Nitrogen cycle major pathways

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. 

Figure 1. A simplified watershed nitrogen cycle, with major pathways (arrows) and their effects on the watershed hydrogen budget (numbers in circles) shown. Circled numbers represent the number of hydrogen ions transferred to the soil solution or surface water ( +1) or from the soil solution or surface water (-l)for every molecule of N03 or NH4 + that follows a given pathway. For example, nitrification follows the pathway for NH4 + assimilation into microbial biomass ( + l) and is leached out as N03 ( + 1), for a total hydrogen ion production of +2 for every molecule of N03 produced.

Fig. 1. Biological pathways and major participants of the nitrogen cycle.

The nitrogen source in the medium is the amino add glutamate. There are several cations K Mn2, Cn2, Zn2, Mg2, Co2, Fe2, Ca2 Mo6. Phosphate (POi") is the major anionic component. Fumaric add is a TCA cycle intermediate and may improve metabolic balance through the catabolic pathways and oxidation through the TCA cyde. Peptone may improve growth through the provision of growth factors (amino acids, vitamins, nudeotides). 

In mammals, hepatic arginase is the terminal enzyme of the urea cycle, which represents the major end-product of nitrogen metabolism — the average adult human excretes some 10 kg of urea per year. The enzyme is not restricted to the liver, since ornithine is a precursor of the nonessential amino acid proUne, and a biosynthetic precursor of polyamines, required for rapidly dividing tissues. Arginine is also the precursor of the important messenger in many vertebrate signal-transduction pathways nitric oxide, NO (Scheme 16.1), of which more shortly. 

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