Biogeochemical Cycles
How nutrients circulate through the living and non-living world — the carbon, nitrogen and phosphorus cycles, and the difference between gaseous and sedimentary cycles.
Key Takeaways
- Biogeochemical cycles recycle nutrients between organisms and the environment.
- Gaseous cycles (carbon, nitrogen) have an atmospheric reservoir; sedimentary cycles (phosphorus) have a rock reservoir.
- Nitrogen fixation converts inert atmospheric N₂ into usable forms.
Core concept
A biogeochemical cycle is the pathway by which a nutrient moves through the biotic (bio-) and abiotic (geo-) parts of an ecosystem. Unlike energy (which flows one way and is lost), nutrients are recycled endlessly between organisms, the atmosphere, water and soil.
Static foundation — two types of cycle
- Gaseous cycles: the main reservoir is the atmosphere — e.g., carbon, nitrogen, oxygen. They are relatively fast.
- Sedimentary cycles: the main reservoir is the Earth's crust (rocks) — e.g., phosphorus, sulphur. They are slow and have little or no gaseous phase.
The Nitrogen Cycle
Nitrogen fixation
Inert atmospheric N₂ is converted to ammonia by bacteria (Rhizobium in root nodules), lightning, or industrially (the Haber process).
Nitrification
Soil bacteria convert ammonia → nitrites → nitrates, the form plants can absorb.
Assimilation
Plants take up nitrates and build proteins; animals get nitrogen by eating plants.
Ammonification
Decomposers break down dead organisms and waste, releasing ammonia back to the soil.
Denitrification
Denitrifying bacteria convert nitrates back to N₂ gas, returning nitrogen to the atmosphere.
The carbon cycle in brief
Carbon moves via photosynthesis (plants absorb CO₂), respiration and decomposition (release CO₂), and combustion of fossil fuels (a huge extra release). Oceans are a vast carbon sink. Human burning of fossil fuels has raised atmospheric CO₂, intensifying the greenhouse effect — the root of climate change.
Relevance & current linkage
Humans have doubled the natural rate of nitrogen fixation (fertilisers), causing eutrophication and pollution, and have disrupted the carbon cycle (climate change). These form the basis of two of the nine planetary boundaries. (Add a current linkage — e.g., nitrogen pollution or a carbon-sink study.)
Prelims trap zones
- Atmospheric nitrogen (N₂) is inert — organisms cannot use it directly; it must be 'fixed'.
- The phosphorus cycle is SEDIMENTARY (no gaseous phase) — a favourite trap.
- Denitrification (not nitrification) returns N₂ to the atmosphere.
Prelims Pointers
- The atmosphere is ~78% nitrogen, but it is inert (N₂) and must be 'fixed' to be usable.
- Nitrogen fixation is done by bacteria (Rhizobium), lightning, and industrially (the Haber process).
- The phosphorus cycle is sedimentary — its main reservoir is rock, and it has no significant gaseous phase.
- Denitrification returns nitrogen to the atmosphere.
Mains Angle
- 'Human activity has profoundly altered the carbon and nitrogen cycles.' Discuss.
- Compare gaseous and sedimentary biogeochemical cycles.
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