Air Is Mostly Nitrogen and Almost No Plant Can Touch It
Learn the processes of the water cycle, how photosynthesis and respiration link the carbon and oxygen cycles, how nitrogen-fixing bacteria make nitrogen usable, and how human activity unbalances all three.
If air is mostly nitrogen, why do crops need nitrogen fertiliser?
Because nitrogen gas is almost unusable. There is plenty of it and a plant cannot take any of it directly.
Nitrogen makes up the largest share of the air, and it exists there as a molecule held together by an exceptionally strong bond. That makes it chemically unreactive — it will not simply join with anything, and plant roots have no way to break it apart.
So a plant surrounded by nitrogen can still starve of it. What plants can absorb are nitrogen compounds dissolved in the soil, chiefly nitrates, and something has to make those from the gas first.
That conversion is called nitrogen fixation, and the organisms that do most of it are bacteria — some of them living in swellings on the roots of pea and gram plants.
A farmer applying fertiliser, and a farmer rotating a crop of gram, are solving exactly the same problem by two different routes. This page covers the third part of the CBSE Class 9 Science chapter on the Earth as a system.
Nitrogen makes up the largest share of the air, and it exists there as a molecule held together by an exceptionally strong bond. That makes it chemically unreactive — it will not simply join with anything, and plant roots have no way to break it apart.
So a plant surrounded by nitrogen can still starve of it. What plants can absorb are nitrogen compounds dissolved in the soil, chiefly nitrates, and something has to make those from the gas first.
That conversion is called nitrogen fixation, and the organisms that do most of it are bacteria — some of them living in swellings on the roots of pea and gram plants.
A farmer applying fertiliser, and a farmer rotating a crop of gram, are solving exactly the same problem by two different routes. This page covers the third part of the CBSE Class 9 Science chapter on the Earth as a system.
What are the processes of the water cycle?
Six processes, each with a name, and together they form a loop with no beginning and no end.
- Evaporation — liquid water from oceans, lakes and rivers becomes vapour, using energy from the Sun
- Transpiration — water vapour is lost from the leaves of plants. Together with evaporation this is called evapotranspiration
- Condensation — the rising vapour cools and becomes tiny droplets, forming clouds
- Precipitation — the droplets merge and fall as rain, snow, sleet or hail
- Infiltration — water soaks into the ground and becomes groundwater
- Surface runoff — water flows over the ground into streams and rivers and back to the sea
Sublimation also contributes, where snow and ice turn directly into vapour without melting first.
Everyday evidence for three of the steps. A puddle after rain disappears within hours on a hot day — evaporation. Droplets form on the outside of a cold glass of water — condensation, from the vapour already in the air. The monsoon brings onto the land water that was evaporated from the ocean — the whole cycle, at the scale of a subcontinent.
The Sun drives the whole cycle. Evaporation and transpiration both need energy, and that energy is solar. So the water cycle is not a separate topic from the uneven heating of the first part of this chapter — it is one of the things the received energy does.
Water is not created or destroyed in the cycle, only moved and changed in state. The total quantity of water on Earth is essentially constant, and every drop is recycled endlessly.
**So what does running out of water mean? It means running out of accessible fresh water in a particular place at a particular time**. Most of the planet's water is salty, and much of the fresh water is frozen or deep underground. The total is unchanged; what changes is how much of it is where people need it — and that distinction is the whole basis of the water-conservation measures in the last section of this page.
- Evaporation — liquid water from oceans, lakes and rivers becomes vapour, using energy from the Sun
- Transpiration — water vapour is lost from the leaves of plants. Together with evaporation this is called evapotranspiration
- Condensation — the rising vapour cools and becomes tiny droplets, forming clouds
- Precipitation — the droplets merge and fall as rain, snow, sleet or hail
- Infiltration — water soaks into the ground and becomes groundwater
- Surface runoff — water flows over the ground into streams and rivers and back to the sea
Sublimation also contributes, where snow and ice turn directly into vapour without melting first.
