The nitrogen cycle can be split up into five main processes.

• Assimilation: this is where organic molecules are built up inside of organisms. Plants absorb nitrates through their roots where they’re converted into nitrites. These nitrites are transported by the xylem to the leaves. Here they’re used for protein synthesis: they’re converted into ammonium ions and make amino acids after reacting with triose sugars. Animals eat plants for their nitrogen. They excrete amino acids as urea, uric acid or ammonia.
• Ammonification: nitrogen compounds reach the soil via waste products, plants and dead animals. Saprophytes convert these into ammonia which then dissolves in water to create ammonium ions.
• Nitrification: chemoautotrophic bacteria convert the ammonium first into nitrites and then into nitrates using oxidative reactions.
• Denitrification: N₂ gas is formed when anaerobes remove the nitrogen from the nitrites and nitrates.
• Nitrogen fixation: nitrogen fixing bacteria trap the N₂ gas in the soil with the enzyme nitrogenase. These bacteria can be either free-living within the soil or symbiots which live in the root nodules found on leguminous plants.

The nitrogen cycle then goes as follows:

• N₂ gas found in the atmosphere can be fixed into the biotic environment by nitrogen fixing bacteria.
• Ammonification converts organic waste into ammonium ions which is then converted into nitrates and nitrites by nitrification.
• Plants absorb and assimilate nitrates which are passed onto the animals that eat the plants.
• Nitrogen returns to the soil through excretion and death and the nitrites and nitrates are converted back into N₂ gas by denitrifying bacteria.