ScienceCBSE Class 9 · Earth as a System: Energy, Matter, and Life

Biogeochemical Cycles: Water, Carbon, Nitrogen, Oxygen and Human Impact (Class 9)

Updated 17 October 2026 · Fact-checked

Biogeochemical cycles move water, carbon, nitrogen and oxygen between the living (biotic) and non-living (abiotic) parts of the Earth. In the nitrogen cycle, nitrogen fixation, nitrification, assimilation, ammonification and denitrification pass nitrogen through bacteria, plants and animals; in the carbon cycle photosynthesis takes CO₂ from the air while respiration, decomposition and burning of fossil fuels return it. Human activities (fossil fuels, deforestation, fertilisers) disturb these cycles and cause global warming, ocean acidification and eutrophication.

Understand Water, Carbon and Nitrogen Cycles and Human Impact

Why cycles matter. Matter is not lost on Earth; it cycles between living and non-living parts. These biogeochemical cycles keep carbon, nitrogen, oxygen and water available for life and help ecosystems recover from disturbance.

Water cycle. Evaporation (oceans, lakes) and transpiration (plants) lift water vapour into the air, where it condenses into clouds. Precipitation (rain, hail, snow) brings it back, and runoff carries water to rivers and oceans while infiltration recharges groundwater. Water dissolves minerals and carries nutrients to the oceans. A warmer climate means warmer air holding more moisture, so some areas get heavier rain and others drought; melting glaciers raise rivers and, over time, sea level; intense bursts of rain increase erosion and reduce groundwater recharge.

Carbon cycle. Carbon is in every protein, carbohydrate, fat and DNA molecule. The fast cycle (days to years): photosynthesis takes in CO₂, 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂; respiration and decomposition return it. The slow cycle (millions of years): buried remains become coal, oil and gas; burning them releases CO₂ rapidly, for example CH₄ + 2O₂ → CO₂ + 2H₂O. The oceans hold about 71% of the world's carbon and the atmosphere about 1%. The Keeling curve shows CO₂ rising from about 315 ppm (1960) to 420 ppm (2025).

Nitrogen cycle. The air is about 78% nitrogen, but N₂ is unreactive and cannot be used directly by most organisms. Nitrogen fixation turns N₂ into ammonia: by Rhizobium in root nodules of legumes, by Azotobacter in soil, by lightning (oxides), and in industry (Haber-Bosch). Nitrification converts ammonia to nitrite (Nitrosomonas) and then to nitrate (Nitrobacter). Assimilation: plants absorb nitrates and ammonium and make proteins; animals get nitrogen by eating plants. Ammonification: decomposer bacteria and fungi turn dead remains and wastes into ammonia. Denitrification (Pseudomonas) converts nitrates back to N₂, returning it to the air. The Haber-Bosch process, N₂ + 3H₂ → 2NH₃, supplies most fertiliser but uses 1 to 2% of world energy, and overuse of fertiliser harms soil and water.

Oxygen cycle. About 21% of air is O₂. It is used in respiration, combustion and formation of oxides (rusting), and restored mainly by photosynthesis of land plants and ocean phytoplankton.

Human impact. Burning fossil fuels and deforestation add CO₂ faster than forests and oceans can absorb it, strengthening the greenhouse effect: global warming, melting ice, rising seas and changing monsoons. Oceans absorb extra CO₂ and become more acidic, which harms plankton, shell-builders and coral reefs. Eutrophication: excess nitrate and phosphate from fertiliser washes into lakes and rivers and causes algal blooms; when the algae die, decomposers use up the oxygen and fish die. Deforestation reduces photosynthesis and transpiration, lowers local rainfall, changes albedo and increases soil erosion. Vehicle exhaust reacts in sunlight to form smog and ground-level ozone. Solutions: the Montreal Protocol (ozone recovering), renewable energy, tree planting, sustainable farming and Mission LiFE (save energy, water and food; reduce, reuse, recycle).