Everyday evidence for three of the steps. A puddle after rain disappears within hours on a hot day — evaporation. Droplets form on the outside of a cold glass of water — condensation, from the vapour already in the air. The monsoon brings onto the land water that was evaporated from the ocean — the whole cycle, at the scale of a subcontinent.
The Sun drives the whole cycle. Evaporation and transpiration both need energy, and that energy is solar. So the water cycle is not a separate topic from the uneven heating of the first part of this chapter — it is one of the things the received energy does.
Water is not created or destroyed in the cycle, only moved and changed in state. The total quantity of water on Earth is essentially constant, and every drop is recycled endlessly.
**So what does running out of water mean? It means running out of accessible fresh water in a particular place at a particular time**. Most of the planet's water is salty, and much of the fresh water is frozen or deep underground. The total is unchanged; what changes is how much of it is where people need it — and that distinction is the whole basis of the water-conservation measures in the last section of this page.
How do photosynthesis and respiration link the carbon and oxygen cycles?
Each process supplies exactly what the other consumes, which is why the two cycles cannot be separated.
Read those two lines against each other. Photosynthesis takes in carbon dioxide and gives out oxygen; respiration takes in oxygen and gives out carbon dioxide. Each is the other's supplier.
Where carbon is found. As carbon dioxide in the air; as carbonates in rocks and shells; dissolved as carbon dioxide in water; in every organic molecule in every living thing; and in coal, petroleum and natural gas.
The carbon cycle:
- Plants take carbon dioxide from the air in photosynthesis and build it into glucose
- Animals eat plants, so the carbon passes along food chains
- All living things return carbon dioxide to the air by respiration
- Decomposers break down dead matter and release carbon dioxide
- Combustion of wood and fossil fuels releases carbon dioxide
The oxygen cycle:
- Photosynthesis releases oxygen into the air
- Respiration and combustion consume it
- Oxygen is also consumed in the rusting of metals and in the decay of organic matter
Everyday evidence. A sealed jar with a green plant and enough light stays breathable far longer than a sealed jar without one, because the plant returns the oxygen the respiring organisms use. A bonfire consumes oxygen and releases carbon dioxide, exactly as respiration does but far faster.
Together the two processes keep the proportions of the gases in the air roughly steady. Neither is perfectly reversed by the other, and at the level of the whole cycle each consumes the other's product, so the composition of the atmosphere has stayed remarkably constant.
Burning fossil fuels breaks that balance, and the reason is rate. Coal and petroleum hold carbon that was removed from the air and locked away for very long periods. Burning them returns that carbon to the atmosphere far faster than photosynthesis and the oceans can take it up again. So carbon dioxide accumulates — and, as the first part of this chapter explained, more carbon dioxide means a stronger greenhouse effect.
Nothing has been created; the timing has been changed. That framing matters, and the last section of this page returns to it.
Read those two lines against each other. Photosynthesis takes in carbon dioxide and gives out oxygen; respiration takes in oxygen and gives out carbon dioxide. Each is the other's supplier.
Where carbon is found. As carbon dioxide in the air; as carbonates in rocks and shells; dissolved as carbon dioxide in water; in every organic molecule in every living thing; and in coal, petroleum and natural gas.
The carbon cycle:
- Plants take carbon dioxide from the air in photosynthesis and build it into glucose
- Animals eat plants, so the carbon passes along food chains
- All living things return carbon dioxide to the air by respiration
- Decomposers break down dead matter and release carbon dioxide
- Combustion of wood and fossil fuels releases carbon dioxide
The oxygen cycle:
- Photosynthesis releases oxygen into the air
- Respiration and combustion consume it
- Oxygen is also consumed in the rusting of metals and in the decay of organic matter
Everyday evidence. A sealed jar with a green plant and enough light stays breathable far longer than a sealed jar without one, because the plant returns the oxygen the respiring organisms use. A bonfire consumes oxygen and releases carbon dioxide, exactly as respiration does but far faster.
