What is the Nitrogen Cycle? (DP IB Environmental Systems & Societies (ESS)): Revision Note

The Nitrogen Cycle

Nitrogen Cycle

• Nitrogen is the most abundant gas in the atmosphere, though is relatively inert. It does have a role in biology thanks to its ability to be converted (fixed) into biomass

• Bacteria and other microorganisms play a vital role in the nitrogen cycle

• Nitrogen-fixing bacteria are able to convert gaseous nitrogen into ammonium ions (NH₄⁺)

• Ammonium ions are nitrified by nitrifying bacteria into nitrite (NO₂^-) and nitrate (NO₃^-), which are highly soluble and can be absorbed by roots

• Denitrifying bacteria use nitrates for respiration and return gaseous nitrogen to the atmosphere

• Other bacteria and fungi are involved in saprobiotic decomposition e.g. decay of dead biomass or excreta

Nitrogen cycle diagram

Storage and Flows in the Nitrogen Cycle

• Storages in the nitrogen cycle include:
  

  • Organisms (organic)

  • Soils ( inorganic)

  • Fossil fuels (organic)

  • Atmosphere (inorganic)

  • Water bodies (inorganic)

• Flows in the nitrogen cycle include:
  

  • Nitrogen fixation by bacteria and lightning

  • Absorption

  • Assimilation

  • Consumption (feeding)

  • Excretion

  • Death and decomposition

  • Denitrification by bacteria in water logged soils

Human Impacts on the Nitrogen Cycle

What is the human impact on the nitrogen cycle?

• Increased use of fertilisers:
  

  • Fertilisers, especially nitrogen fertilisers, are widely used in agriculture to increase crop yield

  • However, excess nitrogen can leach into waterways, leading to eutrophication and harmful algal blooms

• Burning of fossil fuels:
  

  • Burning fossil fuels releases nitrogen oxides into the atmosphere, which can lead to the formation of acid rain

  • Acid rain can increase soil acidity, which can affect the ability of plants to take up nitrogen

• Industrial nitrogen fixation:
  

  • Humans have developed methods to fix nitrogen industrially, for example, in the production of fertilisers and explosives

  • This has greatly increased the amount of fixed nitrogen available for use in human activities

Photo by James Park on Unsplash 

Wetlands are important nitrogen sinks

• Land-use changes:
  

  • Conversion of natural landscapes, such as forests and wetlands, into agricultural or urban areas can lead to changes in nitrogen cycling

  • For example, wetlands are important nitrogen sinks, and their loss can result in nitrogen being released into waterways and the atmosphere

• Livestock farming:
  

  • Livestock farming produces large amounts of manure and urine, which can contribute to increased nitrogen inputs to ecosystems

  • This can lead to eutrophication and other environmental problems if not managed properly

• Wastewater treatment:
  

  • Wastewater treatment plants can be a source of nitrogen pollution if they do not effectively remove nitrogen from treated water before releasing it into the environment