The nitrogen cycle

The nitrogen cycle

1. Nitrogen fixation

   • Mutualistic nitrogen fixing bacteria, otherwise known as bacteria in root nodules (legumes, peas, beans, lentils and clover) reduce gaseous nitrogen to ammonia, similarly to the free living nitrogen fixing bacteria.
     • The bacteria synthesises the nitrogen into amino acids, and the plant utilieses this. It is mutualistic as both the plant and the bacteria benefit from this. The bacteria recieves carbohydrates.
   • Free living nitrogen fixing bacteria, otherwise known as bacteria in the soil, reduce gaseous nitrogen to ammonia.
     • Bacteria use ammonia to synthesise amino acids and internalise them.
     • The bacteria then dies, and decays, releasing the nitrogen compounds into the soil.

2. Ammonification/decomposition

   • Plants absorb nitrates from the soil, which are used to form amino acids (proteins).
   • Plants are eaten - therefore the amino acids are absorbed and assimilated into animals amino acids.
     • Assimilation - The absorption of nutrients into the body after digestion in the intestine and its transformation in biological tissues and fluids.
   • Both the plant (producer) and animal (consumer) die. Dead matter/detritus still contain nitrogen still fixed in organic molecules.
     • Decomposers release ammonia into the soil

3. Nitrification

   • Nitrifying bacteria oxidise ammonia to nitrites.
   • Different nitrifying bacteria oxidise nitrites to nitrates.
   • Plants absorb nitrates from the soil.

4. Denitrification

   • Reduces nitrates to nitrogen gas, replenishing the atmosphere.
   • Problems in waterlogged soil and aquatic conditions, as the sediment is in anaerobic conditions. The anaerobic basterica reduces nitrates and ammonium back to nitrogen using nitrates as oxygen.