Together the two processes keep the proportions of the gases in the air roughly steady. Neither is perfectly reversed by the other, and at the level of the whole cycle each consumes the other's product, so the composition of the atmosphere has stayed remarkably constant.
Burning fossil fuels breaks that balance, and the reason is rate. Coal and petroleum hold carbon that was removed from the air and locked away for very long periods. Burning them returns that carbon to the atmosphere far faster than photosynthesis and the oceans can take it up again. So carbon dioxide accumulates — and, as the first part of this chapter explained, more carbon dioxide means a stronger greenhouse effect.
Nothing has been created; the timing has been changed. That framing matters, and the last section of this page returns to it.
How do nitrogen-fixing bacteria make nitrogen available to plants?
By converting unreactive nitrogen gas into compounds that roots can absorb.
The nitrogen cycle has five named steps, and fixation is the first.
Nitrogen fixation. Atmospheric nitrogen is converted into nitrogen compounds by three routes:
- Nitrogen-fixing bacteria such as Rhizobium, living in nodules on the roots of leguminous plants — gram, pea, beans, groundnut — convert nitrogen into ammonium compounds
- Free-living soil bacteria and some blue-green algae fix nitrogen independently of any plant
- Lightning supplies enough energy to make nitrogen and oxygen combine, and the products dissolve in rain and reach the soil
Nitrification. Soil bacteria convert ammonium compounds first into nitrites and then into nitrates, which is the form plant roots absorb most readily.
Assimilation. Plants absorb nitrates and use the nitrogen to build proteins and nucleic acids. Animals obtain their nitrogen by eating plants or by eating animals that ate plants.
Ammonification. Decomposers break down dead organisms and excreta, returning nitrogen to the soil as ammonium compounds.
Denitrification. Other soil bacteria convert nitrates back into nitrogen gas, which returns to the atmosphere and completes the cycle.
Everyday evidence — crop rotation. Farmers grow a leguminous crop such as gram, groundnut or arhar in rotation with wheat or rice, because the Rhizobium in its root nodules leaves the soil richer in nitrogen for the crop that follows. Pull up a gram plant carefully and the nodules are visible on the roots as small pink or white swellings. That practice is nitrogen fixation put to work, and it is why rotation improves yields without fertiliser.
The relationship is mutual, and worth stating as such. The plant supplies the bacteria with food and a protected place to live; the bacteria supply the plant with usable nitrogen. Neither is doing the other a favour, and neither could do the job alone — the plant cannot fix nitrogen and the bacteria cannot photosynthesise.
Why nitrogen matters at all. It is needed for proteins and for DNA, so no organism can do without it. That is why a nitrogen-poor soil limits growth however much water, light and carbon dioxide are available — a plant short of nitrogen cannot build the proteins that photosynthesis itself depends on.
The nitrogen cycle has five named steps, and fixation is the first.
Nitrogen fixation. Atmospheric nitrogen is converted into nitrogen compounds by three routes:
- Nitrogen-fixing bacteria such as Rhizobium, living in nodules on the roots of leguminous plants — gram, pea, beans, groundnut — convert nitrogen into ammonium compounds
- Free-living soil bacteria and some blue-green algae fix nitrogen independently of any plant
- Lightning supplies enough energy to make nitrogen and oxygen combine, and the products dissolve in rain and reach the soil
Nitrification. Soil bacteria convert ammonium compounds first into nitrites and then into nitrates, which is the form plant roots absorb most readily.
Assimilation. Plants absorb nitrates and use the nitrogen to build proteins and nucleic acids. Animals obtain their nitrogen by eating plants or by eating animals that ate plants.
Ammonification. Decomposers break down dead organisms and excreta, returning nitrogen to the soil as ammonium compounds.
Denitrification. Other soil bacteria convert nitrates back into nitrogen gas, which returns to the atmosphere and completes the cycle.
Everyday evidence — crop rotation. Farmers grow a leguminous crop such as gram, groundnut or arhar in rotation with wheat or rice, because the Rhizobium in its root nodules leaves the soil richer in nitrogen for the crop that follows. Pull up a gram plant carefully and the nodules are visible on the roots as small pink or white swellings. That practice is nitrogen fixation put to work, and it is why rotation improves yields without fertiliser.
