Biogeochemical Cycles for UPSC: Carbon, Nitrogen, Phosphorus, Sulphur and Oxygen, and How Humans Disturb Them
How UPSC Tests Nutrient Cycles and the Gaseous/Sedimentary Framework
🎯 Exam priority: Tier-3, but it feeds three high-yield areas: climate change (carbon), water pollution (nitrogen and phosphorus) and air pollution (sulphur). UPSC has asked this chapter directly only a few times: in 2021 which cycle takes its nutrient from the weathering of rocks (the phosphorus cycle), in 2018 whether soil plays any role in the sulphur cycle, and in 2025 the oxygen budget. Its larger value is that eutrophication, acid rain, nitrous oxide and carbon sinks all hang on these cycles.
The unit of thinking is a reservoir (where an element is stored), a flux (how it moves) and a disturbance (what humans changed). If you can state those three for each element, you can answer almost any question.
The framework
The storage and movement of nutrient elements through the parts of an ecosystem is called nutrient cycling, and the nutrients are reused over and over. NCERT divides it into two types, which is the single most useful split for this article:
Gaseous cycle | Sedimentary cycle | |
Reservoir | Atmosphere or hydrosphere | Earth's crust (rocks, sediments) |
Main example | Carbon (also nitrogen and oxygen) | Phosphorus (also, in part, sulphur) |
Speed | Fast exchange with air and water | Slow; released by weathering, uplift and mining |
Typical human disturbance | Fossil-fuel burning, deforestation, fertiliser nitrogen | Phosphate fertiliser runoff, mining of phosphate rock |
The Carbon Cycle: Reservoirs, Fluxes and the Climate Link
The carbon cycle: the climate link
Reservoirs: the atmosphere (as carbon dioxide), the oceans (dissolved, and locked in shells and limestone), living biomass, soil organic matter and fossil fuels. Fluxes:
Photosynthesis pulls carbon dioxide out of the air and water into plant tissue; the plant itself returns some by respiration.
Decomposition turns dead matter into carbon dioxide, water and inorganic nutrients; it is largely an oxygen-requiring process, so waterlogged or cold conditions slow it and let carbon pile up as peat.
The ocean exchanges carbon dioxide with the air; marine organisms build shells of calcium carbonate that sink and become limestone, one of the biggest long-term stores.
Combustion of wood and fossil fuels returns carbon dioxide quickly that was stored for millions of years.
Human disturbance: burning fossil fuels moves carbon from a slow reservoir (the ground) into a fast one (the air), and clearing forests removes a sink at the same time. The extra carbon dioxide has two effects the exam keeps returning to: warming and ocean acidification, because dissolved carbon dioxide forms a weak acid and makes it harder for corals and shellfish to build calcium-carbonate skeletons. These are developed in the climate-science article.
The Nitrogen Cycle: Five Steps, Five Bacterial Roles
The nitrogen cycle: the fertiliser and water-quality link
Nitrogen is the most abundant gas in the air but is chemically inert, so almost no organism can use it until it is 'fixed' into ammonia or nitrate. That is why a few bacteria are central to life. Learn the cycle as five steps and one organism group for each.
Step | What happens | Who does it (standard examples) |
Fixation | Atmospheric nitrogen is converted to ammonia | Symbiotic Rhizobium in root nodules of legumes; free-living Azotobacter and Azospirillum; cyanobacteria such as Anabaena and Nostoc (the Azolla-Anabaena partnership in rice fields); a little by lightning and, industrially, by the Haber-Bosch process |
Ammonification | Dead matter and wastes are broken down to ammonia | Decomposers (bacteria and fungi) |
Nitrification | Ammonia to nitrite, then nitrite to nitrate | Nitrosomonas (ammonia to nitrite); Nitrobacter (nitrite to nitrate) |
Assimilation | Plants take up nitrate or ammonium and build proteins; animals eat the plants | Plants |
Denitrification | Nitrate is returned to nitrogen gas, closing the cycle | Pseudomonas and other denitrifying bacteria, mainly in waterlogged, oxygen-poor soil |
What humans have changed
Industrial fixation for fertiliser now rivals natural fixation, so far more reactive nitrogen enters soils and water than before.
