Nitrogen Cycle

This lesson covers: 

  1. The steps of the nitrogen cycle
  2. The importance of the nitrogen cycle

Importance of the nitrogen cycle

The nitrogen cycle is vital for recycling nitrogen, an essential element for life on Earth. Without this cycle, ecosystems would run out of usable nitrogen, leading to a decline in vital biological molecules needed by organisms.


These molecules include:

  • Proteins
  • DNA
  • RNA


Human activities, like agriculture and the combustion of fossil fuels, have significantly altered the natural nitrogen cycle, leading to various environmental issues. Preserving the balance of the nitrogen cycle is therefore essential for maintaining ecosystem health and sustainability.

The nitrogen cycle

The atmosphere is composed of 78% nitrogen gas, which is inaccessible to most organisms. The nitrogen cycle transforms nitrogen gas into various chemical forms that organisms can use, facilitating its continuous movement through ecosystems.

Diagram illustrating the nitrogen cycle, including processes such as nitrogen fixation, ammonification, nitrification, and denitrification.

The nitrogen cycle comprises four key processes:

  1. Nitrogen fixation
  2. Ammonification
  3. Nitrification
  4. Denitrification


The nitrogen cycle also involves:

  • Decomposition by detritivores - They speed up decay by feeding on detritus, breaking it down into smaller pieces of organic material.
  • Decomposition by saprobionts - They convert nitrogen-containing molecules in dead organisms, faeces, and urine into substances like ammonia.
  • Assimilation by producers like plants - This is when producers absorb useful substances and incorporate them into other compounds that aid their survival.

Nitrogen fixation

This process converts atmospheric nitrogen gas (N2) into nitrogen-containing compounds like ammonia (NH3).


Atmospheric nitrogen gas can be fixed by:

  • Mutualistic bacteria in the root nodules of leguminous plants - They fix N2 into NH3, which is converted into amino acids for the plant, while the plant provides carbohydrates to the bacteria.
  • Free-living bacteria - They fix N2 into NH3 and then amino acids. When they die and are decomposed by saprobionts, nitrogen-rich compounds are released and converted into a form that can be assimilated by plants.
  • Abiotic processes - Lightning, artificial fertilisers, and the Haber process also fix N2.

Ammonification

This process converts organic nitrogen-containing compounds in dead organisms or waste into substances like ammonia or ammonium ions (NH4+).


How ammonification makes these substances available to plants:

  1. Death or excretion by living organisms releases nitrogen-rich organic substances like urea, proteins, nucleic acids, and vitamins.
  2. Saprobionts decompose these into simpler substances like NH4+.
  3. NH4+ can be absorbed and assimilated by plants.

Nitrification

NH3 and NH4+ are first oxidised to nitrites (NO2-) and then to nitrates (NO3-), which plants can absorb through their root hair cells via active transport and assimilate.


Nitrification occurs in two stages:

  1. Nitrifying bacteria oxidise NH3 and NH4+ into NO2-.
  2. Other nitrifying bacteria further oxidise NO2- into NO3-.

Denitrification

This process converts NO3- back into N2. Excessive denitrification can deplete soil nitrogen, negatively impacting plant growth.


How denitrification converts nitrates into atmospheric nitrogen:

  1. It is carried out by denitrifying bacteria.
  2. It occurs under anaerobic conditions, such as in waterlogged soils.