Water, Carbon and Nitrogen Cycles and Human Impact: formulas and key facts

Photosynthesis: $6 C O_{2} + 6 H_{2} O \to C_{6} H_{12} O_{6} + 6 O_{2} (ma sses 264 g + 108 g \to 180 g + 192 g)$
Burning methane: $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O (16 g + 64 g \to 44 g + 36 g)$
Haber-Bosch process: $N_{2} + 3 H_{2} \to 2 N H_{3} (28 g + 6 g \to 34 g)$
Nitrogen cycle steps in order: $F i x a t i o n ∣ ni t r i f i c a t i o n ∣ a ss imi l a t i o n ∣ amm o ni f i c a t i o n ∣ d e ni t r i f i c a t i o n$
Percentage change: $(f ina l - ini t ia l) \div ini t ia l \times 100$

How to solve Water, Carbon and Nitrogen Cycles and Human Impact questions

Use this method for cycle diagrams, equation-based mass questions and cause-and-effect questions.

1For a cycle question, name the stores (air, soil, water, living things) and the process that moves matter between them.
2For a mass question, write the balanced equation first and the relative masses (using C = 12, H = 1, O = 16, N = 14).
3Set up a ratio: mass wanted = mass given × (relative mass wanted ÷ relative mass given).
4Check the conservation of mass: total on the left equals total on the right.
5For human-impact questions, link cause, process and effect in a chain (for example excess fertiliser, algal bloom, oxygen depletion, fish death).
6Give one named example (bacterium, gas, process) to support each step.

Water, Carbon and Nitrogen Cycles and Human Impact in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Nitrogen fixation and crop rotation

Suppose a farmer grows wheat on a field for several years and finds that the yield is falling. The next season she grows a legume such as gram on the same field, and the soil is richer afterwards.

Explain this with the nitrogen cycle, naming the bacteria and the form of nitrogen involved.

Try it first, then show the solutionSolution

Wheat takes up nitrates and ammonium from the soil to make proteins, so the soil gradually loses usable nitrogen.
Legume roots have nodules that house Rhizobium bacteria, which carry out nitrogen fixation: N₂ from the air is converted to ammonia.
Nitrifying bacteria (Nitrosomonas, then Nitrobacter) turn the ammonia into nitrite and nitrate, which the next crop can absorb. The soil is therefore enriched.

Answer: Rhizobium in legume nodules fix N₂ into ammonia; nitrification then makes nitrates the next crop can use.

Memory hook: Beans bring back the nitrogen.

A pond turning green

Suppose heavy rain washes fertiliser from a nearby field into a village pond, and within two weeks the pond is covered with green scum and many fish float dead.

Explain the sequence of events using the right technical terms.

Try it first, then show the solutionSolution

Excess nitrate and phosphate from the fertiliser enters the pond (eutrophication begins).
The nutrients make algae grow rapidly, forming an algal bloom.
When the algae die, decomposers use large amounts of the dissolved oxygen to break them down, and the oxygen level falls so low that fish die.

Answer: Fertiliser runoff causes an algal bloom; decomposers then use up the oxygen, killing the fish.

Memory hook: Too much food, then no air.

Tricks to remember Water, Carbon and Nitrogen Cycles and Human Impact

Short memory aids. Say each one aloud once, then use it on a question the same day.

FNAAD
Fixation, Nitrification, Assimilation, Ammonification, Denitrification: 'Few Nice Animals Always Dance'.
Use it when: Writing the nitrogen cycle steps in order.
Fast cycle is life, slow cycle is rock
Fast: photosynthesis and respiration (days to years). Slow: fossils and rocks (millions of years). Fuel burning short-circuits the slow cycle.
Use it when: Carbon cycle answers.
Eutrophication chain
Nutrients, algae, death, decomposers, no oxygen, fish die.
Use it when: Writing a 3-mark answer.
Bacteria matching
Rhizobium fixes in nodules; Nitrosomonas makes nitrite; Nitrobacter makes nitrate; Pseudomonas denitrifies.
Use it when: Naming bacteria in the nitrogen cycle.

Water, Carbon and Nitrogen Cycles and Human Impact: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Steps and bacteria of the nitrogen cycle · 3 marks · Short answerBoard level

Name the process and one organism (or agent) responsible for each: (a) N₂ to ammonia, (b) ammonia to nitrite and then to nitrate, (c) nitrates back to N₂.