Runoff carries nitrate into rivers, lakes and coastal seas and causes eutrophication: algal blooms, oxygen depletion and fish kills.
Nitrous oxide, a long-lived greenhouse gas, escapes from fertilised soils and flooded rice fields (the reason rice is the crop UPSC named in 2022 as the main anthropogenic source of both methane and nitrous oxide).
Legume crops and green manures restore nitrogen without chemical fertiliser, which is why crop rotation with pulses is an eco-friendly practice (see the agro-ecology article).
Phosphorus, Sulphur and Oxygen: Sedimentary, Acid-Rain and Budget Links
The phosphorus cycle: the sedimentary model
Phosphorus has no significant gaseous phase. Its natural reservoir is rock, in the form of phosphates. When rocks weather, small amounts dissolve in soil water and plant roots absorb them; herbivores and carnivores get phosphorus from plants; wastes and dead remains return it to the soil through decomposers. Phosphorus is needed for cell membranes, nucleic acids, energy transfer (ATP), bones, teeth and shells. Because the cycle is so slow, phosphorus is often the limiting nutrient in freshwater, and that is why phosphate runoff has such a strong effect on lakes.
The 2021 PYQ in one line: in the phosphorus cycle the weathering of rocks is the main source of the nutrient that enters the cycle. Carbon and nitrogen go through the atmosphere instead.
Disturbance: mined phosphate rock goes into fertiliser; surplus washes into water, drives eutrophication, and a large fraction ends up in ocean sediments, a one-way loss on human timescales.
The sulphur cycle: the acid-rain link
Sulphur has both a sedimentary store (sulphate minerals, pyrite, gypsum) and a gaseous phase (hydrogen sulphide from decay, sulphur dioxide from volcanoes and combustion). Soil plays a real role: plants take up sulphate from soil, and soil microbes convert sulphur between organic, sulphide and sulphate forms. (UPSC 2018 used the opposite claim, 'soil does not play any role in the sulphur cycle', as a wrong statement.) Burning coal and oil releases sulphur dioxide, which forms sulphuric acid in the air and causes acid rain (formed, with nitrogen oxides, as the 2022 PYQ tested).
The oxygen budget
Oxygen is made by photosynthesis and used by respiration, decomposition and combustion. The 2025 PYQ established the point worth remembering: marine phytoplankton and photosynthetic bacteria produce about half of the world's oxygen, so the oceans, not just rain forests, are the planet's main oxygen source; and water, even when well oxygenated, holds far less oxygen than air.
Revision Table and How UPSC Could Ask This Next
One-page revision table
Cycle | Type | Main reservoir | Key human disturbance | Exam hook |
Carbon | Gaseous | Atmosphere, oceans, fossil fuels | Fossil fuels, deforestation | Climate change, carbon sinks, ocean acidification |
Nitrogen | Gaseous | Atmosphere (as N2) | Fertiliser, vehicle and power-plant emissions | Eutrophication, nitrous oxide, legumes |
Phosphorus | Sedimentary | Rocks | Phosphate fertiliser and detergents | Lake eutrophication; 2021 PYQ |
Sulphur | Both | Rocks and sediments; air as SO2 | Burning coal and oil, smelting | Acid rain; role of soil |
Oxygen | Gaseous | Atmosphere, water | Deforestation, ocean warming | Plankton as the main source; 2025 PYQ |
How UPSC Could Ask This Next
Match the organism and the process: nitrogen fixers, nitrifiers and denitrifiers; the question can be a 'how many of the pairs' form.
Type of cycle: which cycle is sedimentary, which has no atmospheric phase.
Consequences of disturbance: a statement question linking fertiliser use to eutrophication or nitrous oxide, or sulphur dioxide to acid rain.
Link to news: blue carbon, algal blooms in lakes, deep-sea hypoxia.
Further Reading
NCERT Biology (Class 12) and Geography textbooks, the India State of Forest Report, the MoEFCC Annual Report, the Economic Survey and the official lists cited in this note — the primary sources behind every figure above.
