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Last exams 2025

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

Alistair Marjot

Written by: Alistair Marjot

Reviewed by: Bridgette Barrett

Updated on

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

The nitrogen cycle

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

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies Content Creator

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Bridgette Barrett

Reviewer: Bridgette Barrett

Expertise: Geography Lead

After graduating with a degree in Geography, Bridgette completed a PGCE over 25 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.

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