Try it first, then show the solutionSolution

(a) Nitrogen fixation, by Rhizobium (in root nodules of legumes) or Azotobacter (free in soil); lightning also fixes nitrogen. (1)
(b) Nitrification, by Nitrosomonas (ammonia to nitrite) and Nitrobacter (nitrite to nitrate). (1)
(c) Denitrification, by Pseudomonas, returning nitrogen gas to the air. (1)

Answer: (a) fixation, Rhizobium; (b) nitrification, Nitrosomonas and Nitrobacter; (c) denitrification, Pseudomonas.

Example 2: The water cycle and a warming climate · 3 marks · Short answerBoard level

(a) Name the processes by which water goes from the surface into the air, and the process by which it returns as snow or rain. (1)
(b) Give two effects of a warmer climate on the water cycle. (2)

Try it first, then show the solutionSolution

(a) Evaporation (from oceans and lakes) and transpiration (from plants) put water into the air; precipitation returns it. (1)
(b) Warmer air holds more moisture, so some areas get heavier rain while others get drought. (1)
Melting glaciers raise river flow and, over time, sea level; intense rain also increases erosion and reduces groundwater recharge. (1)

Answer: (a) evaporation and transpiration; precipitation; (b) heavier rain in some places and drought in others; glacier melt and sea-level rise.

Example 3: Carbon numbers: ppm, photosynthesis and methane · 5 marks · Long answerHarder board level

(Relative masses: C = 12, H = 1, O = 16.)
(a) CO₂ in the air rose from about 315 ppm to 420 ppm. Find the increase and the percentage increase. (1)
(b) What mass of CO₂ is absorbed in making 90 g of glucose by 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂? (1)
(c) What mass of oxygen is released in (b)? (1)
(d) What mass of CO₂ is released by burning 8 kg of methane (CH₄ + 2O₂ → CO₂ + 2H₂O)? (2)

Try it first, then show the solutionSolution

(a) Increase = 420 − 315 = 105 ppm. Percentage = 105 ÷ 315 × 100 = 33.3%. (1)
(b) Masses: 6CO₂ = 6 × 44 = 264 g; C₆H₁₂O₆ = 180 g. CO₂ absorbed = 90 × 264 ÷ 180 = 132 g. (1)
(c) 6O₂ = 6 × 32 = 192 g per 180 g glucose, so O₂ released = 90 × 192 ÷ 180 = 96 g. Check: 132 g CO₂ + water 54 g = 186 g = 90 g + 96 g. (1)
(d) CH₄ = 16 and CO₂ = 44, so 16 g of methane gives 44 g of CO₂. 8 kg gives 8 × 44 ÷ 16 = 22 kg of CO₂. (2)

Answer: (a) 105 ppm and 33.3%; (b) 132 g; (c) 96 g; (d) 22 kg.

Example 4: Haber-Bosch ammonia and the nitrogen cycle · 5 marks · Long answerHarder board level

(Relative masses: N = 14, H = 1.)
A factory makes ammonia by N₂ + 3H₂ → 2NH₃ from 14 kg of nitrogen gas.
(a) Find the mass of ammonia formed. (1)
(b) Find the mass of hydrogen needed. (1)
(c) Why can plants not use the nitrogen gas of the air directly? (1)
(d) Name the natural process in a legume's root nodules that does the job of the factory. (1)
(e) Give one harm of overusing nitrogen fertiliser. (1)

Try it first, then show the solutionSolution

(a) 28 g N₂ gives 34 g NH₃, so 14 kg gives 14 × 34 ÷ 28 = 17 kg. (1)
(b) 6 g H₂ for 28 g N₂, so hydrogen = 14 × 6 ÷ 28 = 3 kg. Check: 14 + 3 = 17 kg. (1)
(c) N₂ molecules are very unreactive, so plants cannot take them in and use them; the nitrogen must first be fixed into ammonia or nitrate. (1)
(d) Biological nitrogen fixation by Rhizobium bacteria. (1)
(e) Excess fertiliser washes into lakes and rivers, causing eutrophication (algal blooms, oxygen depletion and fish death), and can harm soil. (1)

Answer: (a) 17 kg; (b) 3 kg; (c) N₂ is unreactive; (d) nitrogen fixation by Rhizobium; (e) eutrophication.