The relationship is mutual, and worth stating as such. The plant supplies the bacteria with food and a protected place to live; the bacteria supply the plant with usable nitrogen. Neither is doing the other a favour, and neither could do the job alone — the plant cannot fix nitrogen and the bacteria cannot photosynthesise.
Why nitrogen matters at all. It is needed for proteins and for DNA, so no organism can do without it. That is why a nitrogen-poor soil limits growth however much water, light and carbon dioxide are available — a plant short of nitrogen cannot build the proteins that photosynthesis itself depends on.
How do human activities unbalance the cycles, and what can be done?
By adding material to one part of a cycle faster than the other parts can move it on. Nothing is destroyed — the timing is broken.
The carbon cycle. Burning fossil fuels and clearing forests raise the concentration of atmospheric carbon dioxide, strengthening the greenhouse effect and warming the planet. The oceans absorb some of the surplus and become more acidic as a result, which harms corals and shelled organisms.
The nitrogen cycle. Heavy use of nitrogenous fertilisers adds far more fixed nitrogen than the natural cycle handles. Rain washes nitrates off the fields into rivers, lakes and ponds, where they act as a nutrient supply. Algae multiply explosively in an algal bloom, then die, and the decomposers breaking them down consume the dissolved oxygen — so the fish suffocate. That whole sequence is called eutrophication.
The water cycle. Deforestation reduces transpiration and increases runoff, so less water soaks in and more is lost quickly — lowering the water table and worsening floods. Concreting over land does the same. Over-extraction of groundwater draws it down faster than rain can recharge it.
The oxygen cycle. Deforestation removes the photosynthesis that replenishes oxygen, while combustion consumes it.
Measures to reduce the harm:
- Reduce fossil fuel use and shift to renewable sources — solar, wind and hydro
- Afforestation, and protecting the forests that remain
- Use fertilisers judiciously, and prefer organic manure and crop rotation with legumes, which fix nitrogen in place rather than adding it in excess
- Rainwater harvesting and deliberate recharge of groundwater
- Treat sewage and industrial effluent before discharging it
- Reduce, reuse and recycle, to cut both extraction and waste
The problem is rate, not quantity. A cycle can absorb a great deal so long as each step keeps pace with the one before it. Carbon released by burning does not vanish and does not add to the planet's total — it accumulates in the atmosphere because photosynthesis and the oceans cannot take it up as fast as it arrives. Nitrates washed into a lake are not new nitrogen — they arrived somewhere the cycle cannot process them quickly.
That single framing explains why every measure on the list is what it is. Each one either slows the input — burning less, fertilising less — or speeds the uptake — planting more, harvesting rain, treating effluent. None of them destroys anything, because nothing needs destroying. The cycles still work; they are simply being fed faster than they turn.
The carbon cycle. Burning fossil fuels and clearing forests raise the concentration of atmospheric carbon dioxide, strengthening the greenhouse effect and warming the planet. The oceans absorb some of the surplus and become more acidic as a result, which harms corals and shelled organisms.
The nitrogen cycle. Heavy use of nitrogenous fertilisers adds far more fixed nitrogen than the natural cycle handles. Rain washes nitrates off the fields into rivers, lakes and ponds, where they act as a nutrient supply. Algae multiply explosively in an algal bloom, then die, and the decomposers breaking them down consume the dissolved oxygen — so the fish suffocate. That whole sequence is called eutrophication.
The water cycle. Deforestation reduces transpiration and increases runoff, so less water soaks in and more is lost quickly — lowering the water table and worsening floods. Concreting over land does the same. Over-extraction of groundwater draws it down faster than rain can recharge it.
The oxygen cycle. Deforestation removes the photosynthesis that replenishes oxygen, while combustion consumes it.
