B. Biology: Sustainable Ecosystems and Climate Change · Sustainable Ecosystems and Climate ChangeThe Nitrogen and Phosphorus Cycles
B2.2 A1.2 A2.5About 70 minutes
Learning goals- I can explain why living things need nitrogen and phosphorus.
- I can describe nitrogen fixation, nitrification, assimilation, decomposition (ammonification) and denitrification, and the role of bacteria.
- I can describe the phosphorus cycle and explain why it is so slow.
- I can explain what a limiting nutrient is, using evidence from a famous Canadian experiment.
Words to know
nitrogen fixation
Changing nitrogen gas (N₂) from the air into compounds such as ammonium that living things can use. Done by certain bacteria, by lightning, and in fertilizer factories.
nitrification
The process in which soil bacteria change ammonium (NH₄⁺) into nitrite and then nitrate (NO₃⁻).
denitrification
The process in which bacteria in low-oxygen soil or water change nitrate back into nitrogen gas, returning it to the air.
ammonification
Decomposers breaking down proteins in dead matter and wastes into ammonium.
legume
A plant in the pea and bean family (such as soybeans, clover, alfalfa) with root nodules that hold nitrogen-fixing bacteria.
phosphate
A compound of phosphorus and oxygen (PO₄³⁻); the form in which living things take in phosphorus.
limiting nutrient
The nutrient in shortest supply compared with what organisms need; it limits how much they can grow.
Remember from Grade 7
In Grade 7 you learned that bacteria in the roots of legumes fix nitrogen and that farmers use fertilizer to add nutrients. This lesson adds the full set of nitrogen processes, introduces the phosphorus cycle (new this year), and explains how these two nutrients can control the growth of a whole ecosystem.
Let's start with a puzzle
Every breath you take is about 78% nitrogen. Yet nitrogen is often the nutrient that limits how fast crops grow, and farmers spend a lot of money on nitrogen fertilizer. How can something so common be in short supply?
The answer: nitrogen gas (N₂) has two atoms joined by a very strong triple bond. Plants and animals cannot break it. Only a few kinds of bacteria, lightning, and high-temperature factories can. Living things need nitrogen to build proteins and DNA, so they depend on bacteria to change N₂ into a usable form.
The nitrogen cycle, step by step
- Nitrogen fixation. Bacteria change N₂ into ammonium (NH₄⁺). Some live free in soil or water (including some cyanobacteria). Others, such as Rhizobium, live inside nodules on the roots of legumes (beans, peas, clover, alfalfa, soybeans). The plant gives the bacteria sugar; the bacteria give the plant nitrogen. Lightning and fertilizer factories also fix nitrogen.
- Nitrification. Nitrifying bacteria change ammonium into nitrite (NO₂⁻) and then nitrate (NO₃⁻).
- Assimilation. Plants absorb nitrate (and some ammonium) through their roots and use it to build proteins and DNA. Animals get nitrogen by eating plants or other animals.
- Ammonification (decomposition). When organisms die or produce wastes, decomposers break down the proteins and release ammonium back into the soil.
- Denitrification. In waterlogged, low-oxygen soil, denitrifying bacteria change nitrate back into N₂ gas (and a little nitrous oxide, N₂O, a greenhouse gas), which returns to the air.
The phosphorus cycle
Phosphorus is needed for DNA, ATP (the molecule cells use to carry energy), cell membranes, and bones and teeth.
Unlike carbon and nitrogen, phosphorus has no important gas form in the atmosphere. Its cycle is centred on the lithosphere:
- Weathering of rock slowly releases phosphate into soil and water.
- Plants absorb dissolved phosphate through their roots; animals get it by eating.
- Decomposers return phosphate to the soil when organisms die.
- Runoff carries some phosphate into lakes and oceans, where it settles into sediments.
- Over millions of years, sediments become rock, and geological uplift can bring that rock back to the surface.
Because weathering is so slow, phosphate is often scarce in fresh water, so it is frequently the limiting nutrient for algae in Canadian lakes.
A famous Canadian experiment: Lake 226
In the 1960s, many lakes were turning green with algae. Was the cause carbon, nitrogen or phosphorus? Detergents contained a lot of phosphate, but some companies argued that carbon was to blame.
At the Experimental Lakes Area in northwestern Ontario, scientists led by David Schindler tested whole lakes. In 1973 they divided Lake 226 in two with a plastic curtain. One half got carbon, nitrogen and phosphorus; the other half got only carbon and nitrogen. Only the half with phosphorus turned bright green. This striking evidence (a famous aerial photo shows the two halves) strengthened the case for limiting phosphorus, including phosphates in laundry detergents, in Canada and elsewhere. The Experimental Lakes Area is still running whole-lake experiments today.
