremendous progress has been achieved in the analysis of the isotope composition of important trace compounds in the atmosphere. The major elements - nitrogen, oxygen, carbon - continually break apart and recombine in a multitude of photochemical reactions, which have the potential to produce isotope fractionations (Kaye 1987). Isotope analysis is increasingly employed in studies of the cycles of atmospheric trace gases e.g., CH4 and N2O, which can give insights into sources and sinks and transport processes of these compounds. The rationale is that various sources have characteristic isotope ratios and that sink processes are accompanied by isotope fractionation. 

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