Measures to reduce the harm:
- Reduce fossil fuel use and shift to renewable sources — solar, wind and hydro
- Afforestation, and protecting the forests that remain
- Use fertilisers judiciously, and prefer organic manure and crop rotation with legumes, which fix nitrogen in place rather than adding it in excess
- Rainwater harvesting and deliberate recharge of groundwater
- Treat sewage and industrial effluent before discharging it
- Reduce, reuse and recycle, to cut both extraction and waste
The problem is rate, not quantity. A cycle can absorb a great deal so long as each step keeps pace with the one before it. Carbon released by burning does not vanish and does not add to the planet's total — it accumulates in the atmosphere because photosynthesis and the oceans cannot take it up as fast as it arrives. Nitrates washed into a lake are not new nitrogen — they arrived somewhere the cycle cannot process them quickly.
That single framing explains why every measure on the list is what it is. Each one either slows the input — burning less, fertilising less — or speeds the uptake — planting more, harvesting rain, treating effluent. None of them destroys anything, because nothing needs destroying. The cycles still work; they are simply being fed faster than they turn.
Exam tip
Exam tip: name every process in the cycle, not just the diagram
Name each water-cycle process: evaporation, transpiration (together evapotranspiration), condensation, precipitation, infiltration, surface runoff, and sublimation. A labelled arrow without a process name earns less.
Say that the Sun drives the water cycle, and that water is moved and changed in state, never created or destroyed.
For carbon and oxygen, write both equations and then state the link: photosynthesis takes in carbon dioxide and gives out oxygen; respiration does the reverse.
Name all the carbon reservoirs — air, rocks and shells as carbonates, dissolved in water, in living things, and in fossil fuels.
For the nitrogen cycle, name all five steps in order: fixation, nitrification, assimilation, ammonification, denitrification.
Name three routes of fixation: Rhizobium in root nodules of legumes, free-living soil bacteria and blue-green algae, and lightning.
Say that plants absorb nitrates, and that nitrogen is needed for proteins and nucleic acids.
State the Rhizobium relationship as mutual — food and shelter one way, usable nitrogen the other.
For human impact, name the process: eutrophication for fertiliser runoff, with algal bloom, decomposition and oxygen depletion in that order.
And frame the whole answer as rate: material is added faster than the cycle can move it on, so every remedy either slows the input or speeds the uptake.
Say that the Sun drives the water cycle, and that water is moved and changed in state, never created or destroyed.
For carbon and oxygen, write both equations and then state the link: photosynthesis takes in carbon dioxide and gives out oxygen; respiration does the reverse.
Name all the carbon reservoirs — air, rocks and shells as carbonates, dissolved in water, in living things, and in fossil fuels.
For the nitrogen cycle, name all five steps in order: fixation, nitrification, assimilation, ammonification, denitrification.
Name three routes of fixation: Rhizobium in root nodules of legumes, free-living soil bacteria and blue-green algae, and lightning.
Say that plants absorb nitrates, and that nitrogen is needed for proteins and nucleic acids.
State the Rhizobium relationship as mutual — food and shelter one way, usable nitrogen the other.
For human impact, name the process: eutrophication for fertiliser runoff, with algal bloom, decomposition and oxygen depletion in that order.
And frame the whole answer as rate: material is added faster than the cycle can move it on, so every remedy either slows the input or speeds the uptake.
Did you know
Why a lake can die of too much fertiliser
It seems contradictory. Fertiliser makes plants grow, and a lake is full of plants — so surely fertiliser reaching a lake should make it more alive, not less.
It does make it grow, and that is precisely the problem.
Nitrates washed off the fields arrive in the water and the algae respond as any plant would, multiplying rapidly into a thick green bloom across the surface. For a short while the lake is enormously productive.
Then two things happen. The bloom at the surface blocks the light from reaching anything below it, so the submerged plants die. And the algae themselves, having used up the nutrient, die in vast numbers too.
Now the decomposers take over, and this is the step that kills the lake. Breaking down that quantity of dead material is respiration on an enormous scale, and respiration consumes oxygen dissolved in the water. The dissolved oxygen falls, and the fish — which cannot leave — suffocate.
