4.11B Nitrogen Cycle (Edexcel IGCSE Biology)

• Nitrogen as an element is required to make proteins
• Neither plants nor animals can absorb it from the air as N₂ gas is very stable and cannot be easily broken down
• Two ways that Nitrogen gas in the air can be converted into a usable form are:
  • 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 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
• Plants absorb nitrogen from the soil in the form of nitrates and use it to build proteins
• Animals eat the plants and get the nitrogen they need from the plant proteins
  • This nitrogen is passed up the food chain when secondary consumers eat primary consumers, etc.
• Waste (urine and faeces) from animals sends nitrogen back into the soil in the form of ammonium compounds
  • E.g. the urea in urine contains nitrogen
• The bodies of dead plants and animals decay and all the proteins inside them are broken down into ammonium compounds by decomposers
• The plants can’t absorb ammonium compounds, so nitrifying bacteria convert the ammonium compounds to nitrites and then to nitrates, which can then be absorbed by plants – and so the cycle goes on
• Denitrifying bacteria take nitrates out of the soil and convert them back into N₂ gas
  • This process reduces soil fertility and is bad for plant growth 
  • Denitrifying bacteria are anaerobic so aerating the soil, e.g. by reducing waterlogging and turning over the soil during ploughing, can reduce the rate of denitrification

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