Inorganic nutrients occur in limited quantities and their loss to an ecosystem or retention and re-use is of great importance. The cycles of chemical elements in an ecosystem are known as nutrient cycles. If there is no loss to the ecosystem the cycle is said to be a 'perfect cycle' and if loss does occur the cycle is said to be 'imperfect'. The decomposers play an important role in these cycles because they break down dead organisms and make the nutrient components available once more to other organisms.

The carbon and nitrogen cycle are two such cycles.

• The Carbon Cycle
  

  All organic compounds contain carbon and the most important sources of all inorganic carbon is carbon dioxide in the atmosphere.

  • carbon dioxide is taken up by autotrophic organisms during photosynthesis and the carbon is incorporated into carbohydrates and other compounds , such as proteins and fats;
  • consumers (heterotrophic organisms) feed on plants, and their bodies assimilate carbon compounds derived from the plants;
  • all organisms, including plants, release carbon dioxide during respiration as a by product. (Fermentation releases of carbon dioxide);
  • when autotrophic and heterotrophic organisms die or lose body parts such as leaves, carbon dioxide is released as a result of decomposition;
  • combustion of dead animal and plant material also releases carbon dioxide;
  • under high pressures, dead plants and animals are carbonized, forming fossil-fuels, such as coal and crude-oil. These release carbon dioxide during combustion.

  A Diagrammatic representation of the Carbon Cycle

• The Nitrogen cycle
  

  Nitrogen is an element essential in all organisms, occurring in proteins and other nitrogenous compounds, e.g. nucleic acids. Although organisms live in nitrogen-rich environments (78% of the atmosphere is nitrogen) the gaseous forms of nitrogen can only be used by certain organisms. Free nitrogen must first be fixed into a useable form.

  • free nitrogen in the atmosphere is mainly fixed by two groups of bacteria, nl. Azotobacter and Clostridium. The nitrogen is then used to manufacture proteins in their bodies, when they die, their proteins are broken down by decomposers (mainly bacteria and other micro-organisms), and converted into ammonia (blue-green algae, cyanobacteria, can also be use free nitrogen from the atmosphere);
  • during electrical changes in the atmosphere(e.g. lightning), free nitrogen is fixed (combined) finally forming nitrate;
  • nitrates are taken up by plants which use them to manufacture proteins;
  • animals (herbivores) eat plants and convert plant proteins to animal proteins, while carnivores obtain their plant proteins by indirect means (by eating herbivores);
  • when plants and animals die, the proteins in their bodies are broken down into ammonia by decomposers. The process is known as ammonification;
  • ammonia is converted to nitrites by nitrite bacteria (Nitrosomonas and Nitrosococcus). Nitrites are again converted to nitrates by nitrate bacteria (Nitrobacter )This process is known as nitrification;
  • different types of bacteria are also able to break down nitrates, nitrites and ammonia which results in the release of nitrogen. This process is known as denitrification.

A Diagrammatic Representation of the Nitrogen Cycle.

These two cycles emphasizes the mutual interdependence of producers, consumers and decomposers in an ecosystem.

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