What is the Nitrogen Cycle? (SL IB Environmental Systems & Societies (ESS))

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

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