The Nitrogen Cycle (Edexcel GCSE Biology)

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)

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