Humans change the nutrient cycles
- Fertilizer: factories fix huge amounts of nitrogen from the air, and phosphate is mined from rock. This has greatly increased crop yields, but extra nitrogen and phosphorus wash into water.
- Manure and sewage add nitrogen and phosphorus to lakes and rivers.
- Burning fuels releases nitrogen oxides, which contribute to smog and acid rain.
- Fertilized soils release more nitrous oxide (N₂O), a strong greenhouse gas.
Too much nitrogen and phosphorus in water leads to eutrophication: algae blooms, low oxygen and fish die-offs. You will study this in Lesson 9, and how farmers reduce it in Lesson 15.
Keep the nitrogen words straight
- Fixation = N₂ from the air → ammonium (fixes it into a usable form).
- Nitrification = ammonium → nitrate.
- Denitrification = nitrate → N₂ gas (de- means undo).
- Ammonification = dead stuff → ammonium.
All four are done mainly by bacteria.
Worked exampleLet's trace a nitrogen atom
Trace a nitrogen atom from the air into a deer and back into the air. Name each process.
- N₂ in the air is changed into ammonium by Rhizobium bacteria in clover root nodules: nitrogen fixation.
- The clover uses the ammonium to build protein: assimilation.
- A deer eats the clover and uses the nitrogen to build its own proteins: eating.
- The deer's wastes are broken down by decomposers into ammonium: ammonification.
- Nitrifying bacteria change the ammonium into nitrate: nitrification.
- In a soggy, low-oxygen spot, denitrifying bacteria change the nitrate to N₂: denitrification. The atom is back in the air.
Fixation → assimilation → eating → ammonification → nitrification → denitrification.
Worked exampleLet's identify a limiting nutrient
A student grows duckweed in four jars of pond water: (A) nothing added, (B) nitrate added, (C) phosphate added, (D) nitrate and phosphate added. After two weeks the duckweed covers: A 10%, B 12%, C 45%, D 80%. Which nutrient was limiting at first?
- Compare each jar to the control (A, 10%).
- Adding nitrate alone (B) made almost no difference (12%), so nitrogen was not the first limit.
- Adding phosphate alone (C) more than quadrupled growth (45%), so phosphorus was limiting.
- Adding both (D) gave the most growth (80%): once phosphate was supplied, nitrogen became the next limit.
Phosphorus was the limiting nutrient at first. After phosphorus was added, nitrogen limited growth next.
Hands-on activityHunt for root nodules (A1.2)
You need: a small trowel, a bucket of water, a magnifying glass (or phone camera zoom), paper towel. Ask a grown-up first and only dig where you have permission.
- Find white clover in a lawn or park edge, or a pea or bean plant in a garden.
- Carefully dig up one plant with a ball of soil around the roots (dig wide so you don't strip the roots).
- Gently swirl the roots in the bucket of water to wash off soil.
- Look for small round bumps on the roots: nodules. Count them.
- With a grown-up, cut one large nodule in half with a clean knife. Is the inside pinkish or white/green? (Active nitrogen-fixing nodules are often pink inside.)
- Compare with the roots of a grass plant dug up nearby. Does grass have nodules?
- Record your observations with sketches or photos, and write a conclusion.
What to look for: clover and bean roots usually have nodules; grass roots do not, because grasses are not legumes. Pink nodules are actively fixing nitrogen.
Grown-up note: an adult should handle the knife. Wash hands after handling soil. Replant the clover if you can.
Just for funAir is about 78% nitrogen, but most living things can't use any of it. It's like sitting at a table piled with sealed snacks and no way to open them. Luckily, nitrogen-fixing bacteria are the planet's snack openers.
Your turn
1Why can't plants and animals use nitrogen gas directly?
N₂ has a very strong triple bond that their cells cannot break. Only certain bacteria, lightning and industrial processes can fix it.
2Name the process and the organisms responsible: ammonium → nitrate.
Nitrification, carried out by nitrifying bacteria.
3Describe the relationship between legumes and Rhizobium bacteria.
The bacteria live in root nodules and fix nitrogen for the plant; the plant supplies the bacteria with sugar from photosynthesis. Both benefit (mutualism).
4What does denitrification do, and where does it happen?
