Circulation in Ecosystems

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Circulation in Ecosystems

1. Decomposition
2. Nutrient Cycling

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• Materials in ecosystems need to be recycled and
  reused. Decomposition and decay play a role in
  this.

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Decomposition

• Decomposition is the process whereby organisms
  obtain their energy through breaking down dead
  organic matter, this releases inorganic
  nutrients. These are then available for uptake
  by plants and other primary producers.

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• 2 main types of decomposer
• - detritivores and decomposers

• Detritivores
• These are detritis eating invertebrates that
  break detritis down into humus.

Detritis organic debris e.g. leaf litter Humus
organic portion of soil that makes it fertile
and productive
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• Decomposers
• These are usually bacteria and fungi, they
  breakdown dead and decaying material into
  inorganic nutrients, carbon dioxide and water.
• e.g. nitrogen in protein is brokendown to
• ammonium or nitrate ions.

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• Decomposition is dependant upon and limited by

• Type of detritis. Deciduous tree material is
  faster at decaying than coniferous material.
• Type and abundance of decomposer
• Conditions e.g. temperature, moisture level,
  oxygen availability

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Nutrient Cycling

• The continual production and growth of new
  organisms requires a constant supply of raw
  materials. Resources on the planet are not
  infinite. The processes of death, decomposition
  and decay are equally as important as the
  processes of assimilation, production and growth

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• Nitrogen and phosphorous are essential elements
  in the construction of all organisms on earth.

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Nitrogen Cycle

• Nitrogen is found in 3 main forms in the
  ecosystem

• - nitrogen gas in the atmosphere
• - inorganic nitrogen compounds in the
  soil e.g. nitrate
• - organic compounds in organisms
  (protein)

The nitrogen cycle involves the transfer of
nitrogen between these 3 forms.
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• Nitrogen cycle basic steps

• Plants take up ammonium and nitrate minerals

• Cyanobacteria and leguminous plants e.g. clover,
  are able to take up atmospheric nitrogen and
  convert it into organic nitrogen (nitrogen
  fixation). The enzyme responsible for this
  conversion is nitrogenase. In cyanobacteria
  nitrogenase is contained in structures called
  heterocysts, in leguminous plants it is found in
  root nodules.

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Anabaena (cyanobacteria)
root nodules
heterocyst
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• Ammonia in the soil is converted into nitrite by
  bacteria e.g. nitrosomonas and nitrococcus.
  Nitrobacter then converts nitrite into nitrate.
  These 2 processes are referred to as
  nitrification.

• Nitrate is taken up by plants/primary producers
  and can be assimilated into amino acids or
  nucleic acids. These assimilated nitrogen
  products then pass along the food chain from one
  trophic level to another.

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• Bacteria are also responsible for decomposing
  dead organisms and animal waste into ammonia,
  called ammonification. This makes the ammonia
  available for bacteria carrying out the
  nitrification reactions.

• A process of denitrification can also occur by
  anaerobic denitrifying bacteria. This is an
  example of a loss of nitrogen from the cycle.

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Water Saturation and Oxygen Levels

• Decomposition and nitrification require oxygen.
  Therefore in water saturated areas little
  nitrogen is transferred into the nitrate and
  ammonia forms, making nitrogen slow down or stop.
  Under anaerobic conditions denitrification is
  favoured .

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• In aquatic systems the level of productivity is
  limited by the nutrient level of that system. Low
  phosphate levels, for example, will limit the
  rate of primary productivity. Phosphates are used
  by organisms to construct nucleic acids, ATP and
  membranes.

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• Phosphates have often been added to an aquatic
  system e.g. through fertilizer run off or sewage,
  and it has as a result increased the primary
  productivity levels of the aquatic system
  (increase in plant growth and decay). This
  produces large populations of algae algal
  blooms, which can lead to deoxygenation and
  consequently a loss of diversity.