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IB DemystifiedMYP Sciences
Nutrient cycles
The carbon atoms in your body may once have been in a dinosaur, a tree or the air above an ancient ocean. Carbon and nitrogen are recycled endlessly between the air, living things and the soil, and decomposers you never see keep the whole system running.
Recommended for MYP 4 · eAssessment priority · About 3 lessons · Criteria A, B, C and D
Figure 1. Carbon moves between the air, living things, dead matter and fossil fuels.
describe the nitrogen cycle and the roles of different bacteria
explain decomposition and the factors that affect the rate of decay
explain how human activities change nutrient cycles
analyse decay and crop-rotation data
discuss composting, fertiliser use and farming choices
Before you start
You will use these skills. If any feel shaky, review them first.
photosynthesis and respiration
food chains and ecosystems (see Human impact on ecosystems)
enzymes and temperature
Key vocabulary
Decomposer
A microorganism (bacterium or fungus) that breaks down dead matter and waste.
Nitrogen fixation
Converting nitrogen gas into compounds living things can use, by bacteria or lightning.
Nitrifying bacteria
Bacteria that convert ammonium into nitrate.
Denitrifying bacteria
Bacteria that convert nitrate into nitrogen gas, mainly in waterlogged soil.
Legume
A plant, such as lentils or chickpeas, with root nodules containing nitrogen-fixing bacteria.
Combustion
Burning; it releases carbon stored in fuels as carbon dioxide.
Understanding the ideas
What is it?
Living things are built from a small number of elements that are constantly recycled. In the carbon cycle, photosynthesis removes carbon dioxide from the air; respiration by plants, animals and decomposers returns it; and combustion releases carbon stored for millions of years in fossil fuels.
Why does it happen?
Nitrogen is needed to make proteins and DNA, but most organisms cannot use nitrogen gas. Nitrogen-fixing bacteria, some living in legume root nodules, convert it into usable compounds. Decomposers turn proteins in dead matter into ammonium, nitrifying bacteria convert it into nitrate for plants, and denitrifying bacteria return nitrogen to the air.
How do we know?
Scientists measure the carbon cycle directly: continuous CO₂ records show a yearly rise and fall as northern forests grow and rest. In the laboratory, you can measure decay by mass loss or by the carbon dioxide decomposers release.
Why does it matter?
Humans have changed both cycles: burning fuels and clearing forests add carbon dioxide to the air, while synthetic fertilisers have doubled the nitrogen entering ecosystems, causing water pollution. Composting and crop rotation are ways of working with the cycles.
What does it connect to?
This topic links to photosynthesis, respiration and ecosystems in biology, to fuels and fertilisers in chemistry, and to climate change in Earth science.
The nitrogen cycle in steps
Nitrogen fixation: nitrogen-fixing bacteria (free in soil or in legume root nodules) and lightning turn N₂ into usable compounds.
Absorption: plants take up nitrate from the soil to make proteins.
Feeding: animals get nitrogen by eating plants or other animals.
Decomposition: decomposers turn proteins in dead matter and waste into ammonium.
Nitrification: nitrifying bacteria (which need oxygen) turn ammonium into nitrite and then nitrate.
Denitrification: denitrifying bacteria (in waterlogged soil) turn nitrate back into N₂.
Nutrient cycles in the real world
Farmers across South Asia rotate wheat with lentils and chickpeas to improve soils. Cities are starting to collect food waste for compost instead of landfill. Peat bogs and mangrove soils store vast amounts of carbon because decay there is very slow.
Worked examples
Example 1: decay rates
Leaves lost 6% of their mass in 4 weeks at 10 °C and 18% at 20 °C. Explain.
Decomposers' enzymes work faster at 20 °C.
Microorganisms grow and respire faster, so they break down leaves more quickly.
The loss tripled for a 10 °C rise.
Example 2: a legume
Why do farmers plough lentil plants into the soil after harvest?
The roots and stems contain nitrogen compounds made with help from nitrogen-fixing bacteria.
Decomposers break them down into ammonium, and nitrifying bacteria make nitrate.
The next crop absorbs this nitrate.
In the eAssessment
Nutrient cycle questions often use cycle diagrams and decay experiments. Expect:
Describe and explain each step of the carbon and nitrogen cycles, naming the organisms and processes.
Process decay data, including means and anomalies.
Explain how conditions affect decay and nitrogen availability.
Discuss composting, fertilisers and farming decisions.
Common ways to lose marks: saying plants take nitrogen from the air; mixing up nitrifying and denitrifying bacteria; forgetting that decomposers respire; and saying cold kills decomposers rather than slowing them.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Show
Investigation: what decomposes, and how fast?
Partially guided investigation · set up in 30 minutes, check weekly for 4 weeks · groups
Research question
How does the type of material (apple slice, bread, paper, cotton cloth, plastic bag) affect the percentage of its mass lost after 4 weeks buried in moist soil?
Scientific background
Materials made from living things contain carbon compounds that decomposers can digest; some are broken down much faster than others. Most plastics cannot be broken down by decomposers.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Samples of each material of similar size, balance, garden soil, five identical pots or trays, fine plastic mesh bags or labelled netting, water spray, labels, gloves.
Method
Weigh each dried sample and seal it in a mesh bag.
Bury each bag at the same depth in moist soil in its own pot, kept at room temperature.
Keep the soil moist with the same volume of water each day.
After 4 weeks, dig up the samples, rinse gently, dry and reweigh, and calculate the percentage mass lost.
Safety. Wear gloves when handling soil and decaying material, and wash hands afterwards. Do not open sealed containers of mouldy food near your face; anyone with mould allergies should not handle the samples.
Then evaluate: why were the samples dried before weighing, and what could make your comparison fairer?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
