2.3 Biogeochemical Cycles

What is a biogeochemical cycle?

A system of stores and flows that cycle chemical elements through biotic and abiotic components of ecosystems.

Why are biogeochemical cycles important?

They ensure chemical elements remain available to living organisms over long periods.

What are stores in biogeochemical cycles?

Places where matter accumulates over time.

What is a carbon store?

An area where carbon accumulates through carbon sequestration.

What is carbon sequestration?

The capture of atmospheric CO₂ and storage in solid or liquid form.

What are carbon sinks?

Stores where carbon inflow exceeds outflow.

What are carbon sources?

Stores where carbon outflow exceeds inflow.

Name major carbon stores.

Atmosphere, oceans, organisms, soils, fossil fuels, sediments, sedimentary rocks.

What are organic carbon stores?

Carbon stored in living or once-living organisms (e.g. plants, animals, fossil fuels).

What are inorganic carbon stores?

Carbon stored in abiotic forms (e.g. CO₂, carbonates in oceans and rocks).

How does carbon flow by photosynthesis?

CO₂ is converted into glucose and oxygen by plants.

How does carbon flow by cellular respiration?

Glucose is converted into CO₂ and water by organisms.

How does feeding transfer carbon?

Carbon moves from plants to animals and between animals.

What role does decomposition play in the carbon cycle?

It releases carbon to soil and atmosphere.

What is fossilisation?

The transformation of organic matter into fossil fuels under heat and pressure.

What is combustion?

Burning fossil fuels releases stored carbon as CO₂.

Why are fossil fuels long-term carbon stores?

They store carbon for millions of years.

How do ecosystems act as carbon sinks?

By absorbing more CO₂ than they release.

How can deforestation turn ecosystems into carbon sources?

Stored carbon is released via burning and decay.

Why are oceans carbon sinks?

CO₂ dissolves in water and is assimilated by marine organisms.

What is ocean acidification?

A decrease in ocean pH due to increased dissolved CO₂.

How does ocean acidification affect marine life?

It weakens calcium carbonate shells and coral reefs.

What agricultural practices make farms carbon sources?

Tillage, monoculture, wetland drainage, fossil fuel use.

What agricultural practices make farms carbon sinks?

Crop rotation, cover cropping, composting, reduced tillage.

What is methane (CH₄)?

A greenhouse gas produced in anaerobic conditions.

Where is methane produced naturally?

Wetlands, swamps, marshes, bogs.

How is methane produced biologically?

By methanogenic bacteria during anaerobic respiration.

Why is methane a potent greenhouse gas?

It traps ~28× more heat than CO₂.

What is methane’s atmospheric residence time?

~10 years.

What happens to methane in the atmosphere?

It is oxidised into CO₂ and water.

How does permafrost contribute to methane emissions?

Thawing releases stored methane via positive feedback loops.

What are lithosphere carbon stores?

Fossil fuels and carbonate rocks like limestone.

How does limestone form biologically?

From shells of corals and molluscs containing calcium carbonate.

How does limestone form non-biologically?

By precipitation of calcium carbonate from water.

Why is limestone the largest carbon store?

It stores carbon for geological timescales.

What are fossil fuels formed from?

Partially decomposed plants and marine organisms.

Where does coal form?

From peat forests and bogs.

Where do oil and natural gas form?

From marine organisms buried under sediments.

What is the nitrogen cycle?

The movement of nitrogen through biotic and abiotic components of ecosystems.

Why is nitrogen important?

It is essential for proteins and DNA.

What is the largest nitrogen store?

The atmosphere (N₂ gas).

Why can plants not use atmospheric nitrogen directly?

N₂ is chemically inert.

What are organic nitrogen stores?

Nitrogen in proteins and nucleic acids in organisms.

What are inorganic nitrogen stores?

N₂ gas, ammonia, ammonium, nitrates, nitrites.

What is nitrogen fixation?

Conversion of N₂ into ammonia or ammonium.

Who carries out nitrogen fixation?

Nitrogen-fixing bacteria and lightning.

What is ammonification?

Conversion of organic nitrogen into ammonia during decomposition.

What is nitrification?

Conversion of ammonium into nitrites and nitrates by bacteria.

What conditions are required for nitrification?

Aerobic (oxygen-rich) soils.

What is denitrification?

Conversion of nitrates into nitrogen gas.

When does denitrification occur?

In anaerobic or waterlogged soils.

Why is denitrification significant?

It reduces nitrogen available to plants.

What is assimilation in the nitrogen cycle?

Uptake of nitrates or ammonium by plants.

What is a mutualistic relationship in nitrogen fixation?

Both species benefit.

Which bacteria form mutualisms with legumes?

Rhizobium.

What do plants give nitrogen-fixing bacteria?

Glucose.

What do bacteria give legumes?

Fixed nitrogen compounds.

How does lightning fix nitrogen?

Energy converts N₂ into nitrates and ammonia.

How does deforestation disrupt the nitrogen cycle?

Reduces nitrogen input and increases leaching.

How does agriculture disrupt the nitrogen cycle?

Removes biomass and causes nitrate runoff.

What is eutrophication?

Excess nutrients cause algal blooms and oxygen depletion.

How does urbanisation affect the nitrogen cycle?

Increases nitrogen runoff and pollution.

What is the Haber process?

Industrial synthesis of ammonia from nitrogen and hydrogen.

Why is the Haber process important?

It increased global food production.

What are disadvantages of the Haber process?

Energy use, GHG emissions, eutrophication.

What is the planetary boundary for nitrogen?

A safe limit for nitrogen use in Earth systems.

What is the safe nitrogen use boundary?

~35 million tonnes per year.

How much nitrogen do humans currently use?

~121 million tonnes per year.

Why is crossing the nitrogen boundary dangerous?

It causes irreversible ecosystem damage.

How can nitrogen cycle disruption be mitigated?

Regenerative agriculture and reduced fertiliser use.

What practices prevent nitrogen loss?

Cover cropping, terracing, windbreaks.

What practices replace nitrogen naturally?

Composting and legumes.

How can individuals help restore nitrogen cycles?

Choose sustainably grown food and reduce waste.