Biogeochemical Cycles (Carbon, Nitrogen, Phosphorus): Complete UPSC Notes for Prelims + Mains
Think of Nature like India's supply chain. Raw materials move from one place to another, get used, get recycled, and come back again. In the same way, carbon, nitrogen, and phosphorus keep moving between air, water, soil, rocks, plants, animals, and microbes. This circular movement is called biogeochemical cycling.
UPSC loves this topic because it connects climate change, soil fertility, eutrophication, pollution, agriculture, biodiversity, and ecosystem functioning. If you understand these cycles properly, many Environment questions become easy.
1) What are Biogeochemical Cycles?
📘 Biogeochemical Cycle
The continuous movement and recycling of essential elements (like C, N, P) between living organisms (bio) and the non-living environment such as air, water, soil, and rocks (geo + chemical).
These cycles are powered by three major forces:
- Solar energy (drives photosynthesis, climate, winds, rainfall)
- Gravity (moves water, sediments, nutrients downhill)
- Microbes (do most conversions in soil and water)
Every cycle has:
- Reservoir / Pool: the main storage place (example: atmosphere for nitrogen, rocks for phosphorus)
- Flux: the movement rate between pools (example: photosynthesis moves CO₂ into plants)
- Source: adds the element to a system (example: burning coal adds CO₂ to air)
- Sink: removes/stores the element (example: forests store carbon in biomass)
📘 Reservoir (Pool)
A natural storage place where an element stays for a long time, like atmosphere, oceans, soils, forests, or rocks.
📘 Source and Sink
Source releases an element (or increases its concentration) in a system; Sink absorbs/stores it and reduces its concentration.
2) Types of Biogeochemical Cycles: Gaseous vs Sedimentary
UPSC frequently asks the difference.
| Feature | Gaseous Cycles | Sedimentary Cycles |
|---|---|---|
| Main reservoir | Atmosphere / Oceans | Rocks / Soil / Sediments |
| Speed | Generally faster | Generally slower |
| Examples | Carbon, Nitrogen | Phosphorus |
| Why slower? | Quick exchange through air-water | Needs weathering, erosion, sedimentation, uplift |
📘 Limiting Nutrient
A nutrient that is in shortest supply compared to demand, and therefore limits plant/algal growth (like the "weakest link").
Quick UPSC point: Phosphorus is often the limiting nutrient in many freshwater ecosystems, so even small phosphate addition can trigger algal bloom.
3) Carbon Cycle (C Cycle)
Carbon is the backbone of life: carbohydrates, fats, proteins, DNA—all are carbon-based. Carbon moves through atmosphere, biosphere, hydrosphere, and lithosphere.
📘 Carbon Cycle
The circulation of carbon between atmosphere (CO₂), living organisms (biomass), oceans (dissolved carbon), soils, and rocks (carbonates and fossil fuels) through processes like photosynthesis, respiration, decomposition, ocean exchange, and combustion.
3.1 Major Carbon Reservoirs
| Reservoir | In what form? | UPSC Relevance |
|---|---|---|
| Atmosphere | CO₂, CH₄ (small share) | Greenhouse effect, climate change |
| Plants and forests | Biomass carbon | Carbon sink, afforestation |
| Soil | Soil organic carbon (humus) | Soil fertility, carbon sequestration |
| Oceans | Dissolved CO₂, bicarbonate, carbonate | Ocean acidification, blue carbon |
| Rocks / sediments | Carbonates (limestone), fossil fuels | Long-term carbon storage |
📘 Carbon Sequestration
Long-term storage of carbon in forests, soil, oceans, or geological formations so that it does not remain in the atmosphere as CO₂.
3.2 The Core Steps of the Carbon Cycle
A) Photosynthesis (Atmosphere → Biosphere)
- Green plants take CO₂ from air and convert it into glucose using sunlight.
- This is the biggest natural pathway that removes CO₂ from atmosphere.
B) Respiration (Biosphere → Atmosphere)
- Plants, animals, and microbes break down food and release CO₂ back.
C) Decomposition (Dead matter → Soil + Atmosphere)
- When plants/animals die, decomposers convert organic carbon into CO₂ (and sometimes CH₄ in low-oxygen areas).
📘 Methane (CH₄) and Anaerobic Conditions
In oxygen-poor conditions (like wetlands, paddy fields, landfill), microbes produce methane, a powerful greenhouse gas.
D) Ocean–Atmosphere Exchange
- CO₂ dissolves in ocean water and can also come out depending on temperature and concentration.
