Nitrogen cycle ammonium

Manufacture of ammonium nitrate The nitrogen cycle Problems with fertilisers... 

NH4N03), ammonium phosphate ((NH4)3P04) and potassium chloride (KC1) in varying proportions (Figure 11.19). Fertilisers have an important role in the nitrogen cycle (see below). 

Ammonia is found in the environment as the result of natural and industrial processes. It is released into the environment by the breakdown of organic wastes, and it is a constituent of the soil, the atmosphere, and bodies of water. Ammonia is also a key intermediate in the nitrogen cycle and is a product of amino acid metabolism (WHO 1986). Anhydrous ammonia is used in the production of nitric acid, explosives, synthetic fibers, and fertilizers (Budavari 1989). It is used as a refrigerant as a corrosion inhibitor in the purification of water supplies in steel production as a catalyst for polymers as a preservative for latex and in the production of nitrocellulose, urea formaldehyde, sulfite cooking liquors, and nitroparaffins (ACGIH 1991 Lewis 1993). Ammonium hydroxide (10-35% ammonia) is a major constituent of many cleaning solutions. Ammonia... 

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 6 Microbial transformations of the nitrogen cycle. Pathways depicted are 1—N2 fixation 2—DIN assimilation 3—ammonium regeneration 4—nitrification 5—nitrate/nitrite reduction and 6—denitrification.

Figure 12 A diagram of the nitrogen cycle with catalyzing enzymes and metal requirements of each step. NIT, nitrogenase AMO, ammonium mono-oxygenase HAO, hydroxylamine oxidoreductase NAR, membrane-bound respiratory nitrate reductase NAP, periplasmic respiratory nitrate reductase NR, assimila-tory nitrate reductase NIR, respiratory nitrite reductase NiR, assimilatory nitrite reductase NOR, nitric oxide reductase N2OR, nitrous oxide reductase.

Nitrification is an aerobic microbial process by which specialized bacteria oxidize ammonium to nitrite and then to nitrate. Nitrification is a very important part of the nitrogen cycle, because for most plants nitrate is... 

Isotopes—Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties. Nitrogen cycle—series of chemical reactions by which elemental nitrogen in the atmosphere is converted to nitrates and ammonium compounds, those compounds are processed through the plant and animal world, and then are returned to the atmosphere as free nitrogen. 

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. 

Figure 28. Hypothetical anaerobic nitrogen cycle based on the following thermodynamically permissible reactions (1) ammonium oxidation to dinitrogen by carbon dioxide,. sulfate or ferric iron (no evidence at present, possibly kinetically limited) (2) dinitrogen fixation by various organic and inorganic reductants (known) (3) ammonium oxidation by nitrite or nitrate producing dinitrogen (known) (4) denitrification (known) (5) nitrite or nitrate respiration (known) (6) ferric iron oxidation of ammonium to nitrite or nitrate (no evidence at present) (7) nitrate assimilation (known) (8) ammonium assimilation and di.s,similation (known) (Fenchel etai, 1998).

Within the modern nitrogen cycle there is a variety of different metabolic processes, which give rise to a range of isotopic fractionations. These are summarized in Fig. 6.1 (after Papineau et al., 2005). Here we focus on two specific processes within the nitrogen cycle. First, is the production of ammonium ions, and second the process of denitrification. 

Fig. 3.17 Summary of the nitrogen cycle (oxidation states of nitrogen shown in parentheses). Ammonium assimilation and ammonification can occur in oxic and anoxic environments, as can nitrogen fixation (although the most prolific bacteria are aerobes).

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