The lake did not die of poisoning. It died of too much food, processed too fast, by organisms that consumed the oxygen everything else depended on.
And the lower oxygen is made worse by something from the earlier chemistry chapter of this course: the solubility of a gas in water falls as the temperature rises. A warm lake holds less dissolved oxygen to begin with, so a bloom in the hot season does more damage than the same bloom in the cool season.
So eutrophication is not a special kind of pollution with rules of its own. It is the nitrogen cycle and the oxygen cycle, running exactly as described on this page, at a rate neither was built for.
It does make it grow, and that is precisely the problem.
Nitrates washed off the fields arrive in the water and the algae respond as any plant would, multiplying rapidly into a thick green bloom across the surface. For a short while the lake is enormously productive.
Then two things happen. The bloom at the surface blocks the light from reaching anything below it, so the submerged plants die. And the algae themselves, having used up the nutrient, die in vast numbers too.
Now the decomposers take over, and this is the step that kills the lake. Breaking down that quantity of dead material is respiration on an enormous scale, and respiration consumes oxygen dissolved in the water. The dissolved oxygen falls, and the fish — which cannot leave — suffocate.
The lake did not die of poisoning. It died of too much food, processed too fast, by organisms that consumed the oxygen everything else depended on.
And the lower oxygen is made worse by something from the earlier chemistry chapter of this course: the solubility of a gas in water falls as the temperature rises. A warm lake holds less dissolved oxygen to begin with, so a bloom in the hot season does more damage than the same bloom in the cool season.
So eutrophication is not a special kind of pollution with rules of its own. It is the nitrogen cycle and the oxygen cycle, running exactly as described on this page, at a rate neither was built for.
Exam relevance
Why do the biogeochemical cycles keep appearing in NEET?
Because the cycles are the framework that ecology is built on, and NEET examines them from a Class 12 chapter devoted to the subject.
This is the foundation for the Class 12 Biology chapter Ecosystem, a standing part of the NEET syllabus. That chapter treats nutrient cycling formally, distinguishing gaseous cycles, where the reservoir is the atmosphere — carbon, nitrogen and oxygen — from sedimentary cycles, where it is the Earth's crust, such as the phosphorus cycle. The Class 9 versions of the carbon and nitrogen cycles are the same cycles with the reservoirs and fluxes named.
The link through photosynthesis and respiration is developed into energy flow and productivity in that chapter, with the trophic levels and pyramids that follow from it. The statement on this page that each process supplies the other's raw material is what makes a food chain possible in the first place.
Nitrogen fixation reappears in Class 11 Biology Mineral Nutrition, examined in NEET, where the nitrogenase enzyme, the root nodule and the Rhizobium symbiosis are treated in detail, and where the reason nitrogen gas is unreactive is explained by its triple bond. The mutual relationship described above becomes formal symbiosis.
Human impact becomes Class 12 Environmental Issues, which covers eutrophication, biochemical oxygen demand, global warming and ozone depletion. The sequence on this page — nitrates, algal bloom, decomposition, oxygen depletion, fish death — is exactly the sequence that chapter examines.
The chemistry route leads to Class 11 Environmental Chemistry and to the nitrogen chemistry of Class 12 The p-Block Elements, both examined in JEE Main and NEET, where the unreactivity of nitrogen and the industrial fixation of it are covered.
What the questions look like. Match-the-column items pairing a process with its description or a bacterium with its role are the commonest form — and the pairing most often confused is nitrification with nitrogen fixation. Sequence questions give the steps of a cycle out of order. Assertion-reason items favour the statement that plants cannot use atmospheric nitrogen directly, and the statement that eutrophication kills fish by oxygen depletion rather than by poisoning. Statement-count questions suit the topic because each cycle has so many named steps.
How board and competitive emphasis differ. A board paper asks you to draw and label a cycle diagram and explain the role of nitrogen-fixing bacteria. A NEET item gives a step and asks which process it is, or asks which organism carries out a named conversion — so the process names and the organisms matter far more than the diagram.
