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. These cycles 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. In the IB MYP Science curriculum, understanding nutrient cycles helps students appreciate how systems in organisms are interconnected and sustain life on Earth.

How does the carbon cycle function within an ecosystem?

The carbon cycle is a series of processes through which carbon atoms circulate among the earth's atmosphere, oceans, soil, and living organisms. It begins with photosynthesis, where plants absorb carbon dioxide (CO2) and convert it into organic matter. Animals then consume these plants, incorporating carbon into their bodies. Through respiration, decomposition, and combustion, carbon is released back into the atmosphere as CO2. This cycle is essential for regulating climate and supporting life, making it a key topic in the IB MYP Science curriculum under Systems 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 organic matter and waste products. This process releases nutrients back into the soil, making them available for plants to absorb and use for growth. In the context of the IB MYP Science curriculum, understanding the role of decomposers highlights the importance of recycling nutrients within ecosystems, ensuring the sustainability of life.

Can you explain the nitrogen cycle and its significance in ecosystems?

The nitrogen cycle is a complex process that converts nitrogen from the atmosphere into forms usable by living organisms and then back into the atmosphere. It involves processes such as nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. Nitrogen is essential for the production of amino acids and nucleic acids, which are building blocks of life. In the IB MYP Science curriculum, the nitrogen cycle is studied to understand how ecosystems maintain balance and support diverse life forms.

How do human activities 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 water pollution and disrupting aquatic ecosystems. Deforestation can reduce the carbon storage capacity of forests, affecting the carbon cycle. In the IB MYP Science curriculum, students explore these impacts to understand the importance of sustainable practices in maintaining the balance of nutrient cycles and protecting ecosystems.

Why is "Drawing cycles as one-way arrows." a common mistake in Nutrient Cycles?

Why it happens: Easy to forget the return processes. How to avoid it: A cycle is CYCLIC — every gain has a matching loss. Photosynthesis (removes CO₂) is balanced by respiration + combustion (add CO₂).

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

Why it happens: Air is 78% N₂. How to avoid it: Plants can't use N₂. It must be FIXED into nitrate first (by bacteria or lightning) — then plants absorb the nitrate.

Systems in OrganismsIB MYP Sciences (Years 1-5)

Nutrient Cycles

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Nutrient Cycles — IB MYP Sciences: carbon, nitrogen and water on the move

Atoms are recycled through the biosphere. This MYP Sciences note covers the carbon, nitrogen and water cycles and how humans disrupt each.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Identify each stage of the carbon, nitrogen and water cycles.
MYP Sciences A — Identify the role of decomposers.
MYP Sciences C — Explain a specific human disruption (e.g. fossil-fuel burning).
MYP Sciences D — Connect nutrient cycles to climate change.

The carbon cycle

Photosynthesis ↔ respiration ↔ combustion.

Carbon moves between four main reservoirs: ATMOSPHERE (CO₂), LIVING THINGS, OCEANS, and ROCKS/FOSSIL FUELS.

The main flows:

Photosynthesis: plants and algae take CO₂ from the atmosphere → glucose (and other organic compounds).
Respiration: ALL living things (plants, animals, bacteria, fungi) release CO₂ back to atmosphere.
Combustion: burning fuel (wood, coal, oil, gas) releases CO₂.
Decomposition: bacteria and fungi break down dead organisms → release CO₂.
Ocean exchange: oceans absorb ~25% of human CO₂ emissions.
Fossilisation: over millions of years, some dead plants/animals get buried → coal, oil, gas.

The carbon cycle. Photosynthesis is the only major flow OUT of the atmosphere; respiration, combustion and decomposition all put CO₂ back in.

Human disruption: burning fossil fuels (coal, oil, gas) releases carbon that was locked underground for millions of years. Atmospheric CO₂ has risen from ~280 ppm (1800) to ~420 ppm today — the highest in 800,000 years. This extra CO₂ is the main driver of climate change.

Deforestation makes it worse — fewer trees to absorb CO₂ AND released carbon when forests burn.

Photosynthesis takes CO₂ out; respiration + combustion + decomposition put it back.
Atmospheric CO₂ rose from 280 to 420 ppm in 200 years.
Main culprit: fossil fuel burning + deforestation.

The nitrogen cycle

From air to plants to animals — back to air via decomposers.

Nitrogen is 78% of the atmosphere but plants can't use N₂ directly — too unreactive. It has to be FIXED into nitrates (NO₃⁻) first.

Stages:

Nitrogen fixation: N₂ → NO₃⁻ (nitrate).
- LIGHTNING (small amount).
- NITROGEN-FIXING BACTERIA in soil and in root nodules of legumes (peas, beans, clover).
- INDUSTRIAL: Haber process for fertiliser.
Absorption: plant roots absorb nitrate (NO₃⁻) by active transport.
Assimilation: plant uses nitrate to make amino acids → proteins.
Feeding: animals eat plants → animal proteins.
Decomposition: when plants/animals die, DECOMPOSERS (bacteria + fungi) break down protein → release ammonium (NH₄⁺) → NITRIFYING BACTERIA convert it to nitrate. Available again for plants.
Denitrification: DENITRIFYING BACTERIA in waterlogged soil convert nitrate back to N₂ gas → released to atmosphere.

Key bacteria types:

Nitrogen-fixing: N₂ → ammonium / nitrate (good for plants).
Nitrifying: ammonium → nitrate (good for plants).
Denitrifying: nitrate → N₂ (bad for farmers — loses nutrients).

Human disruption:

Synthetic fertilisers (Haber process) hugely boost crop yields but excess runoff causes eutrophication in waterways.
Burning fossil fuels creates NOₓ → acid rain → soil pH disrupted.

