The Nitrogen Cycle and Eutrophication

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The Nitrogen Cycle shows how Nitrogen is Passed on

  • Plants and animals need nitrogen to make proteins and nucleic acids (DNA and RNA)
  • Atmosphere is made up of about 78% nitrogen
  • Plants unable to use it in that form so need bacteria to convert it into nitrogen compounds
  • Shows how nitrogen is converted into a useable form and then passed on between different living organisms and the non-living environment
  • Includes food chains (nitrogen passed on when organisms are eaten) and four different processes that involve bacteria - nitrogen fixation, ammonification, nitrification and denitrification
  • Parts can be carried out artificially and on an industrial scale
  • Haber process produces ammonia from atmospheric nitrogen - used to make fertilisers
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Nitrogen fixation

  • When nitrogen gas in the atmosphere is turned into ammonia by bacteria called Rhizobium - can then be used by plants
  • Bacteria found inside root nodules (growths on the roots) of leguminous plants
  • Form mutualistic relationship with plants - provide plants with nitrogen compounds and plants provide them with carbohydrates
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  • When nitrogen compounds from dead organisms turned into ammonium compounds by decomposers
  • Animal waste also contains nitrogen compounds - turn into ammonium compounds by decomposers
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  • When ammonium compounds in the soil changed into nitrogen compounds which can then be used by plants
  • First nitrifying bacteria (e.g. Nitrosomonas) changed ammonium compounds into nitrites
  • Other nitrifying bacteria (Nitrobacter) change nitrites into nitrates
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  • When nitrites in the soil are converted into nitrogen gas by denitrifying bacteria - use nitrites in soil to carry out respiration and produce nitrogen gas
  • Happens under anaerobic conditions (no oxygen) e.g. waterlogged soils
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Nitrogen Fertilisers can Leach into Water and Caus

Leaching: when water-soluble compounds in the soil are washed away, e.g. by rain or irrigation systems into nearby ponds and rivers. - if nitrogen fertiliser is leached into waterways it can cause eutrophication

  • Nitrates leached from fertilised fields stimulate the growth of algae in ponds and rivers
  • Large amounts of algae block light from reaching plants below
  • Eventually plants die because they are unable to photosynthesise enough
  • Bacteria feed on the dead plant matter
  • Increased numbers of bacteria reduce oxygen concentration by carrying out aerobic respiration
  • Fish and other organisms die as there isn't enough dissolved oxygen
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