The single trap that costs the most marks. Confusing nitrogen fixation with nitrification. Fixation converts nitrogen gas into ammonium compounds; nitrification converts those ammonium compounds into nitrites and then nitrates. Both involve bacteria, both produce nitrogen compounds, and they are different steps at different points in the cycle. Learn the five steps as an ordered list and the confusion cannot arise.
A second trap worth naming. Saying that fertiliser runoff poisons fish. It does not — it feeds algae, whose decomposition removes the dissolved oxygen the fish need. A question asking how eutrophication kills fish wants oxygen depletion named, and an answer saying the chemicals are toxic has described a different problem entirely.
This is the foundation for the Class 12 Biology chapter Ecosystem, a standing part of the NEET syllabus. That chapter treats nutrient cycling formally, distinguishing gaseous cycles, where the reservoir is the atmosphere — carbon, nitrogen and oxygen — from sedimentary cycles, where it is the Earth's crust, such as the phosphorus cycle. The Class 9 versions of the carbon and nitrogen cycles are the same cycles with the reservoirs and fluxes named.
The link through photosynthesis and respiration is developed into energy flow and productivity in that chapter, with the trophic levels and pyramids that follow from it. The statement on this page that each process supplies the other's raw material is what makes a food chain possible in the first place.
Nitrogen fixation reappears in Class 11 Biology Mineral Nutrition, examined in NEET, where the nitrogenase enzyme, the root nodule and the Rhizobium symbiosis are treated in detail, and where the reason nitrogen gas is unreactive is explained by its triple bond. The mutual relationship described above becomes formal symbiosis.
Human impact becomes Class 12 Environmental Issues, which covers eutrophication, biochemical oxygen demand, global warming and ozone depletion. The sequence on this page — nitrates, algal bloom, decomposition, oxygen depletion, fish death — is exactly the sequence that chapter examines.
The chemistry route leads to Class 11 Environmental Chemistry and to the nitrogen chemistry of Class 12 The p-Block Elements, both examined in JEE Main and NEET, where the unreactivity of nitrogen and the industrial fixation of it are covered.
What the questions look like. Match-the-column items pairing a process with its description or a bacterium with its role are the commonest form — and the pairing most often confused is nitrification with nitrogen fixation. Sequence questions give the steps of a cycle out of order. Assertion-reason items favour the statement that plants cannot use atmospheric nitrogen directly, and the statement that eutrophication kills fish by oxygen depletion rather than by poisoning. Statement-count questions suit the topic because each cycle has so many named steps.
How board and competitive emphasis differ. A board paper asks you to draw and label a cycle diagram and explain the role of nitrogen-fixing bacteria. A NEET item gives a step and asks which process it is, or asks which organism carries out a named conversion — so the process names and the organisms matter far more than the diagram.
The single trap that costs the most marks. Confusing nitrogen fixation with nitrification. Fixation converts nitrogen gas into ammonium compounds; nitrification converts those ammonium compounds into nitrites and then nitrates. Both involve bacteria, both produce nitrogen compounds, and they are different steps at different points in the cycle. Learn the five steps as an ordered list and the confusion cannot arise.
A second trap worth naming. Saying that fertiliser runoff poisons fish. It does not — it feeds algae, whose decomposition removes the dissolved oxygen the fish need. A question asking how eutrophication kills fish wants oxygen depletion named, and an answer saying the chemicals are toxic has described a different problem entirely.
Key takeaways
The water, carbon, oxygen and nitrogen cycles: quick revision
- Water cycle processes: evaporation, transpiration (together evapotranspiration), condensation into clouds, precipitation, infiltration to groundwater, surface runoff, and sublimation from snow and ice.
- The Sun drives the cycle, and water is moved and changed in state, never created or destroyed — so running out of water means running out of accessible fresh water in a place at a time.
-
-
- Each supplies what the other consumes, which is why the carbon and oxygen cycles are linked.
