The Nitrogen Cycle (Edexcel GCSE Biology: Combined Science)

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The 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

  • The nitrogen cycle shows this conversion, as well as how the nitrogen in the nitrogen-containing compounds is then passed between trophic levels or between living organisms and the non-living environment

The role of bacteria in the nitrogen cycle

  • There are four key processes in the nitrogen cycle that are carried out by different types of bacteria
  • Nitrogen fixation:
    • Nitrogen fixing bacteria convert N2 gas into ammonium compounds, which can then be converted to usable nitrates
      • Nitrogen fixing bacteria can be free-living in the soil or can live within the root nodules of some plants
    • Lightning can also split the bond between the two N atoms, turning them into nitrous oxides like N2O and NO2 that dissolve in rainwater and leach into the soil
  • Ammonification:
    • Nitrogen compounds in waste products (e.g. urine and faeces) and dead organisms are converted into ammonia by saprobionts
      • These are decomposers, e.g. fungi and bacteria
    • This ammonia forms ammonium ions in the soil
  • Nitrification:
    • The ammonium ions in the soil are converted by nitrifying bacteria into nitrogen compounds that can be used by plants, known as nitrates
      • Initially, nitrifying bacteria convert ammonium ions into nitrites
      • Different nitrifying bacteria then convert these nitrites into nitrates
  • Denitrification:
    • Denitrifying bacteria use nitrates in the soil during respiration
    • This process produces nitrogen gas, which returns to the atmosphere
    • This process occurs in anaerobic conditions (when there is little or no oxygen available, such as in waterlogged soil)

nitrogen cycle

The nitrogen cycle involves nitrogen fixation, decomposition, nitrification and denitrification

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.