This MCQ module is based on: Biogeochemical Cycles — Water, Carbon and Nitrogen Cycles
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Biogeochemical Cycles — Water, Carbon and Nitrogen Cycles
🎓 Class 9
Science
CBSE
Theory
Ch 13 — Earth as a System: Energy, Matter, and Life
⏱ ~19 min
🌐 Language: [gtranslate]
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This assessment will be based on: Biogeochemical Cycles — Water, Carbon and Nitrogen Cycles
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Introduction: Matter Goes Round and Round
The atoms in the glass of water you drank this morning may once have flowed in the Ganga, condensed in a Himalayan cloud, sat inside a dinosaur, and eventually returned to your tap. Earth has a fixed amount of water, carbon, nitrogen and oxygen. Nothing new arrives — these substances are simply moved from one sphere to another, again and again. Such repeating journeys are called biogeochemical cycles.
Key Idea: Energy flows through Earth in one direction (Sun → plants → animals → heat lost to space). But matter cycles — atoms are reused over and over.
13.8 The Water Cycle
The water cycle (also called the hydrological cycle) is the journey water takes between the oceans, atmosphere, land and living things. It is powered almost entirely by sunlight.
Stages of the water cycle
- Evaporation: Sunlight heats the seas, rivers and lakes. Liquid water turns to invisible water vapour and rises.
- Transpiration: Plants release water vapour through tiny pores in their leaves. (Together with evaporation, this is called evapotranspiration.)
- Condensation: High in the cool air, vapour clusters around dust specks and forms tiny droplets — clouds.
- Precipitation: When droplets grow heavy enough, they fall back as rain, snow or hail.
- Infiltration: Some rainwater seeps into the soil and refills the groundwater.
- Runoff: Water that does not soak in flows over the surface into streams and rivers, eventually returning to the sea.
💧 Water-Cycle Step-Through — Click each stage in order L3 Apply
Click evaporation, then condensation/clouds, then precipitation, then transpiration, then runoff to step through the hydrological cycle in motion.
Click the labelled stages above (evaporation → condensation → precipitation → transpiration → runoff) to step through the water cycle.
The Indian monsoon is a giant water cycle in motion. The south-west monsoon picks up moisture from the Arabian Sea and Bay of Bengal, drops it as rain over the subcontinent, and the rain ultimately flows back through rivers like the Ganga and Krishna into the ocean.
13.9 The Carbon Cycle
Carbon is the building block of every living cell. The carbon cycle shows how carbon atoms move between the atmosphere (as CO₂), living things, oceans and rocks.
Main pathways
- Photosynthesis: Green plants take in CO₂ from the air and use sunlight to make glucose. Carbon enters the biosphere.
- Respiration: Plants, animals and microbes release CO₂ back into the air while breaking down food for energy.
- Decomposition: When organisms die, decomposers break their bodies down, releasing CO₂.
- Combustion: Burning of wood, coal, petrol or diesel releases CO₂. This includes both natural fires and human activity.
- Ocean exchange: Seawater dissolves CO₂ from the air and releases it back as conditions change.
- Geological storage: Over millions of years, dead plants buried under pressure became coal, petroleum and natural gas — the fossil fuels.
Photosynthesis can be summarised as:
6CO₂ + 6H₂O —(sunlight + chlorophyll)→ C₆H₁₂O₆ + 6O₂
Human disruption: By burning huge amounts of coal, oil and natural gas, humans are pushing fossil-fuel carbon back into the atmosphere far faster than plants and oceans can absorb it. This is the chief cause of global warming.
13.10 The Nitrogen Cycle
About 78% of the air is nitrogen gas (N₂), yet plants and animals cannot use it directly — the two atoms in N₂ are bonded too tightly. The nitrogen cycle describes how nitrogen is converted into usable forms and eventually returned to the air.
Steps of the nitrogen cycle
- Nitrogen fixation: Specialised bacteria living free in the soil (e.g. Azotobacter) or in nodules on legume roots (Rhizobium in chana, moong, soybean) convert N₂ into ammonia (NH₃). Lightning also fixes a small amount of nitrogen by combining N₂ and O₂ in the air to form nitrogen oxides that wash down with rain.
- Nitrification: Soil bacteria turn ammonia first into nitrites (NO₂⁻) and then into nitrates (NO₃⁻), the form taken up by plant roots.
- Assimilation: Plants build proteins and DNA from nitrates. Animals get nitrogen by eating plants.
- Ammonification: Decomposers break down dead remains and waste, releasing ammonia back into the soil.
- Denitrification: Other bacteria turn nitrates back into N₂ gas, completing the cycle.