- Cold water absorbs more CO₂ (so polar oceans can be stronger sinks).
E) Carbonate Formation and Sedimentation (Long-term storage)
- Marine organisms (shells, corals) use carbonate to form hard structures.
- After death, shells settle and form carbonate rocks (like limestone) over long time.
📘 Ocean Acidification
When oceans absorb extra CO₂, carbonic acid forms and lowers pH. This can harm corals and shell-forming organisms by reducing carbonate availability.
F) Combustion (Lithosphere/Biomass → Atmosphere)
- Burning coal, oil, gas, and biomass releases CO₂ quickly.
- This is the major human-driven disturbance of the carbon cycle.
3.3 Human Impacts on Carbon Cycle (UPSC Mains Angle)
- Fossil fuel burning increases atmospheric CO₂ → global warming.
- Deforestation reduces carbon sinks and releases stored carbon.
- Forest fires release large pulses of CO₂ and reduce future absorption.
- Land-use change (wetlands drained, soil erosion) reduces soil carbon.
- Ocean warming reduces CO₂ absorption capacity.
3.4 India-Specific Examples (Use in Answers)
- Mangroves (Sundarbans, Bhitarkanika, Pichavaram) store large carbon in biomass and soil (blue carbon).
- Western Ghats forests act as major carbon sinks and also regulate rainfall.
- Himalayan forests store carbon and protect watersheds.
- India's climate commitments under the Paris Agreement (2015; updated NDC in 2022) include creating additional carbon sink through forest/tree cover by 2030 (commonly cited as 2.5–3 billion tonnes CO₂e).
4) Nitrogen Cycle (N Cycle)
Nitrogen is essential for amino acids (proteins), DNA, RNA, and chlorophyll. The atmosphere has about 78% nitrogen (N₂), but most organisms cannot directly use N₂ because it is very stable (triple bond). So Nature needs special processes to "fix" nitrogen.
📘 Nitrogen Fixation
The conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃) or related usable forms by bacteria, lightning, or industrial processes.
4.1 Main Steps of the Nitrogen Cycle
Step 1: Nitrogen Fixation (N₂ → NH₃/NH₄⁺)
- Biological fixation: by microbes (most important)
- Atmospheric fixation: lightning forms nitrogen oxides that enter soil with rain
- Industrial fixation: Haber–Bosch process (ammonia for fertilizers)
📘 Biofertilizers (Nitrogen)
Living microbes added to soil/seed to increase nutrient availability. Example: Rhizobium in legume roots fixes nitrogen.
Important organisms for fixation (Prelims favourite):
- Rhizobium (symbiotic, legume root nodules: chickpea, groundnut, soybean)
- Azotobacter (free-living in soil)
- Cyanobacteria like Anabaena, Nostoc (common in paddy fields)
- Azolla–Anabaena association used in rice fields as green manure
Step 2: Nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻)
- Happens in aerobic (oxygen-rich) soil.
- Nitrosomonas: converts ammonia to nitrite (NO₂⁻).
- Nitrobacter: converts nitrite to nitrate (NO₃⁻).
📘 Nitrification
An aerobic microbial process where ammonium (NH₄⁺) is converted into nitrite (NO₂⁻) and then nitrate (NO₃⁻).
Step 3: Assimilation (NO₃⁻/NH₄⁺ → Plant proteins)
- Plants absorb nitrate/ammonium and form amino acids and proteins.
- Animals get nitrogen by eating plants/other animals.
Step 4: Ammonification (Organic N → NH₃/NH₄⁺)
- When organisms die or excrete waste, decomposers convert organic nitrogen into ammonia/ammonium.
📘 Ammonification
Decomposer-driven conversion of organic nitrogen (dead bodies, wastes) into ammonia (NH₃) or ammonium (NH₄⁺).
Step 5: Denitrification (NO₃⁻ → N₂/N₂O)
- Occurs in anaerobic (oxygen-poor) conditions like waterlogged soils.
- Bacteria convert nitrate back to nitrogen gas, returning it to atmosphere.
- Also produces nitrous oxide (N₂O), a strong greenhouse gas.
📘 Denitrification
An anaerobic microbial process where nitrate (NO₃⁻) is reduced to nitrogen gas (N₂), returning nitrogen to the atmosphere.
