Nutrient cycles

• introduction to nitrogen cycle

  • The nitrogen cycle shows how nitrogen is recycled in ecosystems
  • Plants and animals require nitrogen in order to produce proteins and nucleic acids (DNA and RNA)
  • About 78% of the atmosphere is actually nitrogen gas but plants and animals cannot access the nitrogen in this gaseous form
  • Instead, they rely on certain bacteria to convert the nitrogen gas into nitrogen-containing compounds, which can be taken up by plants
• What are the stages of the nitrogen cycle

  1. Nitrogen fixation
  2. ammonification
  3. nitrification
  4. denitrification
• Nitrogen fixation

  Nitrogen gas is converted into nitrogen-containing compounds (Amino acids)
• Mutualistic nitrogen fixing bacteria in root nodules

  • Nitrogen is fixed into ammonium ions and then converted into amino acids by bacteria
  • Bacteria provide the plants with nitrogen-containing compounds
  • Plants provide bacteria with organic compounds such as carbohydrates
• Ammonification

  • Nitrogen compounds in waste products (urine and faeces) and dead organism are converted into ammonia by saprobinots (a type of décomposer including some fungi and bacteria)
  • this ammonia forms ammonium ions in the soil
• nitrification 


  The ammonium ions in the soil are oxidised to nitrites then nitrates by nitrifying bacteria.
• denitrification 


  converts nitrogen in compounds back to nitrogen gas.

  this occurs in anaerobic conditions.
• Free living nitrogen fixing bacteria in the soil

  • Nitrogen gas is reduced to ammonium gas in the soil.
  • Amino acids are released when bacteria die