Example 5: Clearing a forested catchment · 4 marks · Case-basedHOTS / competency

Suppose a hill catchment of 1 hectare (10 000 m²) receives 1000 mm of rain in a year. Under forest, 20% of the rain leaves as surface runoff; after clearing the forest, 50% leaves as runoff.
(a) Find the volume of rain on the hectare in m³. (1)
(b) Find the runoff volume before and after clearing. (1)
(c) Find the percentage increase in runoff. (1)
(d) Explain two consequences for the hydrosphere and geosphere, and one for the carbon cycle. (1)

Try it first, then show the solutionSolution

(a) 1000 mm = 1 m of water over 10 000 m², so volume = 10 000 × 1 = 10 000 m³. (1)
(b) Forest: 20% of 10 000 = 2000 m³. Cleared: 50% of 10 000 = 5000 m³. (1)
(c) Increase = 5000 − 2000 = 3000 m³; 3000 ÷ 2000 × 100 = 150%. (1)
(d) More runoff gives more soil erosion (geosphere) and flood risk in rivers, with less infiltration and groundwater recharge (hydrosphere). Fewer trees mean less photosynthesis, so less CO₂ is removed from the air (carbon cycle). (1)

Answer: (a) 10 000 m³; (b) 2000 m³ and 5000 m³; (c) 150%; (d) erosion, flooding and less recharge; less CO₂ taken up.

Where students lose marks in Water, Carbon and Nitrogen Cycles and Human Impact

The slips examiners see most often, and what to write instead.

Mistake: Writing the nitrogen cycle steps in the wrong order or with wrong bacteria.
The names fixation, nitrification and denitrification look alike.
Wrong working
Denitrification converts N₂ to ammonia.
Right working
Fixation converts N₂ to ammonia; denitrification converts nitrates back to N₂.
Do this: 'De' means undo: denitrification undoes fixation.
Usually costs: 1 to 2 marks.
Mistake: Writing that photosynthesis only removes CO₂ and ignoring respiration.
Students see plants as only absorbing CO₂.
Wrong working
Plants only take in CO₂ so they cannot add CO₂.
Right working
Plants take in CO₂ by photosynthesis and release CO₂ by respiration and decomposition.
Do this: Name both processes in the carbon cycle.
Usually costs: 1 mark.
Mistake: Not balancing or not using relative masses in a mass question.
The equation is taken as 1 g reacts with 1 g.
Wrong working
1 kg CH₄ gives 1 kg CO₂.
Right working
16 g CH₄ gives 44 g CO₂, so 1 kg gives 2.75 kg.
Do this: Write relative masses beneath the equation.
Usually costs: 2 marks.
Mistake: Saying eutrophication is caused by too little oxygen.
The cause and the effect are reversed.
Wrong working
Low oxygen causes algal blooms.
Right working
Excess nutrients cause algal blooms; decomposition of dead algae then lowers oxygen.
Do this: Cause is nutrients; oxygen loss is the effect.
Usually costs: 1 mark.
Mistake: Confusing ground-level ozone with the ozone layer.
Both are called ozone.
Wrong working
Smog damages the ozone layer.
Right working
Ground-level ozone in smog is harmful to breathe; the stratospheric ozone layer protects against UV.
Do this: State the place: ground level or stratosphere.
Usually costs: 1 mark.

Exam tips

In a cycle question, name each process and give an organism or agent; a bare list of words earns half marks.
Give the equation for photosynthesis and for burning fossil fuels when answering carbon cycle questions.
For a mass question, write the balanced equation, relative masses and ratio before calculating.
In human-impact answers, write cause, process and effect in order.
If a diagram is asked, label the stores (air, soil, water, plants, animals) and the arrows with process names.

