The Nitrogen Cycle (Cambridge (CIE) O Level Biology): Revision Note

The Nitrogen Cycle

• 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

  • The bonds holding the nitrogen atoms together would need massive amounts of energy to break (the two nitrogen atoms in a nitrogen molecule are held together by a triple covalent bond)

• However, there are two ways it can be taken out of the air and converted into something easier to absorb:

  • Nitrogen-fixing bacteria found ‘free-living’ in soil and also in the root nodules of certain plants (peas, beans, clover – called leguminous plants) take N₂ gas and change it into nitrates in the soil

  • Lightning can ‘fix’ N₂ gas, splitting the bond between the two atoms and turning them into nitrous oxides like N₂O and NO₂ that dissolve in rainwater and ‘leach’ into the soil

• Plants absorb the nitrates they find in the soil and use the nitrogen in them to make proteins

• Animals eat the plants (or other animals) and get the nitrogen they need from the proteins in the plant or animal

• Waste (urine and faeces) from animals sends nitrogen back into the soil as ammonium compounds (the urea in urine contains nitrogen)

• When the animals and plants die, they decay and all the proteins inside them are broken down into ammonium compounds and put back into the soil by decomposers

• The plants can’t absorb ammonium compounds though, so a second type of soil bacteria, 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

• Finally, there is a third, unhelpful type of (anaerobic) bacteria called denitrifying bacteria found in poorly aerated soil (ie not much oxygen)

• These bacteria take the nitrates out of the soil and convert them back into N₂ gas

• Farmers can help reduce the amount of these unhelpful bacteria by ploughing and turning over soil

Nitrogen Cycle Diagram

 The Nitrogen Cycle