The main stores of nitrogen are the atmosphere, soil and water, living biomass, and dead organic matter. Nitrogen moves between them through flows including nitrogen fixation, nitrification, uptake, feeding, excretion, death, decomposition, ammonification and denitrification.
This content belongs to IB ESS subtopic 2.3 Biogeochemical cycles and is HL only.
Main Stores and Flows
| Store | Form of nitrogen and important flows |
|---|---|
| Atmosphere | Contains a very large inorganic store of nitrogen gas. Nitrogen fixation transfers atmospheric nitrogen into biologically available compounds, while denitrification returns it to the atmosphere. |
| Soil and water | Contain inorganic nitrogen compounds, including ammonium, nitrites and nitrates. Nitrification converts ammonium into nitrites and then nitrates. |
| Living biomass | Plants absorb nitrates through mineral uptake and incorporate nitrogen into organic compounds such as proteins. Feeding transfers this organic nitrogen through food chains. |
| Dead organic matter and waste | Dead organisms, faeces and excretory products contain organic nitrogen. Decomposition and ammonification convert this nitrogen into ammonium compounds. |
Bacteria drive several essential transformations. Nitrogen-fixing bacteria make atmospheric nitrogen available to ecosystems; nitrifying bacteria produce nitrates; decomposers contribute to ammonification; and denitrifying bacteria convert nitrates back into atmospheric nitrogen under anaerobic conditions, such as in waterlogged soils.
A common misconception is that plants absorb atmospheric nitrogen directly. They generally cannot fix nitrogen themselves: they absorb mineral nitrogen from soil, although some form mutualistic associations with nitrogen-fixing bacteria.
Human activity changes these stores and flows. The Haber process fixes atmospheric nitrogen industrially to produce ammonia for fertilizers. Fertilizer runoff can increase nitrate flows into aquatic systems, contributing to eutrophication, oxygen depletion and environmental injustice where downstream communities bear pollution costs.
Exam Technique
For a systems-diagram question, draw stores as rectangular boxes and flows as labelled arrows. Distinguish a store, such as soil nitrate, from a process, such as nitrification, and ensure every arrow points in the correct direction.
