The Nitrogen Cycle

Nitrogen has five valence electrons and can take on oxidation states between -t-5 and — 3. Most of the nitrogen compounds we will discuss either have nitrogen bonded to carbon and hydrogen, in which case the oxidation state of the nitrogen is negative (N is more electronegative than 

To fully understand some of the major players in the nitrogen cycle, we should also consider some of the industrial and social implications of these compounds. 

Nitric acid is a very strong acid about 6.8 million metric tons per year are manufactured for industrial purposes in the US. Most of it is produced from ammonia by the catalytic oxidation to NO, which is then further oxidized to NO2. Addition of water forms HNO3. Most of the nitric acid produced is used in the manufacture of fertilizers, and a lesser amount is used to make explosives. 

In the troposphere, nitrogen oxides react to also produce HNO3. The oxidants are free radicals produced photochemically, such as HO2, RO2, and OH. The HNO3 produced in this manner is an important contributor to acid rain.  

In its pure form, nitric acid is a liquid with a high vapor pressure (47.6 torr at 20°C), so that in the lower atmosphere HNO3 exists as a gas, in an aerosol or in a cloud droplet. When nitric acid reacts with a base a nitrate salt is produced, if 

The various redox reactions of inorganic nitrogen containing species are presented in Table 7-1. These reactions can be used to construct a pC- 

Denitrification [the conversion of NOs to N2(ag ] is a common reaction catalyzed by many bacteria and proceeds rapidly. However, it appears that microbial catalysts are unable to catalyze the reverse of the denitrification reaction (Ngca, ) — NOs ). 

The nitrifying bacteria, universally found in aerobic soil and aquatic environments, derive energy from the oxidation of reduced inorganic nitrogen compounds (ammonia and nitrite). As do autotrophic bacteria, they obtain carbon from carbon dioxide in the atmosphere. 

The process of dissimilatory dentrification occurs anaerobically and is mediated by bacteria that use nitrate in place of oxygen as an acceptor of electrons during respiration. The result is the formation of molecular nitrogen and nitrous oxide. The nitrous oxide plays a role in the chemistry of stratospheric ozone and is, therefore, extremely important bio-geochemically. These bacteria are heterotrophic and derive energy from the anaerobic oxidation of organic compounds. 

Many organisms, especially bacteria, decompose organic material with the release of ammonia, a process referred to as ammonification. 

The cyclic transformation of nitrogenous compounds is of great importance in the total turnover of this element in the biosphere. The main features of the biological nitrogen cycle are illustrated schematically in Fig. 4.16. Plants and algae assimilate nitrogen as either nitrate or ammonia to form 

Nitrification, followed by denitrification, is an important process in the removal of nitrogen from wastewaters. Removal of the nitrogenous constituents helps to minimize the toxicity of the water and to reduce its oxygen demand. The removal of nitrogenous material commences when the sewage is formed and almost all the urea is decomposed to ammonia and carbon dioxide. The toxic ammonia in a waste is first nitrified to nitrite and nitrate by aerobic biological processes and then denitrified anaerobically to molecular nitrogen. 

The maximum rates of nitrification for Nitrosomonas and Nitrobacter occur in the pH range 7-9. The nitrifiers may be sensitive to temperature since the extent of nitrification is usually less in winter than in summer. Nitrification in the activated sludge process can be controlled by a long aeration period and by retention of the solids. Nitrification also takes place during the treatment of organic wastes by the biological filtration process. 

The ability to carry out denitrification is known among.a broad range of facultative anaerobic and anaerobic bacteria such as Pseudomonas. Denitrification requires a source of carbon, and methanol is often used for this 

Symbiotic nitrogen fixation results from a mutualistic partnership between bacteria belonging to the genus Rhizobium and leguminous plants such as peas, beans, alfalfa, clover and lupine. 

An alternative bacterial route from NO to N2, where NH oxidation is coupled to NOj reduction in a process called anammox (anaerobic ammonium oxidation), dominates N2 production in many marine environments, but, unlike classical denitrification, it does not lead to the production of N2O. Together, denitrification and anammox close the nitrogen cycle by returning N2 gas back to the atmosphere. 

Amino acids Nucleotides Amino sugars Coenzymes Porphyrins 

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One of the things that environmental scientists do IS to keep track of important elements in the biosphere—in what form do these ele ments normally occur to what are they transformed and how are they returned to their normal state Careful studies have given clear although compli cated pictures of the nitrogen cycle the sulfur cy cle and the phosphorus cycle for example The carbon cycle begins and ends with atmospheric carbon dioxide It can be represented in an abbrevi ated form as... 

Fig. 1. Biological pathways and processes iavolved ia the nitrogen cycle.

The Royal Society, The Nitrogen Cycle of the United Kingdom A Study Group Report, The Royal Society, London, 1983. 

The continuous interchange of nitrogen between the atmosphere and the biosphere is called the nitrogen cycle. Global estimates are difficult to obtain and there are frequently regional and local impacts which vary greatly from the mean. However, some indication of the size of the various reservoirs of nitrogen in the atmosphere, on land, and in the seas is... 

C. C. Delwiche, The nitrogen cycle. Chap. 5 in C. L. Hamilton (ed.), Chemistry in the Environment, Readings from Scientific American, W. H. Freeman, San Francisco, 1973. 

There is also considerable current environmental interest in hyponitrite oxidation because it is implicated in the oxidation of ammonia to nitrite, an important step in the nitrogen cycle (p. 410). Specifically, it seems likely that the oxidation proceeds from ammonia through hydroxylamine and hyponitrous acid to nitrite (or N2O). 

The natural supply of nitrogen available to plants from the nitrogen cycle is limited. To meet the growing demands for agriculture crops, nitrogen is added to soil in the form of fertilizers. An estimated one-third of the human population is fed as a result of the use of synthetic fertilizers. 

Like sulfur, nitrogen has stable compounds in a wide range of oxidation states and many of them are foimd in the atmosphere. Again, both gaseous and particulate forms exist as do a large number of water-soluble compounds. Table 7-5 lists the gaseous forms. The nitrogen cycle is discussed in Chapter 12. 

The global nitrogen cycle is often referred to as the nitrogen cycles, since we can view the overall process as the result of the interactions of various biological and abiotic processes. Each of these processes, to a first approximation, can be considered as a self-contained cycle. We have already considered the biological cycle from this perspective (Fig. 12-1), and now we will look at the other processes, the ammonia cycle, the cycle, and the fixation/denitrification cycle. 

Fig. 12-2 Partitioning of the various forms of nitrogen in the atmosphere. Units are Tg N. (Reprinted with permission from R. Soderlund and T. Rosswall, The nitrogen cycles. In O. Huntizger (1982). "The Natural Environment and the Biogeochemical Cycles," p. 70, Springer-Verlag, Heidelberg.)...

How would the nitrogen cycle change if life on Earth were suddenly absent What would be the time scale for these changes ... 

Delwiche, C. C. (1981). The nitrogen cycle and nitrous oxide. In "Denitrification, Nitrification, and Atmospheric Nitrous Oxide" (C. C. Delwiche, ed.). Wiley, New York. 

Jaffe, D. A. (1992). The nitrogen cycle in global biogeochemical cycles. In "Global Biogeochemical Cycles" (S. S. Butcher, R. J. Charlson, G. H. Orians, G. V. Wolfe, eds). Academic Press, New York. 

Smil, V. (1997). Global Population and the Nitrogen Cycle. Sclent. Am. July, 76-81. 

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