Test Yourself: Practice Questions and PYQs
Twelve practice questions on the carbon, nitrogen, phosphorus, sulphur and oxygen cycles, followed by the real UPSC Prelims questions that tested them.
Practice Questions
Q1. Which one of the following correctly pairs a nutrient cycle with its type, as classified in NCERT?
(a) Carbon: sedimentary; phosphorus: gaseous
(b) Carbon: gaseous; phosphorus: sedimentary
(c) Both carbon and phosphorus: gaseous
(d) Both carbon and phosphorus: sedimentary
Show answer
Answer: (b) — NCERT's two examples: carbon (reservoir in the atmosphere or hydrosphere) is gaseous and phosphorus (reservoir in the Earth's crust) is sedimentary.
Q2. Consider the following pairs:
Bacterium : Process
1. Rhizobium : Fixation of atmospheric nitrogen in root nodules
2. Nitrosomonas : Conversion of ammonia to nitrite
3. Nitrobacter : Conversion of nitrate to nitrogen gas
4. Pseudomonas : Denitrification
How many of the above pairs are correctly matched?
(a) Only three
(b) Only one
(c) Only two
(d) All four
Show answer
Answer: (a) — Pairs 1, 2 and 4 are right. Nitrobacter converts nitrite to nitrate; turning nitrate into nitrogen gas is denitrification.
Q3. Consider the following statements about the phosphorus cycle:
1. It has no significant gaseous phase.
2. Weathering of rocks is the main source of phosphorus entering the cycle.
3. Phosphate runoff from agricultural land can cause eutrophication in lakes.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 1 and 3 only
(c) 1, 2 and 3
(d) 2 and 3 only
Show answer
Answer: (c) — All three hold: phosphorus is a sedimentary cycle, weathering releases it from rock, and surplus phosphate in runoff drives algal blooms.
Q4. Eutrophication of a lake is most directly caused by
(a) Excess carbon dioxide in the water
(b) Lack of nitrogen-fixing bacteria
(c) Increased sunlight at the water surface alone
(d) Excess nitrates and phosphates that trigger algal blooms and oxygen depletion
Show answer
Answer: (d) — Nutrient enrichment from fertiliser and sewage fuels algal growth; when the algae die, their decomposition uses up dissolved oxygen.
Q5. Which of the following nitrogen-cycle processes returns nitrogen gas to the atmosphere?
(a) Nitrification
(b) Denitrification
(c) Ammonification
(d) Nitrogen fixation
Show answer
Answer: (b) — Denitrifying bacteria convert nitrate back to nitrogen gas, mainly in waterlogged, oxygen-poor soils. Fixation does the opposite.
Q6. Consider the following statements about the oxygen budget:
1. Marine phytoplankton and photosynthetic bacteria produce about half of the world's oxygen.
2. Rain forests produce more oxygen than the oceans.
3. Well-oxygenated surface water holds several times more oxygen than air.
Which of the statements given above is/are correct?
(a) 2 and 3
(b) 1 and 2
(c) None of the above statements is correct
(d) 1 only
Show answer
Answer: (d) — Only 1 is correct: oceans, through plankton and bacteria, make about half the world's oxygen, more than rain forests, and water holds far less oxygen than the same volume of air.
Q7. Statement-I: Burning of fossil fuels disturbs the carbon cycle.
Statement-II: It transfers carbon from a store that exchanges slowly with the atmosphere (the ground) to the atmosphere itself.
Which one of the following is correct in respect of the above statements?
(a) Both statements are correct but Statement-II is not the correct explanation of Statement-I
(b) Statement-I is correct but Statement-II is incorrect
(c) Statement-I is incorrect but Statement-II is correct
(d) Both statements are correct and Statement-II is the correct explanation of Statement-I
Show answer
Answer: (d) — Fossil carbon stored underground for millions of years is released in decades, which is exactly why the natural balance of the cycle is disturbed.
Q8. Acid rain is caused mainly by emissions of
(a) Carbon dioxide and methane
(b) Carbon monoxide and ozone
(c) Chlorofluorocarbons and halons
(d) Sulphur dioxide and oxides of nitrogen
Show answer
Answer: (d) — Sulphur dioxide and nitrogen oxides form sulphuric and nitric acid in the air. Carbon dioxide and methane are greenhouse gases; CFCs and halons deplete ozone.
