What are nutrient cycles and why are they important in ecosystems?

Nutrient cycles, also known as biogeochemical cycles, refer to the movement and exchange of organic and inorganic matter back into the production of living matter. They are crucial in ecosystems because they ensure the continuous supply of essential nutrients like carbon, nitrogen, and phosphorus, which are vital for the growth and survival of organisms. These cycles help maintain ecosystem stability and support life by recycling nutrients through the environment.

How does the nitrogen cycle function in an ecosystem?

The nitrogen cycle is a process that converts nitrogen from the atmosphere into forms that can be used by living organisms and then back into the atmosphere. It involves several steps: nitrogen fixation, where atmospheric nitrogen is converted into ammonia by bacteria; nitrification, where ammonia is converted into nitrites and then nitrates; assimilation, where plants absorb nitrates; and finally, denitrification, where nitrates are converted back into nitrogen gas by bacteria, completing the cycle. This cycle is essential for producing proteins and nucleic acids in organisms.

What role do decomposers play in nutrient cycles?

Decomposers, such as bacteria and fungi, play a critical role in nutrient cycles by breaking down dead organisms and organic waste. This process releases nutrients back into the soil and atmosphere, making them available for uptake by plants. Decomposers help recycle essential nutrients like carbon and nitrogen, ensuring the sustainability of ecosystems by maintaining the flow of energy and matter.

How does human activity impact nutrient cycles?

Human activities, such as agriculture, deforestation, and industrial processes, can significantly impact nutrient cycles. For example, the excessive use of fertilizers can lead to nutrient runoff, causing eutrophication in water bodies. Deforestation disrupts the carbon cycle by reducing the number of trees that absorb carbon dioxide. Industrial emissions can increase atmospheric nitrogen, altering the nitrogen cycle. These disruptions can lead to environmental issues like climate change and loss of biodiversity.

What is the carbon cycle and its significance in the environment?

The carbon cycle is the process through which carbon is exchanged among the atmosphere, oceans, soil, and living organisms. It involves processes such as photosynthesis, where plants convert carbon dioxide into organic matter, and respiration, where organisms release carbon dioxide back into the atmosphere. The carbon cycle is significant because it regulates the Earth's climate by controlling the levels of carbon dioxide, a greenhouse gas. It also supports life by providing the carbon necessary for building organic molecules.

Why is "Saying carbon flows one way (like energy)." a common mistake in Nutrient Cycles?

Why it happens: Energy flows through ecosystems. How to avoid it: Carbon (and nitrogen) CYCLE — the same atoms are reused. Only ENERGY flows one way.

Why is "Saying nitrogen-fixing bacteria release nitrogen gas into the air." a common mistake in Nutrient Cycles?

Why it happens: Confusing it with denitrifying bacteria. How to avoid it: Nitrogen FIXING: nitrogen gas → usable compounds (air to soil). DENITRIFYING: nitrate → nitrogen gas (soil to air). Opposite directions.

Why is "Saying plants take in nitrogen gas from the air." a common mistake in Nutrient Cycles?

Why it happens: The air is mostly nitrogen. How to avoid it: Plants cannot use nitrogen gas. They absorb NITRATE IONS from the soil.

Cambridge IGCSE Biology (0610)

Nutrient Cycles

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Short Notes - Nutrient Cycles

Detailed Study Notes

Detailed notes on Organisms and their Environment for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.

Nutrient Cycles — Cambridge IGCSE 0610 Biology Extended (2026)

Carbon, water, nitrogen — atoms recycled through ecosystems. Bacteria do most of the unseen work.

At a glance

Carbon cycle: photosynthesis ↔ respiration; combustion adds CO₂.
Water cycle: evaporation → condensation → precipitation.
Nitrogen cycle: bacterial-mediated; 4 types of bacteria.
Plants absorb NITRATE from soil; not atmospheric N₂.
Decomposers recycle nutrients back to soil/atmosphere.
Atoms cycle; energy flows.

What you’ll learn

Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).

19.3 — Describe the carbon cycle.
19.3 — Describe the water cycle.
19.3 — Describe the nitrogen cycle, including bacterial roles.

Carbon cycle

CO₂ ↔ glucose via photosynthesis and respiration. Combustion + decomposition add CO₂.

The cycle:

PHOTOSYNTHESIS — plants take CO₂ from air, fix into glucose.
FEEDING — animals eat plants, taking in carbon.
RESPIRATION — all organisms break down glucose, releasing CO₂.
DECOMPOSITION — when organisms die, decomposers break them down, releasing CO₂.
FOSSILISATION — over millions of years, dead organisms become coal, oil, gas (carbon stored).
COMBUSTION — burning fossil fuels and wood releases stored CO₂ rapidly.

The balance.

Without human activity, in/out are balanced.
HUMANS BURN fossils faster than photosynthesis can absorb → CO₂ rising in atmosphere → GLOBAL WARMING.

Worked qualitative. Why does deforestation make global warming worse?

Trees absorb CO₂ via photosynthesis.
Cutting trees REMOVES this CO₂ sink.
BURNING the wood RELEASES the stored carbon.
Double effect: less absorption + more release.
Replanting trees is one strategy to mitigate.

Cambridge tip. Memorise the 6 steps. Always include both PHOTOSYNTHESIS and RESPIRATION (the main flux).

Photosynthesis removes CO₂; feeding and death move carbon through organisms; respiration, decomposition and combustion return it, while fossilisation locks it away.

