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

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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