Practice questions on Water, Carbon and Nitrogen Cycles and Human Impact

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
Name two processes that add carbon dioxide to the atmosphere and one process that removes it.
Hint
Think of what living things and fuels do.
Answer
Any two of respiration, decomposition, burning of fossil fuels (1 mark). Photosynthesis removes CO₂ from the atmosphere (1 mark).
Q2 · 3 marks · Short answerHarder board level
A power plant burns 4.8 kg of methane (CH₄ + 2O₂ → CO₂ + 2H₂O). With C = 12, H = 1, O = 16, find (a) the mass of CO₂ released, (b) the mass of oxygen used, and (c) verify the conservation of mass using the water formed.
Hint
16 g of CH₄ reacts with 64 g of O₂.
Answer
(a) 4.8 × 44 ÷ 16 = 13.2 kg (1 mark). (b) 4.8 × 64 ÷ 16 = 19.2 kg (1 mark). (c) Water = 4.8 × 36 ÷ 16 = 10.8 kg; reactants 4.8 + 19.2 = 24 kg, products 13.2 + 10.8 = 24 kg (1 mark).
Q3 · 2 marks · Assertion-reasonBoard level
Assertion (A): Plants cannot use the nitrogen gas of the air directly. Reason (R): Nitrogen gas (N₂) is very unreactive, so it must first be converted to ammonia or nitrates. Choose: (a) A and R both true, R explains A; (b) A and R both true, R does not explain A; (c) A true, R false; (d) A false, R true.
Hint
Think of what nitrogen fixation achieves.
Answer
(a). Both are true and the unreactive nature of N₂ is why it must first be fixed by bacteria, lightning or industry into a usable form.
Q4 · 3 marks · Short answerHarder board level
The oceans hold about 71% of the world's carbon and the atmosphere about 1%. (a) How many times more carbon do the oceans hold than the atmosphere? (b) Explain how extra atmospheric CO₂ makes the oceans more acidic. (c) State one effect of this on marine life.
Hint
Think of CO₂ dissolving in water.
Answer
(a) 71 ÷ 1 = 71 times (1 mark). (b) The oceans absorb the extra CO₂, and the dissolved CO₂ makes the water more acidic (1 mark). (c) Plankton, shell-builders and coral reefs are harmed (1 mark).
Q5 · 4 marks · Case-basedHOTS / competency
In a lake the dissolved oxygen before a fertiliser spill is 8 mg per litre. After an algal bloom has died back it falls to 2 mg per litre. Assume the fish in this lake need at least 5 mg per litre. (a) Find the percentage fall in dissolved oxygen. (b) Will the fish survive? (c) Describe the chain of events leading to this fall. (d) Suggest two ways to prevent a repeat.
Hint
Fall ÷ original × 100.
Answer
(a) (8 − 2) ÷ 8 × 100 = 75% (1 mark). (b) No, 2 mg per litre is below 5 mg per litre (1 mark). (c) Fertiliser nitrates and phosphates cause an algal bloom; when the algae die, decomposers use up the oxygen (eutrophication) (1 mark). (d) Use less fertiliser or time its use, and plant buffer strips of vegetation to catch runoff (1 mark).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which bacteria fix nitrogen in root nodules of legumes?Show answer
Rhizobium.
2.Which bacteria carry out denitrification?Show answer
Pseudomonas, which return nitrogen to the air as N₂.
3.Write the Haber-Bosch reaction.Show answer
N₂ + 3H₂ → 2NH₃.
4.What percentage of the world's carbon do the oceans hold?Show answer
About 71%.
5.What is the main source that restores oxygen in the air?Show answer
Photosynthesis of land plants and ocean phytoplankton.
6.What is eutrophication?Show answer
Nutrient enrichment of water that causes algal blooms and then oxygen loss as dead algae decompose.

Water, Carbon and Nitrogen Cycles and Human Impact: frequently asked questions

Why is the carbon cycle called fast and slow?

Photosynthesis, respiration and decomposition move carbon within days to years (fast), while burial of remains as coal, oil and gas and their formation takes millions of years (slow).

Why do legumes enrich the soil?

Rhizobium bacteria in their root nodules fix atmospheric nitrogen into ammonia, which becomes nitrate for later crops.

How did ammonia from air change farming?

Making ammonia from nitrogen and hydrogen supplied most fertiliser, which raised crop yields, but it needs a lot of energy and careless use harms soil and water.

What is the Keeling curve?

A graph of rising atmospheric CO₂, from about 315 ppm in 1960 to 420 ppm in 2025, with yearly dips when northern plants grow.

How does deforestation affect rainfall?

Fewer trees mean less transpiration, so less moisture goes into the air and local rainfall can fall.