Photosynthesis: CO₂ → glucose.
Respiration: glucose → CO₂.
Decomposition releases CO₂.
Combustion (fossils, wood) → CO₂.
Fossilisation locks carbon up.

Water cycle

Evaporation → clouds → rain → runoff → ocean. Powered by the sun.

Steps:

EVAPORATION — water from oceans, lakes, rivers turns to vapour (sun's heat).
TRANSPIRATION — plants release water vapour through stomata.
CONDENSATION — vapour rises into cooler air → forms clouds (tiny water droplets).
PRECIPITATION — droplets combine, fall as rain, snow, hail, sleet.
RUNOFF — rain runs into rivers → returns to ocean.
GROUNDWATER — some water seeps into soil, replenishes aquifers, slowly returns to oceans.

Sometimes called the HYDROLOGICAL CYCLE.

The sun drives evaporation and transpiration; vapour condenses into clouds, falls as precipitation, and runs off back to the ocean.

Worked qualitative. Why does cutting forests sometimes change LOCAL rainfall?

Trees pump huge amounts of water from soil to atmosphere via transpiration.
This water often falls back as RAIN nearby.
Cutting trees → less transpiration → less local rainfall.
Amazon rainforest 'creates' much of its own rain.

Cambridge tip. Don't forget TRANSPIRATION as a source of water vapour, alongside evaporation. Cambridge marks both.

Evaporation + transpiration → vapour.
Condensation → clouds.
Precipitation → land/ocean.
Runoff + groundwater return.

Nitrogen cycle — the bacterial cycle

Four types of bacteria. Plants need nitrate; air is mostly N₂ (unusable directly).

The challenge. Air is 78% N₂, but plants can't use N₂ directly. They need NITRATE (NO₃⁻). Bacteria do the conversions.

Steps in the cycle:

1. Nitrogen fixation (atmosphere → soil).

NITROGEN-FIXING BACTERIA in soil + ROOT NODULES of legumes (peas, beans, clover).
Convert N₂ → ammonia (NH₃) or ammonium (NH₄⁺).
Now usable by plants.

Legumes have root nodules containing nitrogen-fixing bacteria that turn nitrogen gas into ammonium the plant can use.

2. Decomposition (dead matter → soil).

DECOMPOSERS (saprotrophs — bacteria + fungi) break down dead organisms + animal waste.
Release ammonium back to soil.

3. Nitrification (ammonium → nitrate).

NITRIFYING BACTERIA in soil.
Two-step: ammonium → nitrite → NITRATE.
Plants absorb nitrate.

4. Plant uptake.

Roots absorb nitrate via active transport.
Nitrate used to make amino acids → proteins.

5. Animal feeding.

Animals eat plants → get nitrogen as proteins.

6. Denitrification (soil → atmosphere).

DENITRIFYING BACTERIA, especially in waterlogged anaerobic soil.
Convert nitrate back to N₂ → released to atmosphere.
Returns nitrogen to start of cycle.

The four bacterial jobs: fixing (N₂→ammonium), decomposing (dead matter→ammonium), nitrifying (ammonium→nitrate) and denitrifying (nitrate→N₂).

Why farmers add NITRATE FERTILISER:

Crop plants remove nitrate from soil.
If not replaced, soil becomes nitrogen-depleted.
Adding nitrate (or planting legumes) replenishes.

Why LIGHTNING also fixes nitrogen:

High-energy electrical discharge breaks N₂ apart.
Nitrogen oxides form → dissolve in rain → reach soil as nitrate.
A small natural fix.

Worked qualitative. Why do farmers ROTATE crops, including LEGUMES (e.g. beans)?

Other crops deplete soil nitrogen.
Legumes (peas, beans) have root nodules with nitrogen-fixing bacteria → ADD nitrogen to soil.
Rotating: grow legumes one year, other crops next → soil refilled.
Practised for thousands of years.

Cambridge tip. Memorise the FOUR bacteria types. Cambridge marks each name + its conversion.

Four bacteria groups drive the cycle — fixing, nitrifying and denitrifying bacteria plus decomposers.

Fixing: N₂ → ammonia.
Decomposition: dead → ammonium.
Nitrifying: ammonium → nitrate.
Plants absorb nitrate.
Denitrifying: nitrate → N₂.

Quick recap

Carbon: photosynthesis ↔ respiration; combustion adds CO₂.
Water: evaporation, transpiration, condensation, precipitation.
Nitrogen: fixing, decomposition, nitrifying, denitrifying bacteria.
Plants need nitrate; can't use atmospheric N₂.
Bacteria do most of the work in nitrogen cycle.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Carbon cycle — photosynthesis ↔ respiration + decomposition + combustion.
Water cycle — evaporation/transpiration → condensation → precipitation.
Nitrogen-fixing bacteria — N₂ → ammonia.
Nitrifying bacteria — ammonium → nitrate.
Denitrifying bacteria — nitrate → N₂.

How it’s examined

Nutrient cycles appear every Paper 4 (8-12 marks: label diagrams, name bacteria + their conversions). Examiner reports flag students confusing nitrifying with denitrifying.

Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (19.3); 0610/22 May/Jun 2024 — Q12 (cycles); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

Take this whole topic with you

Step-by-step worked examples — Nutrient Cycles

Step-by-step solutions to past-paper-style questions on nutrient cycles, written exactly the way a tutor would explain them at the board.

