Nitrogen Cycle

The nitrogen cycle is the biogeochemical cycle that describes the gradual transformation of nitrogen and nitrogen-containing compounds in nature. It is the means by which the supply of nitrogen is distributed in nature.

The Earth’s atmosphere, containing about 79percent nitrogen, constitutes the largest pool of nitrogen. Nitrogen is crucial to all life processes on earth. It is present in all amino acids, proteins and nucleic acids (RNA and DNA). Although nitrogen is abundant in the atmosphere and the majority of the air we breathe in is nitrogen (oxygen constitutes only 21percent of the air we breathe in), nitrogen is not readily available for cellular utilization. This is because the strong triple covalent bonds between the N atoms in N2 molecules make it relatively inert. By implication, biochemically available nitrogen is in short supply in natural ecosystems. Hence, plant growth and biomass accumulation are limited. In order for plants and animals to use nitrogen for their metabolic processes, N2 gas must be converted to a chemically available form such as ammonium (N H +4

), nitrate (N O 3

) or organic nitrogen such as urea,
(NH2)CO. The nitrogen cycle shown on the diagram on the post describes the movement of nitrogen among the atmosphere, biosphere and geosphere in different forms.

Basic Processes of the Nitrogen Cycle

(a) Nitrogen Fixation: This is the process by which the atmospheric nitrogen is converted into a form that is readily available to plants and subsequently to animals and humans. There are four ways of converting atmospheric nitrogen (N2) into more biochemically available forms.

  1. Biological Fixation: Symbiotic bacteria, e.g. Rhizobium, associated with the root nodules of leguminous plants and some free-living bacteria, e.g. Azotobacter, are able to covert (fix) free nitrogen to organic nitrogen.
  2. Industrial Fixation: In the industrial Haber-Bosch process, atmospheric nitrogen and hydrogen (obtained from natural gas or petroleum) are combined to form ammonia, NH3

  3. Combustion of fossil fuels: The exhaust fumes from internal combustion engines are made up of volatile matters including oxides of nitrogen.
  4. Electrical storms (lightning) and photolysis: During electrical storms, nitrogen is oxidized to NO, which is oxidized by ozone in the atmosphere to form NO2. NO2 in turn is reduced back to NO by photolysis. These reactions are important aspects of atmospheric chemistry, but they are inadequate for both terrestrial and aquatic nitrogen turnover.
  1. Assimilation: Plants can absorb N O 3
    or NH 4+
    ions from the soil

(Nitrogen uptake) through their roots. Absorbed nitrate is first reduced to nitrite ions and then ammonium ions for subsequent incorporation into amino acids, nucleic acids and chlorophyll. In leguminous plants with root nodules, nitrogen in the form of ammonium ions can readily be assimilated. Animals and human beings are incapable of utilizing nitrogen from the atmosphere or inorganic compounds hence, they depend on plants or other animals (except ruminants) that feed on plants, for their protein.

  1. Ammonification: At death, the proteins stored in the body of plants and animals become waste materials. Urine contains the nitrogen resulting from the metabolic breakdown of proteins in form of urea, (NH2)2CO. Urea is rapidly hydrolyzed by the enzyme to ammonium carbonate, (NH4)2CO3.

  1. Nitrification: The excess ammonia released by bacterial action on urea and proteins that are not used by plants is oxidized by the autotrophic nitrifying bacteria-Nitrosomonas and Nitrobacter. Under aerobic conditions, Nitrosamines convert ammonia to nitrite while nitrite is further oxidized to nitrate by Nitrobacter.

The bacteria derive energy from the oxidation processes. Some of the nitrate formed is used by plants while the excess is carried away in water percolating through the soil because the soil does not have the ability to hold nitrate for long.

It is important for the nitrite ions to be converted to nitrate ions because accumulated nitrites are toxic to plant life.

  1. Denitrification: Nitrate and nitrite are reduced under anaerobic conditions by pseudomonas and clostridium bacteria. Nitrate is reduced to nitrite while nitrite is reduced to ammonia. Most of the nitrate is later reduced to nitrogen thus, completing the nitrogen cycle. This constitutes a serious loss of fertilizing matter in soil when anaerobic conditions develop. Also, some

    Denitrifying bacteria produce N20 from nitrate reduction. The N20 produced enters the atmosphere and is reduced through photolysis to produce N2 and an excited state of oxygen, which oxidizes N2O to NO.

The Nitrogen Cycle

Source: C.N. Sawyer et al., 2006

Answer the following Question

1(a) what are the basic processes of nitrogen cycle?

(b) Discuss fully anyone of the processes named in 1(a) above.

  1. List and discuss the four ways by which atmospheric nitrogen can be converted into more biochemically available forms.

  2. In not more than two sentences, discuss the reason(s) why atmospheric nitrogen is not readily available for plants and animals metabolism despite its large atmospheric reservoir.
  3. Ground waters in most of the agricultural soils are noted for very high nitrate content. Briefly explain this observation.


CHM314 Environmental chemistry.

See also  Physical and Biological Basis of Man's Adaptation to His Environment

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