4.2 Nitrogen Cycle Summary Table (Prelims Ready)
| Process | Conversion | Condition | Key Agents |
|---|---|---|---|
| Fixation | N₂ → NH₃/NH₄⁺ | Varies | Rhizobium, Azotobacter, cyanobacteria; lightning; industry |
| Nitrification | NH₄⁺ → NO₂⁻ → NO₃⁻ | Aerobic | Nitrosomonas, Nitrobacter |
| Assimilation | NO₃⁻/NH₄⁺ → organic N | Normal | Plants (then animals) |
| Ammonification | Organic N → NH₃/NH₄⁺ | Normal | Decomposers (bacteria, fungi) |
| Denitrification | NO₃⁻ → N₂/N₂O | Anaerobic | Denitrifying bacteria |
4.3 Human Impacts on Nitrogen Cycle
- Overuse of urea and fertilizers increases nitrate runoff → eutrophication in lakes and reservoirs.
- Nitrate leaching contaminates groundwater (public health issue).
- More N₂O emissions from fertilized soils → climate change.
- NOx emissions from vehicles/industries contribute to smog and acid rain-like effects.
📘 Eutrophication
Excess nutrients (mainly nitrogen and phosphorus) in water cause rapid algal growth. When algae die, decomposition reduces oxygen, harming fish and other aquatic life.
4.4 India Examples + Solutions (Answer Enrichment)
- Green Revolution belt saw heavy fertilizer use; improving nitrogen use efficiency is crucial.
- Neem-coated urea helps slow nitrogen release and reduces diversion/wastage.
- Legume crop rotation (like pulses) naturally improves soil nitrogen.
- Biofertilizers and organic manures reduce chemical load.
- Wetlands can reduce nitrate pollution through natural denitrification (so wetland conservation also supports nutrient control).
5) Phosphorus Cycle (P Cycle)
Phosphorus is essential for ATP (energy currency), DNA/RNA, and bones/teeth. Unlike carbon and nitrogen, phosphorus does not have a significant gaseous phase. So the phosphorus cycle is mainly a sedimentary cycle and is slower.
📘 Phosphorus Cycle
The movement of phosphorus mainly between rocks, soil, water, and living organisms through weathering, absorption by plants, food chains, decomposition, and sedimentation—without a major atmospheric phase.
5.1 Major Reservoir
- Phosphate rocks and marine sediments are the biggest reservoirs.
- Soil has phosphorus, but much of it is locked and not easily available to plants.
📘 Phosphate (PO₄³⁻)
The main usable form of phosphorus for plants. It is commonly present as phosphate salts in soil and water.
5.2 Steps of the Phosphorus Cycle
Step 1: Weathering (Rocks → Soil/Water)
- Weathering releases phosphate ions from rocks into soil and water.
- This is slow, so phosphorus supply is naturally limited.
Step 2: Absorption by Plants (Soil → Plants)
- Plants absorb phosphate from soil solution.
- Many plants depend on mycorrhizal fungi to increase phosphorus uptake.
📘 Mycorrhiza
A symbiotic association between fungi and plant roots. Fungi increase water and nutrient (especially phosphorus) absorption, while plants give sugars to fungi.
Step 3: Food Chain Transfer (Plants → Animals)
- Animals get phosphorus by eating plants or other animals.
Step 4: Decomposition and Mineralization (Dead matter → Soil)
- Decomposers return phosphorus to soil as inorganic phosphate.
📘 Mineralization
Conversion of organic nutrients (in dead matter and wastes) into inorganic forms that plants can reuse.
Step 5: Runoff and Sedimentation (Soil → Rivers → Lakes/Sea → Sediments)
- Phosphates can wash into water bodies through runoff.
- In oceans, phosphate settles to the bottom and becomes sedimentary rock over long time.
- Geological uplift can bring these rocks back to land (very slow loop).
5.3 Human Impacts on Phosphorus Cycle
- Phosphate fertilizers increase phosphate runoff → algal blooms.
- Sewage discharge adds phosphates to lakes and rivers.
- Detergents (phosphates in some products) can add nutrients to water.
- Phosphate mining disturbs land and creates waste (phosphogypsum issues).
5.4 India Examples (Use in UPSC Answers)
- Urban lakes (example: parts of Bengaluru's lake system) show algal blooms and frothing due to sewage + nutrient load.
- Dal Lake and other lakes face eutrophication pressures due to nutrient inflow (sewage, runoff).
- Rajasthan has major rock phosphate deposits (example: Udaipur region), supporting fertilizer industry.