1Adding and removing carbon dioxide
Getting started• carbon cycle, identify
Question
State which processes (a) remove carbon dioxide from the atmosphere and (b) return carbon dioxide to the atmosphere.
Step-by-step solution
1. Step 1
  (a) Photosynthesis removes carbon dioxide from the atmosphere (plants take it in to make glucose).
2. Step 2
  (b) Respiration (of plants, animals and decomposers) returns carbon dioxide to the atmosphere.
3. Step 3
  (b) Combustion (burning fuels, including fossil fuels) also returns carbon dioxide to the atmosphere.
Answer
Removed by photosynthesis. Returned by respiration and by combustion (burning fuels).
2Microorganisms in the nitrogen cycle
Getting started• Supplement 19.3• nitrogen cycle, identify
Question
Name the type of microorganism responsible for each: (a) breaking down dead organisms and waste; (b) converting ammonium ions into nitrate; (c) converting nitrogen gas into a usable form; (d) converting nitrate back into nitrogen gas.
Step-by-step solution
1. Step 1
  (a) Breaking down dead matter → decomposers (decomposition).
2. Step 2
  (b) Ammonium → nitrate → nitrifying bacteria (nitrification).
3. Step 3
  (c) Nitrogen gas → usable form → nitrogen-fixing bacteria (nitrogen fixation).
4. Step 4
  (d) Nitrate → nitrogen gas → denitrifying bacteria (denitrification).
Answer
(a) Decomposers; (b) nitrifying bacteria; (c) nitrogen-fixing bacteria; (d) denitrifying bacteria.
Examiner tip
Four roles in the syllabus: decomposition, nitrification, nitrogen fixation, denitrification. Note the spellings of 'nitrifying' and 'denitrifying'.
3Describe the carbon cycle
Building confidence• carbon cycle
Question
Describe the main processes of the carbon cycle.
Step-by-step solution
1. Step 1
  Photosynthesis: plants take carbon dioxide from the atmosphere and use it to make carbohydrates, so carbon enters living things.
2. Step 2
  Feeding: carbon passes along food chains when animals eat plants (and other animals).
3. Step 3
  Respiration: plants, animals and decomposers respire, releasing carbon dioxide back to the atmosphere.
4. Step 4
  Decomposition of dead organisms releases carbon dioxide too; and over millions of years some remains form fossil fuels, whose combustion (burning) releases carbon dioxide.
Answer
Photosynthesis fixes CO₂ into plants; feeding moves carbon along food chains; respiration and decomposition return CO₂; fossil fuels form from dead remains and combustion releases CO₂.
4How nitrogen reaches plants
Building confidence• Supplement 19.3• nitrogen cycle
Question
Explain how nitrogen from the air and from dead organisms becomes available to plants as nitrate.
Step-by-step solution
1. Step 1
  Plants cannot use nitrogen gas directly; they absorb nitrate ions from the soil.
2. Step 2
  Nitrogen fixation (by lightning and by nitrogen-fixing bacteria) converts nitrogen gas into compounds that plants can eventually use.
3. Step 3
  Decomposers break down the proteins in dead organisms and waste into ammonium ions.
4. Step 4
  Nitrifying bacteria then convert these ammonium ions into nitrate ions (nitrification), which the plants absorb to make amino acids and proteins.
Answer
Nitrogen fixation (lightning and bacteria) and the decomposition of dead matter to ammonium, followed by nitrification to nitrate, make nitrogen available; plants absorb the nitrate to make proteins.
5The full nitrogen cycle
Stretch• Supplement 19.3• nitrogen cycle
Question
Describe the nitrogen cycle, including the roles of the different microorganisms.
Step-by-step solution
1. Step 1
  Nitrogen fixation: nitrogen gas from the air is converted into usable nitrogen compounds by nitrogen-fixing bacteria (and by lightning).
2. Step 2
  Plants absorb nitrate ions and use them, with glucose, to make amino acids and proteins; animals get their protein by feeding on plants.
3. Step 3
  When organisms die (or excrete), decomposers break down the proteins into ammonium ions, and nitrifying bacteria convert the ammonium into nitrate (nitrification). Excess amino acids in animals are broken down by deamination, forming urea, which also returns nitrogen to the soil.
4. Step 4
  In waterlogged soils, denitrifying bacteria convert nitrate back into nitrogen gas (denitrification), returning it to the atmosphere.
Answer
N₂ fixed (bacteria/lightning) → nitrate absorbed by plants → proteins → feeding → death/excretion → decomposers make ammonium → nitrifying bacteria make nitrate; denitrifying bacteria return nitrate to N₂ gas.
Examiner tip
Include all four microbial roles (fixation, decomposition, nitrification, denitrification) and the absorption of nitrate by plants to make proteins.
6Burning fossil fuels and the carbon cycle
Stretch• carbon cycle, application
Question
Explain how the large-scale burning of fossil fuels has affected the carbon cycle and the atmosphere.
Step-by-step solution
1. Step 1
  Fossil fuels (coal, oil and gas) are stores of carbon formed from the remains of organisms over millions of years.
2. Step 2
  Burning them (combustion) releases this stored carbon as carbon dioxide into the atmosphere.
3. Step 3
  This is happening faster than photosynthesis can remove the carbon dioxide, especially as deforestation reduces the number of trees.
4. Step 4
  So the concentration of carbon dioxide in the atmosphere rises, which increases the greenhouse effect and contributes to global warming.
Answer
Burning fossil fuels releases stored carbon as CO₂ faster than photosynthesis removes it, so atmospheric CO₂ rises — adding to the greenhouse effect and global warming.
Examiner tip
Link the combustion of stored fossil carbon to a faster release of CO₂ than removal by photosynthesis, raising atmospheric CO₂.