- Carbon is found in the air as carbon dioxide, in rocks and shells as carbonates, dissolved in water, in every living thing, and in fossil fuels.
- Carbon cycle: photosynthesis takes it in, food chains pass it along, respiration, decomposition and combustion return it.
- Oxygen cycle: released by photosynthesis, consumed by respiration, combustion, rusting and decay.
- Burning fossil fuels returns long-buried carbon faster than photosynthesis and the oceans can absorb it, so it accumulates and strengthens the greenhouse effect.
- Nitrogen gas is unreactive, so plants cannot use it directly — they absorb nitrates.
- Nitrogen cycle in five steps: fixation, nitrification, assimilation, ammonification, denitrification.
- Fixation happens by Rhizobium in root nodules of legumes (gram, pea, beans, groundnut), by free-living soil bacteria and blue-green algae, and by lightning.
- Nitrification turns ammonium compounds into nitrites and then nitrates. Assimilation builds proteins and nucleic acids. Ammonification returns nitrogen from dead matter. Denitrification returns nitrogen gas to the air.
- Crop rotation with a legume leaves the soil richer in nitrogen — the nodules are visible on the roots.
- The Rhizobium relationship is mutual: food and shelter one way, usable nitrogen the other.
- Human impact: fossil fuels and deforestation raise carbon dioxide and acidify the oceans; excess fertiliser causes eutrophication — nitrates, algal bloom, decomposition, oxygen depletion, fish death; deforestation and concreting increase runoff and lower the water table.
- Measures: renewables, afforestation, judicious fertiliser use with organic manure and legume rotation, rainwater harvesting, effluent treatment, and reduce, reuse, recycle.
- The problem is rate, not quantity — every remedy either slows the input or speeds the uptake.
Draw the nitrogen cycle from memory with all five processes labelled, then cover the labels and name each arrow — the two you hesitate over are the two that get asked.
- The Sun drives the cycle, and water is moved and changed in state, never created or destroyed — so running out of water means running out of accessible fresh water in a place at a time.
-
-
- Each supplies what the other consumes, which is why the carbon and oxygen cycles are linked.
- Carbon is found in the air as carbon dioxide, in rocks and shells as carbonates, dissolved in water, in every living thing, and in fossil fuels.
- Carbon cycle: photosynthesis takes it in, food chains pass it along, respiration, decomposition and combustion return it.
- Oxygen cycle: released by photosynthesis, consumed by respiration, combustion, rusting and decay.
- Burning fossil fuels returns long-buried carbon faster than photosynthesis and the oceans can absorb it, so it accumulates and strengthens the greenhouse effect.
- Nitrogen gas is unreactive, so plants cannot use it directly — they absorb nitrates.
- Nitrogen cycle in five steps: fixation, nitrification, assimilation, ammonification, denitrification.
- Fixation happens by Rhizobium in root nodules of legumes (gram, pea, beans, groundnut), by free-living soil bacteria and blue-green algae, and by lightning.
- Nitrification turns ammonium compounds into nitrites and then nitrates. Assimilation builds proteins and nucleic acids. Ammonification returns nitrogen from dead matter. Denitrification returns nitrogen gas to the air.
- Crop rotation with a legume leaves the soil richer in nitrogen — the nodules are visible on the roots.
- The Rhizobium relationship is mutual: food and shelter one way, usable nitrogen the other.
- Human impact: fossil fuels and deforestation raise carbon dioxide and acidify the oceans; excess fertiliser causes eutrophication — nitrates, algal bloom, decomposition, oxygen depletion, fish death; deforestation and concreting increase runoff and lower the water table.
- Measures: renewables, afforestation, judicious fertiliser use with organic manure and legume rotation, rainwater harvesting, effluent treatment, and reduce, reuse, recycle.
- The problem is rate, not quantity — every remedy either slows the input or speeds the uptake.
Draw the nitrogen cycle from memory with all five processes labelled, then cover the labels and name each arrow — the two you hesitate over are the two that get asked.