Chapters :
  • Biogeochemical Cycles -01
  • Nitrogen Cycle
  • Sulphur cycle
  • Phosphorous cycle

Biogeochemical Cycles -03

Nitrogen Cycle Nitrogen is important for living organisms to produce a number of complex organic compounds like amino acids, building blocks of proteins and nucleic acids (DNA and RNA). Though nitrogen is abundantly available in atmosphere as dinitrogen (N2), it can not be directly used by living organisms. It needs to be fixed before being utilised by the primary producers, i.e. plants. In the same way, before being released in the atmosphere in the end, it should again be converted in the form it originally was. This complete transfer of nitrogen from the atmosphere to living beings and then back to the atmosphere represents the nitrogen cycle.

The main processes involved in the nitrogen cycle

Nitrogen fixation It involves the conversion of gaseous nitrogen into ammonia, which can be used by plants. It can be done by following methods-
  • Atmospheric fixation– This is done by lightening, combustion and volcanic activity.
  • Industrial fixation– This is done in industries at high temperature and high-pressure where nitrogen molecule is broken into atomic nitrogen and combines with hydrogen to form ammonia. This is also known as Haber’s Process.
  • Bacterial fixation– Symbiotic and free-living bacterias can combine atmospheric or dissolved nitrogen to form ammonia. Rhizobium in roots of the leguminous plant is a symbiotic bacterium and nostac, acetobacter is an example of free-living bacteria.
Note: Symbiotic relationships are those relationships where both the organisms are benefitted from each other. The classic example of symbiotic relationship can be seen as Lichen which shows mutualism between algae and fungi.


The process in which ammonia is converted into nitrates and nitrites by Nitrosomonas and Nitrococcus bacteria respectively. Nitrobacter can convert nitrate into nitrites.


Nitrogen fixed by plants is converted into organic molecules such as DNA, RNA etc. which forms plant and animal tissues. 


  • Nitrogenous waste products such as urea and uric acid produced by living organisms, waste products and dead remains of organisms are converted back into inorganic ammonia by the bacteria.
  • Ammonifyingbacterias like Clostridium, Pseudomonas, Streptomyces etc. help in this process.
  • The conversion of nitrates back into gaseous nitrogen is called denitrification. This process is the reverse of nitrogen fixation.
  • This can even lower the fertility of soil because nitrogen, which is essential for the growth of plants, is removed from the soil and is lost to the atmosphere. 
Phosphorous cycle The transport and chemical transformation of phosphorus through the lithosphere, hydrosphere and biosphere are called phosphorus cycle. The atmosphere does not play a significant role in the movement of phosphorus because phosphorus or phosphorus-based compounds are solids available in normal ranges of temperature and pressure of the earth. Most of the phosphorus remains within rocks, sediments, sand and the ocean floor with a fraction in living biomass. Phosphorus moves along trophic levels in an ecosystem by plant growth, herbivores and carnivores. Note- Phosphates are effective fertilizers but they also cause pollution in lakes and streams. Over enrichment of it can lead to algae blooms. This excess of algae causes increased consumption by bacteria which lead to higher bacterial concentration. In this process, bacteria use much of dissolved oxygen in the water for cellular respiration and cause the death of fish due to oxygen deprivation.

Sulphur cycle

Sulphur Cycle is a type of sedimentary cycle which is sediments based. It does not involve circulation (in the form of gases) through the atmosphere as in case of gaseous cycles. It consists of all such processes through which sulphur is transferred from rocks to the living systems and vice versa. Sulphur is used in the process of proteins and vitamins production. Proteins consist of amino acids that contain sulphur atoms like thiophene. When sulphur is dissolved in water, plants absorb them. Animal consume these plants so that they take up enough sulphur to maintain their health.
  • Most of the earth’s sulphur tied up in the rocks and salts or buried deep in the ocean in oceanic sediments.
  • Sulphur can also be found in the atmosphere. It enters the atmosphere by both natural and human sources.
  • Natural sources can be volcanic eruptions, bacterial processes and evaporation from water or decaying organisms.
  • Human activities mainly from industrial purposes where sulphur dioxide and hydrogen sulphide gases are emitted on a wide scale.
  • When sulphur dioxide enters the atmosphere it reacts with oxygen to produce sulphur trioxide or with other chemicals to make sulphur salts. Sulphur dioxide also reacts with water to produce sulphuric acid. All these particles react with rain and fall back onto Earth as acid deposition.
  • The particles then are absorbed by plants again and are released back into the atmosphere and then sulphur cycle will start over again.

 The entire Earth biosphere is a closed system so that nutrients are neither imported nor exported from the biosphere. The biogeochemical cycle also referred to as the cycle of nature because they link together all organisms and abiotic component.
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