Model Answers — Nutrient Cycles

High-scoring sample answers for nutrient cycles on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
Paper 4 short-answer style1 mark
Name the process that removes carbon dioxide from the atmosphere. (1 mark)
Model answer
Photosynthesis.
Why this scores
One mark for 'photosynthesis'.
Question 2
Paper 4 short-answer style2 marks
State two processes that return carbon dioxide to the atmosphere. (2 marks)
Model answer
Respiration (of plants, animals and decomposers) and combustion (the burning of fuels, including fossil fuels). (Decomposition is also accepted.)
Why this scores
Two distinct processes: respiration; combustion; decomposition.
Question 3
Paper 4 structured style3 marks
Describe the carbon cycle. (3 marks)
Model answer
Photosynthesis removes carbon dioxide from the atmosphere as plants use it to make carbohydrates, so carbon passes into living organisms (and along food chains by feeding). Respiration by plants, animals and decomposers returns carbon dioxide to the atmosphere. The decomposition of dead organisms also releases carbon dioxide, and the combustion of fuels (including fossil fuels formed from dead remains) returns even more.
Why this scores
Three marks from: photosynthesis removes CO₂; respiration returns CO₂; decomposition/combustion returns CO₂; feeding moves carbon through food chains.
Question 4
Paper 4 (Extended) structured style4 marks
Explain how nitrogen gas in the air is made available to plants as nitrate. (4 marks)
Model answer
Plants cannot use nitrogen gas directly. Nitrogen-fixing bacteria (and lightning) convert nitrogen gas into nitrogen compounds that can be used. Decomposers also break down the proteins in dead organisms into ammonium ions. Then nitrifying bacteria convert the ammonium ions into nitrate ions (nitrification). The plants then absorb the nitrate ions from the soil to make amino acids and proteins.
Why this scores
Four marks: nitrogen fixation (bacteria/lightning); decomposition to ammonium; nitrification (ammonium → nitrate); plants absorb nitrate.
Question 5
Paper 4 (Extended) structured style5 marks
Describe the roles of the four types of microorganism in the nitrogen cycle. (5 marks)
Model answer
Decomposers break down the proteins in dead organisms and in waste into ammonium ions (decomposition). Nitrifying bacteria convert these ammonium ions into nitrate ions (nitrification), which is the form plants can absorb. Nitrogen-fixing bacteria convert nitrogen gas from the air into nitrogen compounds that can be used (nitrogen fixation). Denitrifying bacteria convert nitrate ions back into nitrogen gas (denitrification), which returns nitrogen to the atmosphere, especially in waterlogged soils.
Why this scores
Five marks: decomposers → ammonium; nitrifying bacteria → nitrate; nitrogen-fixing bacteria fix N₂; denitrifying bacteria → N₂; correct direction for each.
Question 6
Paper 4 (Extended) extended-response style6 marks
Describe the nitrogen cycle, including how nitrogen passes from the air into plants and animals and is eventually returned to the air. (6 marks)
Model answer
Nitrogen gas in the air is converted into usable nitrogen compounds by nitrogen-fixing bacteria and by lightning (nitrogen fixation). Plants absorb nitrate ions from the soil and use them, together with glucose, to make amino acids and proteins. Animals obtain their protein by feeding on plants (or on other animals). When plants and animals die, or when animals produce waste, decomposers break down the proteins into ammonium ions. Nitrifying bacteria then convert these ammonium ions into nitrate ions (nitrification), which can again be absorbed by plants. In animals, excess amino acids are broken down by deamination, forming urea, which also returns nitrogen to the soil. Finally, in waterlogged (oxygen-poor) soils, denitrifying bacteria convert nitrate ions back into nitrogen gas, returning nitrogen to the atmosphere and completing the cycle.
Why this scores
Up to 6 marks: nitrogen fixation (bacteria/lightning); plants absorb nitrate to make proteins; feeding passes nitrogen to animals; decomposition to ammonium; nitrification to nitrate; deamination/urea; denitrification returns N₂ to the air.

Key Definitions and Keywords — Nutrient Cycles

Definitions to memorise and the exact keywords mark schemes credit for nutrient cycles answers — sharpened from recent examiner reports for the 2026 0610 sitting.

Carbon cycle
Examiner keyword
The movement of carbon between the atmosphere, organisms and fossil fuels through photosynthesis, respiration, feeding, decomposition and combustion.
Nitrogen cycle
Examiner keyword
The movement of nitrogen between the air, the soil and organisms, brought about largely by bacteria.
Nitrogen-fixing bacteria
Examiner keyword
Bacteria that convert nitrogen gas into nitrogen compounds plants can use. (Lightning also fixes nitrogen.)
Nitrifying bacteria
Examiner keyword
Bacteria that convert ammonium ions into nitrate ions (nitrification). Plants absorb the nitrate.
Denitrifying bacteria
Bacteria, mostly in waterlogged soils, that convert nitrate ions back into nitrogen gas (denitrification).
Decomposer
An organism (mainly bacteria and fungi) that breaks down dead matter and waste, releasing carbon dioxide and forming ammonium ions.

Common Mistakes and Misconceptions — Nutrient Cycles

The traps other students keep falling into on nutrient cycles questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.

