Nitrogen Cycle

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Why do plants need Nitrogen?


  • ›Plants need a source of Nitrogen as this is used to form amino acids and proteins.


  • ›Plants can’t use N2 from the air as N2 is a very stable molecule – it requires high voltage from lightning to form NO3- naturally.
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Rhizobium - Nitrogen Fixing Bacteria


  • Rhizobium bacteria lives mainly in a symbiotic relationship with leguminous plants in the root nodules (but it can be free living).
  • When legumes germinate they produce Lectins which bind to the bacteria, which colonise the roots, causing the roots to produce nodules
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Ammonia to Nitrate

  • ›Nitrosomonass bacteria oxidises the NH4+ into NO2-  


  •  Nitrobacter bacteria further oxidises the NO2- into NO3-  


  • ›Denitrifying bacteria then converts the NO3- into  N2   ›
  • The above bacteria are free-living in the soil.  Denitrifying bacteria particularly lives in boggy/wet areas -- which explains why Venus Flytraps need to digest other organisms to get a good supply of nitrates as the soil will have very little.
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Biological Oxygen Demand and Eutrophication

When abnormally high levels of aerobic bacterial activity takes place, the level of dissolved oxygen can drop dramatically to hypoxic levels, causing Eutrophication.

Eutrophication is the addition of nitrites/ates to aquatic ecosystems forming a bloom in phytoplankton, algae or cyanobacteria.

Under eutrophic conditions, dissolved oxygen greatly increases during the day, but is greatly reduced after dark by the respiring algae and by microorganisms that feed on the increasing mass of dead algae.

When dissolved oxygen levels decline to hypoxic levels, fish and other marine animals suffocate. As a result, creatures such as fish, shrimp, and especially immobile bottom dwellers die off.

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