Syllabus Edition
First teaching 2015
Last exams 2025
What is the Nitrogen Cycle?
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
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
Last updated:
Did this page help you?