✕Saying carbon flows one way (like energy).
Why it happens
Energy flows through ecosystems.
How to avoid it
Carbon (and nitrogen) CYCLE — the same atoms are reused. Only ENERGY flows one way.
✕Saying nitrogen-fixing bacteria release nitrogen gas into the air.
Why it happens
Confusing it with denitrifying bacteria.
How to avoid it
Nitrogen FIXING: nitrogen gas → usable compounds (air to soil). DENITRIFYING: nitrate → nitrogen gas (soil to air). Opposite directions.
✕Saying plants take in nitrogen gas from the air.
Why it happens
The air is mostly nitrogen.
How to avoid it
Plants cannot use nitrogen gas. They absorb NITRATE IONS from the soil.

Nutrient Cycles — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Cambridge IGCSE Biology (0610)

Nutrient Cycles

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Short Notes - Nutrient Cycles

Detailed Study Notes

Detailed notes on Organisms and their Environment for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.

Nutrient Cycles — Cambridge IGCSE 0610 Biology Extended (2026)

Carbon, water, nitrogen — atoms recycled through ecosystems. Bacteria do most of the unseen work.

At a glance

Carbon cycle: photosynthesis ↔ respiration; combustion adds CO₂.
Water cycle: evaporation → condensation → precipitation.
Nitrogen cycle: bacterial-mediated; 4 types of bacteria.
Plants absorb NITRATE from soil; not atmospheric N₂.
Decomposers recycle nutrients back to soil/atmosphere.
Atoms cycle; energy flows.

What you’ll learn

Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).

19.3 — Describe the carbon cycle.
19.3 — Describe the water cycle.
19.3 — Describe the nitrogen cycle, including bacterial roles.

Carbon cycle

CO₂ ↔ glucose via photosynthesis and respiration. Combustion + decomposition add CO₂.

The cycle:

PHOTOSYNTHESIS — plants take CO₂ from air, fix into glucose.
FEEDING — animals eat plants, taking in carbon.
RESPIRATION — all organisms break down glucose, releasing CO₂.
DECOMPOSITION — when organisms die, decomposers break them down, releasing CO₂.
FOSSILISATION — over millions of years, dead organisms become coal, oil, gas (carbon stored).
COMBUSTION — burning fossil fuels and wood releases stored CO₂ rapidly.

The balance.

Without human activity, in/out are balanced.
HUMANS BURN fossils faster than photosynthesis can absorb → CO₂ rising in atmosphere → GLOBAL WARMING.

Worked qualitative. Why does deforestation make global warming worse?

Trees absorb CO₂ via photosynthesis.
Cutting trees REMOVES this CO₂ sink.
BURNING the wood RELEASES the stored carbon.
Double effect: less absorption + more release.
Replanting trees is one strategy to mitigate.

Cambridge tip. Memorise the 6 steps. Always include both PHOTOSYNTHESIS and RESPIRATION (the main flux).

Photosynthesis removes CO₂; feeding and death move carbon through organisms; respiration, decomposition and combustion return it, while fossilisation locks it away.

Photosynthesis: CO₂ → glucose.
Respiration: glucose → CO₂.
Decomposition releases CO₂.
Combustion (fossils, wood) → CO₂.
Fossilisation locks carbon up.

Water cycle

Evaporation → clouds → rain → runoff → ocean. Powered by the sun.

Steps:

EVAPORATION — water from oceans, lakes, rivers turns to vapour (sun's heat).
TRANSPIRATION — plants release water vapour through stomata.
CONDENSATION — vapour rises into cooler air → forms clouds (tiny water droplets).
PRECIPITATION — droplets combine, fall as rain, snow, hail, sleet.
RUNOFF — rain runs into rivers → returns to ocean.
GROUNDWATER — some water seeps into soil, replenishes aquifers, slowly returns to oceans.

Sometimes called the HYDROLOGICAL CYCLE.

The sun drives evaporation and transpiration; vapour condenses into clouds, falls as precipitation, and runs off back to the ocean.

Worked qualitative. Why does cutting forests sometimes change LOCAL rainfall?

Trees pump huge amounts of water from soil to atmosphere via transpiration.
This water often falls back as RAIN nearby.
Cutting trees → less transpiration → less local rainfall.
Amazon rainforest 'creates' much of its own rain.

Cambridge tip. Don't forget TRANSPIRATION as a source of water vapour, alongside evaporation. Cambridge marks both.

Evaporation + transpiration → vapour.
Condensation → clouds.
Precipitation → land/ocean.
Runoff + groundwater return.

Nitrogen cycle — the bacterial cycle

Four types of bacteria. Plants need nitrate; air is mostly N₂ (unusable directly).

The challenge. Air is 78% N₂, but plants can't use N₂ directly. They need NITRATE (NO₃⁻). Bacteria do the conversions.

Steps in the cycle:

1. Nitrogen fixation (atmosphere → soil).

NITROGEN-FIXING BACTERIA in soil + ROOT NODULES of legumes (peas, beans, clover).
Convert N₂ → ammonia (NH₃) or ammonium (NH₄⁺).
Now usable by plants.

Legumes have root nodules containing nitrogen-fixing bacteria that turn nitrogen gas into ammonium the plant can use.

2. Decomposition (dead matter → soil).

DECOMPOSERS (saprotrophs — bacteria + fungi) break down dead organisms + animal waste.
Release ammonium back to soil.