6) Comparing Carbon, Nitrogen, and Phosphorus Cycles (High-Value UPSC Table)
| Point | Carbon (C) | Nitrogen (N) | Phosphorus (P) |
|---|---|---|---|
| Main reservoir | Atmosphere + oceans + rocks | Atmosphere (N₂) | Rocks and sediments |
| Type | Gaseous | Gaseous | Sedimentary |
| Key entry to biosphere | Photosynthesis | Nitrogen fixation | Weathering + plant uptake |
| Key return pathway | Respiration + decomposition | Denitrification | Sedimentation (slow) + decomposition |
| Major human disturbance | Fossil fuels, deforestation | Fertilizers, NOx emissions | Fertilizers, sewage, detergents, mining |
| Big environmental issue | Climate change, ocean acidification | Water pollution, N₂O emissions | Eutrophication, lake degradation |
7) How These Cycles Link to Major UPSC Topics
7.1 Climate Change
- Carbon cycle disturbance is the main driver of global warming (CO₂).
- Nitrogen fertilizers increase N₂O, a strong greenhouse gas.
- Healthy forests, soils, and mangroves help in carbon sequestration.
7.2 Agriculture and Food Security
- Nitrogen and phosphorus are core nutrients for crop growth.
- Imbalanced fertilizer use reduces soil health and increases pollution.
- Sustainable farming includes integrated nutrient management and biofertilizers.
📘 Integrated Nutrient Management (INM)
Using a balanced mix of chemical fertilizers, organic manures, compost, crop residues, and biofertilizers to maintain soil fertility and reduce pollution.
7.3 Water Pollution and Eutrophication
- N and P runoff cause algal blooms.
- Oxygen depletion creates fish kills and biodiversity loss.
- Solutions: sewage treatment, buffer zones near farms, wetland restoration.
8) Prelims Quick Revision Points (Very Important)
- Carbon and Nitrogen cycles are mainly gaseous; Phosphorus is mainly sedimentary.
- Nitrification is aerobic; Denitrification is anaerobic.
- Nitrosomonas: NH₄⁺ → NO₂⁻; Nitrobacter: NO₂⁻ → NO₃⁻.
- Rhizobium fixes nitrogen in legume root nodules.
- Phosphorus has no major atmospheric phase, hence slower cycle.
- Excess N and P in water → eutrophication.
- Oceans absorb CO₂ but excess absorption causes ocean acidification.
- Mangroves are major blue carbon ecosystems.
📘 Blue Carbon
Carbon stored in coastal ecosystems like mangroves, seagrasses, and salt marshes, often in both biomass and deep sediments.
9) Mains Answer Writing: How to Structure a 150/250 Word Answer
Smart structure for Mains:
- Start: Define biogeochemical cycle in 1 line.
- Body: Explain key steps (2–3 lines each) + human impacts.
- Use example: Indian lake eutrophication / mangrove blue carbon / fertilizer overuse.
- Way forward: INM, wetland restoration, afforestation, sewage treatment.
10) PYQ Themes (UPSC Pattern-Based) with How to Approach
📝 UPSC PYQ Theme - Carbon Cycle: What processes add CO₂ to atmosphere?
Focus on respiration, decomposition, combustion, volcanic activity as additions; photosynthesis as removal. UPSC often tests "adds vs removes".
📝 UPSC PYQ Theme - Nitrogen Cycle: Match processes with conditions and microbes
Remember nitrification is aerobic (Nitrosomonas, Nitrobacter) and denitrification is anaerobic (returns N₂). They also test nitrogen fixation examples (Rhizobium, cyanobacteria).
📝 UPSC PYQ Theme - Eutrophication: Which nutrients cause it and what are impacts?
Identify nitrogen and phosphorus as key nutrients. Explain algal bloom → oxygen depletion → fish kills. Give Indian lake/river examples and solutions.
📝 UPSC PYQ Theme - Sedimentary cycle: Why phosphorus cycle is slower?
Because major reservoir is rocks/sediments; needs weathering and geological uplift; lacks atmospheric phase. UPSC likes "reason-based" conceptual questions.
📝 UPSC PYQ Theme - Climate change linkage: Carbon sinks and land-use change
Explain forests/soils/mangroves as sinks; deforestation and burning as sources. Add one policy line like afforestation or ecosystem restoration.
11) Practice MCQs (Prelims Level) with Answers and Explanations
MCQ 1
Which of the following processes removes carbon dioxide from the atmosphere?