Indian farmers have used the nitrogen cycle for centuries. Growing pulses (chana, urad, moong) restores soil fertility because their root nodules host nitrogen-fixing Rhizobium. Modern farming adds extra nitrogen as urea fertiliser — useful, but excess fertiliser washing into rivers can pollute the water.
13.11 The Oxygen Cycle
The oxygen we breathe is also continuously cycling. Oxygen leaves the atmosphere through three main routes — respiration in living things, combustion of fuels, and rusting and weathering of rocks. It returns to the atmosphere mainly through photosynthesis by plants, algae and cyanobacteria. Of all the photosynthesis on Earth, more than half happens in the oceans, performed by tiny floating algae called phytoplankton — these are sometimes called the "lungs of the planet" along with the rainforests.
Note: The carbon cycle and oxygen cycle are like two sides of the same coin. Photosynthesis takes CO₂ in and releases O₂; respiration and combustion take O₂ in and release CO₂.
13.12 How Cycles Keep Earth in Balance
For most of Earth's history, the cycles have worked in near-perfect balance — what was taken out by one process was put back by another. This balance gave us a stable climate, breathable air and fertile soil. But human activity has begun to push every cycle out of step:
| Cycle | Human disruption | Consequence |
|---|---|---|
| Water | Damming rivers, paving cities, over-pumping groundwater | Floods, droughts, falling water tables |
| Carbon | Burning fossil fuels, cutting forests | Rising CO₂ → global warming, climate change |
| Nitrogen | Excess use of urea fertiliser, vehicle exhaust | Eutrophication of lakes, acid rain |
| Oxygen | Deforestation, ocean pollution | Loss of natural producers; less O₂ replenishment |
Activity 13.2 — Tracking a Carbon AtomL3 Apply
Predict first: Imagine a CO₂ molecule released from your bicycle's tail-pipe today. Where might it travel over the next year?
- Draw five boxes labelled: Atmosphere, Plant, Animal, Soil, Ocean.
- Place a star inside Atmosphere — that is your CO₂ atom.
- Move the star one box at a time, drawing an arrow and writing the process name (photosynthesis, respiration, decomposition, ocean uptake, etc.).
- Continue until your star has visited at least four boxes and returned to the starting one.
Sample path: Atmosphere → (photosynthesis) → Mango tree → (eaten as fruit) → Animal/Human → (respiration) → Atmosphere → (dissolved in rain) → Ocean → (released back) → Atmosphere. Total time can be days, years or even thousands of years if the atom is locked up in a fossil fuel.
Competency-Based Questions L3 Apply
A farmer in Punjab grows wheat year after year on the same field and notices the yield falling. An agricultural officer suggests rotating with a leguminous crop like chana (chickpea) for one season. The farmer is sceptical and asks why a different crop should help his wheat the next year.
1. Which natural cycle is the officer relying on?
(c) Leguminous plants host Rhizobium bacteria which fix atmospheric nitrogen and enrich the soil.
2. Name two processes by which water from the Arabian Sea ends up as rain over Maharashtra.
Evaporation (sea water → vapour) and condensation (vapour → cloud droplets) followed by precipitation as rain.
3. True or False: Burning petrol takes oxygen out of the air and releases carbon dioxide.
True. Combustion of any fossil fuel: hydrocarbons + O₂ → CO₂ + H₂O.
4. Which of the following is NOT a way nitrogen enters the soil in usable form?
(c) Photosynthesis fixes carbon (from CO₂) — not nitrogen. The other three are genuine routes for nitrogen to enter the soil.
5. Briefly explain why the carbon and oxygen cycles are linked.
Photosynthesis removes CO₂ and releases O₂; respiration and combustion remove O₂ and release CO₂. The same chemical reactions that drive one cycle automatically drive the other in the opposite direction.
Assertion–Reason Questions L4 Analyse
Options: (A) Both A and R true, R explains A. (B) Both true, R does not explain A. (C) A true, R false. (D) A false, R true.
A: Plants growing chickpea improve soil fertility for the next crop.
R: Rhizobium bacteria living in legume root nodules fix atmospheric nitrogen into ammonia.
(A) Both true and the reason explains the assertion — fixed nitrogen stays in the soil to feed the next crop.
A: Burning fossil fuels does not affect the atmosphere because the carbon was originally from the air.
R: Coal and oil were formed from ancient plants that absorbed CO₂ from the atmosphere.
(D) Reason is true but the assertion is false — although carbon was once from the air, releasing it back today (over decades) is much faster than nature can reabsorb, so atmospheric CO₂ rises.
A: Lightning plays a role in the nitrogen cycle.
R: The high energy of a lightning bolt allows N₂ and O₂ in the air to combine, forming nitrogen oxides that dissolve in rain.
(A) Both true and the reason directly explains the assertion. Lightning supplies a small but real share of fixed nitrogen to soils.
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