3. Nitrification (ammonium → nitrate).

NITRIFYING BACTERIA in soil.
Two-step: ammonium → nitrite → NITRATE.
Plants absorb nitrate.

4. Plant uptake.

Roots absorb nitrate via active transport.
Nitrate used to make amino acids → proteins.

5. Animal feeding.

Animals eat plants → get nitrogen as proteins.

6. Denitrification (soil → atmosphere).

DENITRIFYING BACTERIA, especially in waterlogged anaerobic soil.
Convert nitrate back to N₂ → released to atmosphere.
Returns nitrogen to start of cycle.

The four bacterial jobs: fixing (N₂→ammonium), decomposing (dead matter→ammonium), nitrifying (ammonium→nitrate) and denitrifying (nitrate→N₂).

Why farmers add NITRATE FERTILISER:

Crop plants remove nitrate from soil.
If not replaced, soil becomes nitrogen-depleted.
Adding nitrate (or planting legumes) replenishes.

Why LIGHTNING also fixes nitrogen:

High-energy electrical discharge breaks N₂ apart.
Nitrogen oxides form → dissolve in rain → reach soil as nitrate.
A small natural fix.

Worked qualitative. Why do farmers ROTATE crops, including LEGUMES (e.g. beans)?

Other crops deplete soil nitrogen.
Legumes (peas, beans) have root nodules with nitrogen-fixing bacteria → ADD nitrogen to soil.
Rotating: grow legumes one year, other crops next → soil refilled.
Practised for thousands of years.

Cambridge tip. Memorise the FOUR bacteria types. Cambridge marks each name + its conversion.

Four bacteria groups drive the cycle — fixing, nitrifying and denitrifying bacteria plus decomposers.

Fixing: N₂ → ammonia.
Decomposition: dead → ammonium.
Nitrifying: ammonium → nitrate.
Plants absorb nitrate.
Denitrifying: nitrate → N₂.

Quick recap

Carbon: photosynthesis ↔ respiration; combustion adds CO₂.
Water: evaporation, transpiration, condensation, precipitation.
Nitrogen: fixing, decomposition, nitrifying, denitrifying bacteria.
Plants need nitrate; can't use atmospheric N₂.
Bacteria do most of the work in nitrogen cycle.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Carbon cycle — photosynthesis ↔ respiration + decomposition + combustion.
Water cycle — evaporation/transpiration → condensation → precipitation.
Nitrogen-fixing bacteria — N₂ → ammonia.
Nitrifying bacteria — ammonium → nitrate.
Denitrifying bacteria — nitrate → N₂.

How it’s examined

Nutrient cycles appear every Paper 4 (8-12 marks: label diagrams, name bacteria + their conversions). Examiner reports flag students confusing nitrifying with denitrifying.

Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (19.3); 0610/22 May/Jun 2024 — Q12 (cycles); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

Take this whole topic with you

Step-by-step worked examples — Nutrient Cycles

Step-by-step solutions to past-paper-style questions on nutrient cycles, written exactly the way a tutor would explain them at the board.