N₂ fixed by lightning, bacteria, or Haber process.
Plants take up NITRATE, build proteins.
Decomposers + nitrifying bacteria return N to nitrate.
Excess fertiliser → eutrophication.

The water cycle

Evaporation, condensation, precipitation, run-off.

The simplest cycle but central to life:

Evaporation: Sun heats oceans, lakes, soil → water vapour rises into atmosphere.
Transpiration: water evaporates from plant leaves (about 10% of atmospheric water vapour comes from plants).
Condensation: water vapour cools at high altitude → tiny droplets → clouds.
Precipitation: droplets join → fall as rain, snow, hail.
Surface flow (run-off): water flows into rivers → eventually back to the sea.
Infiltration: some water soaks into soil → groundwater → wells, springs.

The water cycle is essential for:

Bringing fresh water to land.
Cooling the climate by moving heat from equator to poles.
Carrying nutrients in soil and rivers.

Human disruption:

Climate change intensifies the cycle — more evaporation, more extreme rain in some places, more droughts in others.
Deforestation reduces transpiration → drier local climates downwind.
Urbanisation (concrete instead of soil) reduces infiltration → more flooding.
Pumping groundwater for agriculture is depleting aquifers faster than they refill (e.g. Indian and US plains, North China plain).

Evap + transpiration → condense → precipitate → run-off + infiltration.
Climate change intensifies the cycle (extreme floods + droughts).
Deforestation, concrete cities, groundwater depletion disrupt the cycle.

How it’s examined

Criterion A: name the stages and key bacteria. Criterion C: trace an atom through a cycle; explain a human disruption.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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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.

1Why is atmospheric CO₂ rising?
Building confidence• carbon cycle
Question
Use the carbon cycle to explain why atmospheric CO₂ has been rising steadily since the industrial revolution.
Step-by-step solution
1. Step 1
  Burning fossil fuels (coal, oil, gas) releases CO₂ that had been locked underground for millions of years.
2. Step 2
  Photosynthesis can't remove CO₂ fast enough — deforestation makes this worse by reducing trees.
3. Step 3
  Oceans absorb some CO₂ but at saturation. The result is net CO₂ accumulation in the atmosphere — from ~280 ppm (1800) to ~420 ppm today.
Answer
Combustion of fossil fuels adds 'old' carbon faster than photosynthesis can remove it; deforestation reduces removal further.
2Three types of bacteria in the nitrogen cycle
Building confidence• nitrogen cycle
Question
Name three groups of bacteria involved in the nitrogen cycle and state what each does.
Step-by-step solution
1. Step 1
  NITROGEN-FIXING bacteria: convert N₂ from air into ammonium / nitrate. Often live in legume root nodules.
2. Step 2
  NITRIFYING bacteria: convert ammonium (from decay) into nitrate (NO₃⁻), which plants can absorb.
3. Step 3
  DENITRIFYING bacteria: convert nitrate back to N₂ gas. Lose nitrogen from soil — bad for farmers.
Answer
Nitrogen-fixing (N₂ → nitrate), nitrifying (NH₄⁺ → NO₃⁻), denitrifying (NO₃⁻ → N₂).
3How does deforestation disrupt the water cycle?
Stretch• water cycle
Question
Cutting down a large area of forest changes the local water cycle. Explain how.
Step-by-step solution
1. Step 1
  Trees normally TRANSPIRE huge amounts of water from their leaves — recycling water back into the atmosphere as vapour.
2. Step 2
  Without trees, less transpiration → drier air → less rainfall in the downwind area.
3. Step 3
  Also, exposed soil loses water faster and water runs off rather than infiltrating → groundwater depletes; flooding increases. The Amazon is currently at risk of a 'savannisation' tipping point because of these feedbacks.
Answer
Trees transpire huge amounts of water; clearing them reduces local rainfall, cuts groundwater recharge, and increases runoff. The local climate becomes drier.

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 IB MYP Sciences sitting.

Nutrient cycle
Examiner keyword
The continuous movement of an element (C, N, water) between living things and the environment.
Nitrogen fixation
Examiner keyword
Conversion of N₂ gas to ammonium or nitrate (by lightning, nitrogen-fixing bacteria, or the Haber process).
Decomposer
Examiner keyword
Bacteria and fungi that break down dead organisms — releasing carbon back to atmosphere and nitrogen back to soil.
Transpiration
Evaporation of water from plant leaves; an important contributor to the water cycle.
Eutrophication (nitrogen-cycle disruption)
Excess nitrate (from fertiliser) reaching water → algal bloom → O₂ depletion → fish die.

Common Mistakes and Misconceptions — Nutrient Cycles

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

✕Drawing cycles as one-way arrows.
Why it happens
Easy to forget the return processes.
How to avoid it
A cycle is CYCLIC — every gain has a matching loss. Photosynthesis (removes CO₂) is balanced by respiration + combustion (add CO₂).
✕Saying plants take nitrogen directly from the air.
Why it happens
Air is 78% N₂.
How to avoid it
Plants can't use N₂. It must be FIXED into nitrate first (by bacteria or lightning) — then plants absorb the nitrate.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Nutrient Cycles — frequently asked questions

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Nutrient Cycles

Short Study Notes in the form of Slides

Short Study Notes — Nutrient Cycles

Detailed Notes

What you’ll learn

How it’s examined

1Why is atmospheric CO₂ rising?

2Three types of bacteria in the nitrogen cycle

3How does deforestation disrupt the water cycle?

Nutrient cycle

Nitrogen fixation

Decomposer

Transpiration

Eutrophication (nitrogen-cycle disruption)

✕Drawing cycles as one-way arrows.

✕Saying plants take nitrogen directly from the air.

Practice questions

Past paper style quiz

Assess your understanding

Nutrient Cycles — frequently asked questions