It changes nitrate back into N₂ gas (returning nitrogen to the air). It happens in waterlogged, low-oxygen soil and sediments.
5How is the phosphorus cycle different from the carbon and nitrogen cycles?
It has no important gas stage in the atmosphere; phosphorus cycles mainly through rock, soil, water and living things, and it is much slower.
6Why do living things need phosphorus?
To build DNA, ATP, cell membranes, and bones and teeth.
7What did the Lake 226 experiment show?
Phosphorus was the nutrient that caused the algae bloom (the limiting nutrient). It strengthened the case for limits on phosphates in detergents.
More practice: warm-up
1What percentage of the air is nitrogen gas?
Any two: beans, peas, clover, alfalfa, soybeans, lentils.
3What process returns nitrogen from dead organisms to the soil as ammonium?
Ammonification (decomposition).
4Where is most of Earth's phosphorus stored?
In rocks and sediments (the lithosphere).
5Name one natural, non-living way nitrogen is fixed.
More practice: on your own
1A farmer plants soybeans one year and corn the next. Explain how this helps the corn.
Soybeans are legumes whose root bacteria fix nitrogen. When soybean roots and leftovers decompose, they leave extra nitrogen in the soil for the corn, so less fertilizer is needed.
2Why might a waterlogged field lose nitrogen?
Low oxygen lets denitrifying bacteria turn nitrate into N₂ gas, which escapes to the air.
3Explain why phosphorus is often the limiting nutrient in Canadian lakes.
Phosphorus comes mainly from slow rock weathering and has no gas form to replenish it, so lakes naturally have very little dissolved phosphate. Algae use it up quickly.
4In the duckweed example, why was jar A needed?
Jar A was the control. It shows how much the duckweed grows with no nutrients added, so you can tell whether added nutrients made a difference.
5How does burning fossil fuels affect the nitrogen cycle?
High temperatures in engines and furnaces make nitrogen oxides, which form smog and acid rain and add extra nitrogen to ecosystems when they fall out of the air.
Challenge
1Fertilizer factories now fix about as much nitrogen as all natural land processes combined (a widely reported estimate). Predict two effects on ecosystems.
Possible effects: more nitrogen runoff into lakes and coasts, causing algae blooms and low-oxygen zones; more nitrous oxide released from soils, adding to climate change; changes in plant communities, as fast-growing plants that love nitrogen crowd out others, which can reduce biodiversity.
2Explain why the Lake 226 experiment was more convincing than growing algae in jars.
It tested a whole real lake, with natural water, temperature, light and organisms, so the result applied to actual lakes. The curtain made a controlled comparison in the same lake at the same time: the only difference was phosphorus. Jar results might not apply to real lakes.
3Both carbon and nitrogen cycles involve bacteria. Give one role bacteria play in each cycle and explain why a sterile (bacteria-free) world would stop both cycles.
Carbon: bacteria decompose dead matter and release CO₂. Nitrogen: bacteria fix, nitrify and denitrify nitrogen. Without bacteria (and other decomposers), dead matter would pile up with its carbon and nitrogen locked inside, nitrogen gas could not enter living things (except through lightning), and plants would run out of usable nutrients.
Quiz
1. Which process changes N₂ from the air into ammonium?
2. Where do Rhizobium bacteria live?
3. Which process returns nitrogen gas to the atmosphere?
4. What is unusual about the phosphorus cycle?
5. Which molecule needs phosphorus?
6. What was added to the green half of Lake 226 that was not added to the other half?
Big idea (B2.2): nitrogen and phosphorus are recycled nutrients that control how much life an ecosystem can support. Bacteria drive the nitrogen cycle; the phosphorus cycle is slow and rock-based, which makes phosphorus a common limiting nutrient in lakes. The Lake 226 story shows how a whole-ecosystem experiment by Canadian scientists changed laws (A2.5: contributions to science; the nodule hunt and duckweed example build A1.2 experimentation skills).
How to help: Dig up clover together and look for nodules. If you garden, ask why seed packets for peas sometimes suggest an "inoculant" (it adds the right bacteria).
Common difficulties:
- Mixing up nitrification and denitrification. Use the word clues in the tip box.
- Thinking plants absorb nitrogen gas from the air. They absorb nitrate and ammonium from soil; only legumes benefit from N₂, and only through their bacteria.
- Thinking phosphorus cycles through the air. It does not (except tiny amounts in dust).
Sources: Lake 226 experiment and its influence on detergent phosphate limits are described by IISD Experimental Lakes Area.