Q9. Which of the following crops is often planted to restore soil nitrogen because of its symbiotic nitrogen-fixing bacteria?
(a) Chickpea
(b) Maize
(c) Sugarcane
(d) Wheat
Show answer
Answer: (a) — Chickpea is a legume that hosts Rhizobium in root nodules; wheat, sugarcane and maize are cereals or grasses without such nodules.
Q10. Which of the following is a human activity that moves reactive nitrogen into the environment in large quantities?
(a) Photosynthesis by phytoplankton
(b) Breathing by animals
(c) Leaching of humus
(d) Industrial manufacture of nitrogen fertiliser
Show answer
Answer: (d) — The Haber-Bosch process fixes atmospheric nitrogen into ammonia for fertiliser on a scale that rivals natural fixation, which is why nitrogen pollution of water has grown.
Q11. Why is phosphorus often the limiting nutrient in freshwater ecosystems?
(a) Plants cannot absorb phosphates
(b) It is abundant in the atmosphere
(c) It has no gaseous reservoir and is released only slowly from rock by weathering
(d) It is produced only by marine organisms
Show answer
Answer: (c) — Without an atmospheric reservoir, phosphorus supply depends on slow weathering, so even a small extra input from runoff can trigger an algal bloom.
Q12. Which one of the following is a consequence of the rise in dissolved carbon dioxide in seawater?
(a) Faster growth of calcium-carbonate shells
(b) A rise in the ocean's oxygen content
(c) A fall in the ocean's temperature
(d) Ocean acidification, which hampers corals and shell-forming organisms
Show answer
Answer: (d) — Dissolved carbon dioxide forms carbonic acid, lowers the pH and reduces the carbonate ions that corals and shellfish need to build their skeletons.
UPSC Previous Year Questions (PYQs)
Real UPSC Prelims questions, checked against the official paper and answer key.
Q1. (2021) In case of which one of the following biogeochemical cycles, the weathering of rocks is the main source of release of nutrient to enter the cycle?
(a) Carbon cycle
(b) Sulphur cycle
(c) Nitrogen cycle
(d) Phosphorus cycle
Show answer
Answer: (d) — Phosphorus has no meaningful gas phase. Its reservoir is rock, and weathering releases phosphate into soil and water. Carbon, nitrogen and sulphur are all exchanged through the atmosphere, so the rock-weathering source marks the phosphorus cycle.
Q2. (2025) With reference to the planet Earth, consider the following statements :
I. Rain forests produce more oxygen than that produced by oceans.
II. Marine phytoplankton and photosynthetic bacteria produce about 50% of world's oxygen.
III. Well-oxygenated surface water contains several folds higher oxygen than that in atmospheric air.
Which of the statements given above is/are correct?
(a) None of the above statements is correct
(b) II only
(c) I and III
(d) I and II
Show answer
Answer: (b) — Marine phytoplankton and photosynthetic bacteria make about half of the world's oxygen, more than rain forests, so I is wrong. Even well-oxygenated water holds far less oxygen than the same volume of air, so III is wrong. Only II is correct.
Mains Practice Questions
Use these to frame full-length answers. You don't have to answer one exactly — they show the angles UPSC tests, so let them guide which points you cover.
Distinguish between gaseous and sedimentary nutrient cycles with examples. Why do human disturbances affect them differently?
Describe the nitrogen cycle and the role of bacteria in each step. How has the use of chemical fertilisers altered it, and what are the environmental consequences?
Why is eutrophication a growing problem in Indian lakes and coastal waters, and what role do nitrogen and phosphorus play in it?
Explain how the carbon cycle links forests, oceans and fossil fuels to climate change and ocean acidification.
Rice cultivation is a significant source of methane and nitrous oxide. Discuss how changes in the nitrogen and carbon cycles contribute and suggest ways of reducing them.
UPSC Mains — Previous Years’ Questions on this topic (last 10 years)
No direct GS Mains question has been asked specifically on this topic in the last decade — it is primarily a Prelims area. Use the practice prompts above to stay exam-ready.