1Adding and removing carbon dioxide
Getting started• carbon cycle, identify
Question
State which processes (a) remove carbon dioxide from the atmosphere and (b) return carbon dioxide to the atmosphere.
Step-by-step solution
1. Step 1
  (a) Photosynthesis removes carbon dioxide from the atmosphere (plants take it in to make glucose).
2. Step 2
  (b) Respiration (of plants, animals and decomposers) returns carbon dioxide to the atmosphere.
3. Step 3
  (b) Combustion (burning fuels, including fossil fuels) also returns carbon dioxide to the atmosphere.
Answer
Removed by photosynthesis. Returned by respiration and by combustion (burning fuels).
2Microorganisms in the nitrogen cycle
Getting started• Supplement 19.3• nitrogen cycle, identify
Question
Name the type of microorganism responsible for each: (a) breaking down dead organisms and waste; (b) converting ammonium ions into nitrate; (c) converting nitrogen gas into a usable form; (d) converting nitrate back into nitrogen gas.
Step-by-step solution
1. Step 1
  (a) Breaking down dead matter → decomposers (decomposition).
2. Step 2
  (b) Ammonium → nitrate → nitrifying bacteria (nitrification).
3. Step 3
  (c) Nitrogen gas → usable form → nitrogen-fixing bacteria (nitrogen fixation).
4. Step 4
  (d) Nitrate → nitrogen gas → denitrifying bacteria (denitrification).
Answer
(a) Decomposers; (b) nitrifying bacteria; (c) nitrogen-fixing bacteria; (d) denitrifying bacteria.
Examiner tip
Four roles in the syllabus: decomposition, nitrification, nitrogen fixation, denitrification. Note the spellings of 'nitrifying' and 'denitrifying'.
3Describe the carbon cycle
Building confidence• carbon cycle
Question
Describe the main processes of the carbon cycle.
Step-by-step solution
1. Step 1
  Photosynthesis: plants take carbon dioxide from the atmosphere and use it to make carbohydrates, so carbon enters living things.
2. Step 2
  Feeding: carbon passes along food chains when animals eat plants (and other animals).
3. Step 3
  Respiration: plants, animals and decomposers respire, releasing carbon dioxide back to the atmosphere.
4. Step 4
  Decomposition of dead organisms releases carbon dioxide too; and over millions of years some remains form fossil fuels, whose combustion (burning) releases carbon dioxide.
Answer
Photosynthesis fixes CO₂ into plants; feeding moves carbon along food chains; respiration and decomposition return CO₂; fossil fuels form from dead remains and combustion releases CO₂.
4How nitrogen reaches plants
Building confidence• Supplement 19.3• nitrogen cycle
Question
Explain how nitrogen from the air and from dead organisms becomes available to plants as nitrate.
Step-by-step solution
1. Step 1
  Plants cannot use nitrogen gas directly; they absorb nitrate ions from the soil.
2. Step 2
  Nitrogen fixation (by lightning and by nitrogen-fixing bacteria) converts nitrogen gas into compounds that plants can eventually use.
3. Step 3
  Decomposers break down the proteins in dead organisms and waste into ammonium ions.
4. Step 4
  Nitrifying bacteria then convert these ammonium ions into nitrate ions (nitrification), which the plants absorb to make amino acids and proteins.
Answer
Nitrogen fixation (lightning and bacteria) and the decomposition of dead matter to ammonium, followed by nitrification to nitrate, make nitrogen available; plants absorb the nitrate to make proteins.
5The full nitrogen cycle
Stretch• Supplement 19.3• nitrogen cycle
Question
Describe the nitrogen cycle, including the roles of the different microorganisms.
Step-by-step solution
1. Step 1
  Nitrogen fixation: nitrogen gas from the air is converted into usable nitrogen compounds by nitrogen-fixing bacteria (and by lightning).
2. Step 2
  Plants absorb nitrate ions and use them, with glucose, to make amino acids and proteins; animals get their protein by feeding on plants.
3. Step 3
  When organisms die (or excrete), decomposers break down the proteins into ammonium ions, and nitrifying bacteria convert the ammonium into nitrate (nitrification). Excess amino acids in animals are broken down by deamination, forming urea, which also returns nitrogen to the soil.
4. Step 4
  In waterlogged soils, denitrifying bacteria convert nitrate back into nitrogen gas (denitrification), returning it to the atmosphere.
Answer
N₂ fixed (bacteria/lightning) → nitrate absorbed by plants → proteins → feeding → death/excretion → decomposers make ammonium → nitrifying bacteria make nitrate; denitrifying bacteria return nitrate to N₂ gas.
Examiner tip
Include all four microbial roles (fixation, decomposition, nitrification, denitrification) and the absorption of nitrate by plants to make proteins.
6Burning fossil fuels and the carbon cycle
Stretch• carbon cycle, application
Question
Explain how the large-scale burning of fossil fuels has affected the carbon cycle and the atmosphere.
Step-by-step solution
1. Step 1
  Fossil fuels (coal, oil and gas) are stores of carbon formed from the remains of organisms over millions of years.
2. Step 2
  Burning them (combustion) releases this stored carbon as carbon dioxide into the atmosphere.
3. Step 3
  This is happening faster than photosynthesis can remove the carbon dioxide, especially as deforestation reduces the number of trees.
4. Step 4
  So the concentration of carbon dioxide in the atmosphere rises, which increases the greenhouse effect and contributes to global warming.
Answer
Burning fossil fuels releases stored carbon as CO₂ faster than photosynthesis removes it, so atmospheric CO₂ rises — adding to the greenhouse effect and global warming.
Examiner tip
Link the combustion of stored fossil carbon to a faster release of CO₂ than removal by photosynthesis, raising atmospheric CO₂.

Model Answers — Nutrient Cycles

High-scoring sample answers for nutrient cycles on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
Paper 4 short-answer style1 mark
Name the process that removes carbon dioxide from the atmosphere. (1 mark)
Model answer
Photosynthesis.
Why this scores
One mark for 'photosynthesis'.
Question 2
Paper 4 short-answer style2 marks
State two processes that return carbon dioxide to the atmosphere. (2 marks)
Model answer
Respiration (of plants, animals and decomposers) and combustion (the burning of fuels, including fossil fuels). (Decomposition is also accepted.)
Why this scores
Two distinct processes: respiration; combustion; decomposition.
Question 3
Paper 4 structured style3 marks
Describe the carbon cycle. (3 marks)
Model answer
Photosynthesis removes carbon dioxide from the atmosphere as plants use it to make carbohydrates, so carbon passes into living organisms (and along food chains by feeding). Respiration by plants, animals and decomposers returns carbon dioxide to the atmosphere. The decomposition of dead organisms also releases carbon dioxide, and the combustion of fuels (including fossil fuels formed from dead remains) returns even more.
Why this scores
Three marks from: photosynthesis removes CO₂; respiration returns CO₂; decomposition/combustion returns CO₂; feeding moves carbon through food chains.
Question 4
Paper 4 (Extended) structured style4 marks
Explain how nitrogen gas in the air is made available to plants as nitrate. (4 marks)
Model answer
Plants cannot use nitrogen gas directly. Nitrogen-fixing bacteria (and lightning) convert nitrogen gas into nitrogen compounds that can be used. Decomposers also break down the proteins in dead organisms into ammonium ions. Then nitrifying bacteria convert the ammonium ions into nitrate ions (nitrification). The plants then absorb the nitrate ions from the soil to make amino acids and proteins.
Why this scores
Four marks: nitrogen fixation (bacteria/lightning); decomposition to ammonium; nitrification (ammonium → nitrate); plants absorb nitrate.
Question 5
Paper 4 (Extended) structured style5 marks
Describe the roles of the four types of microorganism in the nitrogen cycle. (5 marks)
Model answer
Decomposers break down the proteins in dead organisms and in waste into ammonium ions (decomposition). Nitrifying bacteria convert these ammonium ions into nitrate ions (nitrification), which is the form plants can absorb. Nitrogen-fixing bacteria convert nitrogen gas from the air into nitrogen compounds that can be used (nitrogen fixation). Denitrifying bacteria convert nitrate ions back into nitrogen gas (denitrification), which returns nitrogen to the atmosphere, especially in waterlogged soils.
Why this scores
Five marks: decomposers → ammonium; nitrifying bacteria → nitrate; nitrogen-fixing bacteria fix N₂; denitrifying bacteria → N₂; correct direction for each.
Question 6
Paper 4 (Extended) extended-response style6 marks
Describe the nitrogen cycle, including how nitrogen passes from the air into plants and animals and is eventually returned to the air. (6 marks)
Model answer
Nitrogen gas in the air is converted into usable nitrogen compounds by nitrogen-fixing bacteria and by lightning (nitrogen fixation). Plants absorb nitrate ions from the soil and use them, together with glucose, to make amino acids and proteins. Animals obtain their protein by feeding on plants (or on other animals). When plants and animals die, or when animals produce waste, decomposers break down the proteins into ammonium ions. Nitrifying bacteria then convert these ammonium ions into nitrate ions (nitrification), which can again be absorbed by plants. In animals, excess amino acids are broken down by deamination, forming urea, which also returns nitrogen to the soil. Finally, in waterlogged (oxygen-poor) soils, denitrifying bacteria convert nitrate ions back into nitrogen gas, returning nitrogen to the atmosphere and completing the cycle.
Why this scores
Up to 6 marks: nitrogen fixation (bacteria/lightning); plants absorb nitrate to make proteins; feeding passes nitrogen to animals; decomposition to ammonium; nitrification to nitrate; deamination/urea; denitrification returns N₂ to the air.