- A) Respiration
- B) Photosynthesis
- C) Combustion
- D) Decomposition
Answer: B) Photosynthesis
Explanation: Photosynthesis uses CO₂ to form sugars, so it acts as a carbon "removal" pathway.
MCQ 2
Nitrification is best described as:
- A) Conversion of nitrate to nitrogen gas in anaerobic soil
- B) Conversion of ammonium to nitrite and nitrate in aerobic soil
- C) Conversion of atmospheric nitrogen to nitrate directly by plants
- D) Conversion of proteins to nitrogen gas by fungi
Answer: B) Conversion of ammonium to nitrite and nitrate in aerobic soil
Explanation: Nitrification is an aerobic microbial process (Nitrosomonas, Nitrobacter).
MCQ 3
Which pair is correctly matched?
- A) Nitrosomonas: NO₂⁻ → NO₃⁻
- B) Nitrobacter: NH₄⁺ → NO₂⁻
- C) Nitrosomonas: NH₄⁺ → NO₂⁻
- D) Rhizobium: NO₃⁻ → N₂
Answer: C) Nitrosomonas: NH₄⁺ → NO₂⁻
Explanation: Nitrosomonas converts ammonium to nitrite; Nitrobacter converts nitrite to nitrate.
MCQ 4
Phosphorus cycle is considered slower mainly because:
- A) Phosphorus is present mainly in the atmosphere
- B) It has no significant gaseous phase and is rock/sediment-based
- C) Plants cannot absorb phosphorus
- D) Decomposition does not return phosphorus to soil
Answer: B) It has no significant gaseous phase and is rock/sediment-based
Explanation: Major pool is rocks/sediments; weathering and uplift take long time.
MCQ 5
Eutrophication is most directly linked to:
- A) Excess nitrogen and phosphorus in water bodies
- B) Excess carbon dioxide in air
- C) Ozone depletion in stratosphere
- D) Increase in soil salinity only
Answer: A) Excess nitrogen and phosphorus in water bodies
Explanation: Nutrient overload causes algal blooms and oxygen depletion.
MCQ 6
Denitrification primarily occurs in:
- A) Oxygen-rich dry soils
- B) Anaerobic, waterlogged conditions
- C) Upper atmosphere
- D) Desert sand dunes
Answer: B) Anaerobic, waterlogged conditions
Explanation: Denitrification is an anaerobic process that returns nitrate nitrogen back to the atmosphere.
MCQ 7
Which ecosystem type is most associated with "blue carbon"?
- A) Alpine grasslands
- B) Mangroves and seagrasses
- C) Desert scrub
- D) Tropical thorn forests only
Answer: B) Mangroves and seagrasses
Explanation: Coastal ecosystems store carbon in biomass and deep sediments for long time.
MCQ 8
Azolla–Anabaena is important in agriculture mainly because it:
- A) Acts as a pesticide
- B) Fixes nitrogen and improves soil fertility in paddy fields
- C) Absorbs heavy metals from air
- D) Converts nitrate into phosphate
Answer: B) Fixes nitrogen and improves soil fertility in paddy fields
Explanation: Anabaena is a nitrogen-fixing cyanobacterium living symbiotically with Azolla.
MCQ 9
Which statement is most correct?
- A) Phosphorus is mainly stored in the atmosphere as P₂O₅
- B) Carbon cycle has no role in oceans
- C) Nitrogen is abundant in air but needs fixation to become usable
- D) Denitrification increases soil nitrate levels
Answer: C) Nitrogen is abundant in air but needs fixation to become usable
Explanation: Atmospheric N₂ is stable; fixation converts it into usable forms like NH₃/NH₄⁺.
MCQ 10
Ocean acidification happens mainly due to:
- A) Increased phosphate mining
- B) Increased CO₂ absorption forming carbonic acid
- C) Increased dissolved oxygen in oceans
- D) Increased nitrogen fixation by cyanobacteria
Answer: B) Increased CO₂ absorption forming carbonic acid
Explanation: Dissolved CO₂ forms carbonic acid, lowering pH and affecting coral and shell formation.
12) One-Page Conclusion (What to Remember)
Biogeochemical cycles are Nature's recycling system. The carbon cycle controls climate and energy flow through photosynthesis and respiration. The nitrogen cycle controls soil fertility and depends heavily on microbes for fixation, nitrification, and denitrification. The phosphorus cycle is slower because it is rock-based and strongly linked to eutrophication when disturbed by humans. For UPSC, always connect these cycles to climate change, sustainable agriculture, water pollution, and ecosystem health.