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Biogeocycles: Water Cycle, Carbon Cycle, Nitrogen Cycle

Mineral nutrients are always in circulation, moving from non-living to living & then back to non-living components of ecosystem in a more or less circular pattern.

Gaseous cycles: When the reservoir is atmosphere or hydrosphere for ex. Water cycle, Carbon cycle, Nitrogen cycle

Sedimentary cycles: When the reservoir is earth crust for ex. phosphorus (Central role in aquatic ecosystem), calcium & sulfur cycle.

The Water Cycle

Water Cycle

The Carbon Cycle

Carbon cycle

 Nitrogen Cycle

  • Nitrogen – an essential constituent of protein & is a basic building block of all living tissues (Constitutes 16% by weight of all proteins approximately)
  • Atmospheric nitrogen is in elemental form which cannot be used directly by plants, hence needs to be fixed i.e. to be converted to ammonia, nitrites or nitrates


Nitrogen fixation methods:

  • By microorganisms (Bacteria, Blue green algae etc)
  • By man-made fertilizers
  • By thunder & lightening
  • By Cosmic Radiations


Microorganisms capable of fixing atmospheric nitrogen into ammonium ions are Azobacter, Clostridium, Rhizobium, Blue green algae (Anabaena, Spirulina)

Ammonium ions can be taken up as a source of nitrogen by some plants & are oxidized to nitrites or nitrates by nitrifying bacteria viz.

  • Ammonia to Nitrites : By Nitrosomonas
  • Nitrites to Nitrates : By Nitrobacter

Nitrogen Cycle

  • Nitrates synthesized by bacteria in the soil are taken up by plants & are converted into amino acids, which are the building blocks of the proteins.
  • These now go to higher tropic levels & during excretion or upon death of all organisms, nitrogen is returned to soil in form of ammonia.
  • In soils as well as oceans there are special denitrifying bacteria (Pseudomonas) which convert nitrates/nitrites into elemental nitrogen, which escapes to atmosphere completing the cycle.
  • Amount of nitrogen fixed by man through industrial processes & through chemical fertilizers has far exceeded the amount fixed by natural cycle. As a result nitrogen has become a pollutant which leads to acid rains, eutrophication & harmful algae blooms.
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