Key Definitions and Keywords — Nutrient Cycles

Definitions to memorise and the exact keywords mark schemes credit for nutrient cycles answers — sharpened from recent examiner reports for the 2026 0610 sitting.

Carbon cycle
Examiner keyword
The movement of carbon between the atmosphere, organisms and fossil fuels through photosynthesis, respiration, feeding, decomposition and combustion.
Nitrogen cycle
Examiner keyword
The movement of nitrogen between the air, the soil and organisms, brought about largely by bacteria.
Nitrogen-fixing bacteria
Examiner keyword
Bacteria that convert nitrogen gas into nitrogen compounds plants can use. (Lightning also fixes nitrogen.)
Nitrifying bacteria
Examiner keyword
Bacteria that convert ammonium ions into nitrate ions (nitrification). Plants absorb the nitrate.
Denitrifying bacteria
Bacteria, mostly in waterlogged soils, that convert nitrate ions back into nitrogen gas (denitrification).
Decomposer
An organism (mainly bacteria and fungi) that breaks down dead matter and waste, releasing carbon dioxide and forming ammonium ions.

Common Mistakes and Misconceptions — Nutrient Cycles

The traps other students keep falling into on nutrient cycles questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.

✕Saying carbon flows one way (like energy).
Why it happens
Energy flows through ecosystems.
How to avoid it
Carbon (and nitrogen) CYCLE — the same atoms are reused. Only ENERGY flows one way.
✕Saying nitrogen-fixing bacteria release nitrogen gas into the air.
Why it happens
Confusing it with denitrifying bacteria.
How to avoid it
Nitrogen FIXING: nitrogen gas → usable compounds (air to soil). DENITRIFYING: nitrate → nitrogen gas (soil to air). Opposite directions.
✕Saying plants take in nitrogen gas from the air.
Why it happens
The air is mostly nitrogen.
How to avoid it
Plants cannot use nitrogen gas. They absorb NITRATE IONS from the soil.

Nutrient Cycles — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Nutrient Cycles

Short Notes - Nutrient Cycles

Detailed Study Notes

At a glance

What you’ll learn

Carbon cycle

Water cycle

Nitrogen cycle — the bacterial cycle

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Adding and removing carbon dioxide

2Microorganisms in the nitrogen cycle

3Describe the carbon cycle

4How nitrogen reaches plants

5The full nitrogen cycle

6Burning fossil fuels and the carbon cycle

Carbon cycle

Nitrogen cycle

Nitrogen-fixing bacteria

Nitrifying bacteria

Denitrifying bacteria

Decomposer

✕Saying carbon flows one way (like energy).

✕Saying nitrogen-fixing bacteria release nitrogen gas into the air.

✕Saying plants take in nitrogen gas from the air.

Nutrient Cycles — frequently asked questions

Nutrient Cycles

Short Notes - Nutrient Cycles

Detailed Study Notes

At a glance

What you’ll learn

Carbon cycle

Water cycle

Nitrogen cycle — the bacterial cycle

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Adding and removing carbon dioxide

2Microorganisms in the nitrogen cycle

3Describe the carbon cycle

4How nitrogen reaches plants

5The full nitrogen cycle

6Burning fossil fuels and the carbon cycle

Carbon cycle

Nitrogen cycle

Nitrogen-fixing bacteria

Nitrifying bacteria

Denitrifying bacteria

Decomposer

✕Saying carbon flows one way (like energy).

✕Saying nitrogen-fixing bacteria release nitrogen gas into the air.

✕Saying plants take in nitrogen gas from the air.

Nutrient Cycles — frequently asked questions