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Biogeochemical Cycle and Important Biogeochemical Cycles

Biogeochemical Cycle and Important Biogeochemical Cycles


  • A biogeochemical cycle or substance turnover or cycling of substances is a pathway by which a chemical substance moves through both the biotic (biosphere) and abiotic (lithosphere, atmosphere and hydrosphere) components of Earth.
  • Carbon, hydrogen, oxygen, nitrogen and phosphorus constitute 97% of the mass of our body and more than 95% of the mass of all the living organisms.
  • In addition, 15 to 25 other elements are needed by plants and animals. These elements keep on circulating in the environment.
  • On the basis of replacement period of nutrients, biogeochemical cycles are classified as perfect and imperfect cycles.
  • A perfect cycle is a cycle in which nutrients are replaced as fast as they are utilised. Most gaseous cycles are perfect cycles.
  • An imperfect cycle is a cycle in which nutrients remain stored for a long period and thus, could not be brought to utilisation.


Water Cycle (Hydrological Cycle)                                                                                                                     

The water cycle, also known as the hydrological cycle or the H2O cycle, describes the continuous movement of water on, above and below the surface of the earth.

The water cycle is explained below:

  • First, water on the surface of the earth evaporates and is converted into water vapour. Similarly, plants, through the process of transpiration convert water into vapour form.
  • Then, water in the form of water vapour gets collected in the sky. This water rises upwards and forms clouds.
  • Next, the water in the clouds gets colder and gets converted into liquid again. This process is called condensation.
  • Then, the water falls from the sky as rain, snow, sleet or hail. This process is called precipitation.
  • The water sinks into the surface and also gets collected in the lakes, oceans, or aquifers. It again gets converted into vapour form through evaporation and transpiration. The process continues forever.                                          Biogeochemical Cycle and Important Biogeochemical Cycles

Nitrogen Cycle

  • Our atmosphere is comprised of a large amount of nitrogen (78% of atmosphere). It is the building block of all the living organisms.
  • Sixteen percent of proteins by weight in the living organisms are comprised of nitrogen.
  • Even though nitrogen is available in the atmosphere in large quantities, it cannot be used directly by most of the living organisms.
  • Atmospheric nitrogen needs to be fixed or converted into ammonia (NH4), nitrites (NO2) and nitrates (NO3) before it is taken up by the plants.
  • Plants further convert these nitrogen compounds into amino acids and proteins which are consumed by human beings and animals.
  • Nitrogen fixation can be accomplished in these ways:
  • By man-made activities such as production of fertilisers.
  • Small amount of nitrogen is also converted through natural processes such as thunder and lightning.
  • Natural processes convert atmospheric nitrogen to ammonia and nitrates which fall upon the earth through precipitation. Ammonia and nitrates are consumed by various plants.
  • The amount of nitrogen fixed by man-made activities far exceeds than the nitrogen fixed by the natural process.
  • Certain microorganisms are capable of fixing atmospheric nitrogen into ammonia such as free-living nitrifying bacteria like aerobic Azotobacter and anaerobic Nitrifying bacteria such as Rhizobium, which converts nitrogen into simple compounds, live in symbiotic relationship with leguminous crops (i.e. pulses, etc.)
  • Nitrifying bacteria such as blue green algae also lives in symbiotic relationship with non-leguminous plants.
  • Ammonia can be directly taken by some plants or oxidised to nitrite or nitrates by some types of bacteria before being utilised by the plants. Ammonia is transformed into nitrites by a bacteria called Nitrosomonas.
  • Nitrites are further transformed into nitrates by the bacteria called Nitrobacter.
  • The nitrates are taken up by the plants and converted into amino acids which are used to build proteins.
  • These amino acids are eaten by herbivores which are further eaten by the organisms present at the higher levels of food chain.
  • During excretion and on the death of an organism, the nitrogen is returned to soil in the form of ammonia.
  • Denitrifying bacteria such as Pseudomonas convert the nitrates in the soil to free atmospheric nitrogen. Pseudomonas can reduce the amount of fixed nitrogen by up to 50 percent, depleting the soil fertility and reducing the agricultural productivity.
  • Without denitrification, the Earth’s supply of nitrogen would eventually accumulate in the oceans, since nitrates are highly soluble and are continuously leached from the soil into the nearby bodies of water.                            Biogeochemical Cycle and Important Biogeochemical Cycles

Carbon (C) Cycle

  • Carbon cycle is classified as a short-term and a long-term cycle. Short-term carbon cycle involves the movement of carbon from animals to plants.
  • Carbon dioxide released by animals through respiration is taken up by plants during the process of photosynthesis. Plants in turn produce carbohydrates and oxygen which are consumed by animals.
  • Some of the carbon enters the long-term carbon cycle. Carbon enters the long-term cycle in the following ways:
  1. It accumulates in soil in the form of organic content.                                                          Biogeochemical Cycle and Important Biogeochemical Cycles
  2. Mineral carbonates such as calcium carbonates (limestone) remain stored in the earth’s crust for a long period of time.
  3. In deep oceans, organic content in the soil and mineral carbonate remain stored over millions of years.
  4. Carbon present in the dead plants and animals remain stored as a part of fossil fuels for millions of years. This carbon is released into the atmosphere only when fossil fuels are burnt.
  5. Moreover, some of the carbon from atmosphere is also transferred to ocean waters.
  6. Some of the atmospheric carbon remains stored in polar ice caps in the form of dry ice.


Environment & Biodiversity

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