📋 What’s Inside
Nutrient Cycling
Why it matters
Carbon Cycle
CO₂ & life
Nitrogen Cycle
Soil → Plants
Methane Cycle
CH₄ sources
Phosphorus
Sedimentary
Sulphur Cycle
Acid rain
What is Biogeochemical / Nutrient Cycling?
A biogeochemical cycle (or nutrient cycle) is the pathway by which a chemical element or compound moves through the biotic (living) and abiotic (non-living) compartments of the Earth — atmosphere, hydrosphere, lithosphere, and biosphere — and back again. The word combines bio (living organisms), geo (Earth/rocks/soil), and chemical (elements involved).
🔑 Key Facts
- Unlike energy (which flows in one direction and is lost as heat), nutrients are recycled indefinitely — they are never destroyed, only transformed.
- The body of every living organism is made of C, H, O, N, P — these elements together make up 97% of all living matter.
- Biogeochemical cycles are driven by biological processes (photosynthesis, respiration, decomposition), geological processes (weathering, erosion, volcanism), and human activities.
🔑 Two Types of Nutrient Cycles
- Gaseous Cycles — Reservoir is the atmosphere. Fast cycles. Examples: Carbon cycle, Nitrogen cycle, Oxygen cycle, Methane cycle, Water cycle.
- Sedimentary Cycles — Reservoir is the Earth’s crust (rocks/soil). Slow cycles — elements can be locked in rocks for millions of years. Examples: Phosphorus cycle, Sulphur cycle, Calcium cycle, Magnesium cycle.
Gaseous = Sky is the reservoir (atmosphere). Elements move through air. Sedimentary = Rock is the reservoir (Earth’s crust). Elements move through soil and water. If a cycle has a major atmospheric phase → Gaseous. If it doesn’t → Sedimentary.
The classification of cycles as gaseous vs sedimentary is a direct UPSC question. Know: Phosphorus cycle has NO atmospheric phase — it is purely sedimentary. Sulphur cycle is mostly sedimentary but has two gaseous compounds (H₂S and SO₂). Carbon and Nitrogen are gaseous. This distinction appears in statement-based UPSC questions.
Carbon Cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the atmosphere, oceans, soil, and living organisms. Carbon is the backbone of all organic molecules — it forms 49% of the dry weight of living organisms. The main atmospheric form is CO₂; the main stored form is organic carbon in biomass and fossil fuels.
Imagine carbon as a traveller with a passport that lets it enter any country — air, water, soil, rocks, and living bodies. Every living thing is made of carbon. When it dies, carbon returns to the soil or air. Plants pick it up again. The journey never stops.
🔑 Steps in the Carbon Cycle
- Photosynthesis (Atmosphere → Biosphere): Green plants absorb CO₂ from the atmosphere and convert it into organic compounds (glucose, starch, cellulose) using sunlight. This is the entry point of carbon into the food web. Removes CO₂ from air — plants are carbon sinks.
- Respiration (Biosphere → Atmosphere): All living organisms (plants, animals, microbes) break down organic compounds and release CO₂ back into the atmosphere during cellular respiration. Carbon exits the food web and returns to air.
- Decomposition (Dead matter → Soil/Atmosphere): When organisms die, decomposers (bacteria and fungi) break down their organic carbon into CO₂ (if aerobic) or methane CH₄ (if anaerobic — in swamps, marshes). Carbon returns to the atmosphere or soil.
- Ocean Exchange (Atmosphere ↔ Ocean): The ocean absorbs huge amounts of CO₂ from the atmosphere (oceans hold 71% of Earth’s total carbon). CO₂ dissolves in seawater and is used by marine organisms to form shells (calcium carbonate). The ocean is the largest carbon reservoir and carbon sink on Earth.
- Fossil Fuel Formation (Biosphere → Lithosphere): Over millions of years, dead organic matter buried under pressure transforms into coal, oil, and natural gas — locking carbon away in the Earth’s crust. This is long-term carbon storage.
- Combustion (Lithosphere → Atmosphere): Burning fossil fuels (coal, oil, gas), forests, and biomass releases stored carbon back into the atmosphere as CO₂ very rapidly. This is the primary driver of the current greenhouse effect and climate change.
- Volcanism: Volcanic eruptions release CO₂ stored in the Earth’s mantle into the atmosphere — a natural, long-term source of carbon.
When humans burn fossil fuels, they release carbon that was locked away for millions of years — all at once. This rapidly increases atmospheric CO₂, enhancing the greenhouse effect and warming the planet. The ocean is absorbing extra CO₂ (becoming more acidic — ocean acidification) and forests are acting as carbon sinks. Deforestation removes these sinks, leaving more CO₂ in the air. Understanding the carbon cycle is essential for understanding climate policy.
⭐ Carbon Cycle — UPSC Must-Know Facts
- 71% of all carbon on Earth is dissolved in the oceans — the ocean is the largest carbon reservoir.
- Photosynthesis removes CO₂ from atmosphere (carbon sink). Respiration & combustion add CO₂ (carbon source).
- Under anaerobic conditions (swamps, marshes), decomposition produces methane (CH₄) instead of CO₂.
- Fossil fuels = ancient carbon locked in the lithosphere; burning them = releasing prehistoric carbon rapidly.
- Carbon cycle type = Gaseous (reservoir = atmosphere + ocean).
Carbon cycle is central to UPSC questions on climate change, greenhouse gases, carbon sinks, ocean acidification, and deforestation. Key statement tested: “The ocean is the largest carbon sink on Earth.” Also know: Forests = carbon sinks. Burning forests = double blow (releases stored carbon + removes the sink). Blue carbon = carbon stored in coastal ecosystems (mangroves, seagrasses, salt marshes) — a growing UPSC topic.
Nitrogen Cycle
Nitrogen makes up 78% of our atmosphere — it is the most abundant gas in air. But here’s the problem: plants and animals cannot use nitrogen in its gaseous form (N₂). The N₂ molecule has an extremely strong triple bond that most organisms cannot break. So nitrogen must be converted into usable forms — this is what the nitrogen cycle is all about.
Imagine nitrogen gas (N₂) as raw iron ore — useful but unusable as-is. Bacteria are the “factories” that process it into steel (usable forms like NH₃ and NO₃⁻). Plants buy this steel to build proteins. Animals eat plants. Everything dies. Decomposers break it all back down to raw ore. Some bacteria return it to pure gas. The factory runs endlessly. Bacteria are the heroes of the nitrogen cycle.
Step 1: Nitrogen Fixation
Atmospheric N₂ gas (unusable) is converted into ammonia (NH₃) or ammonium (NH₄⁺) — which plants can use. This “fixes” nitrogen into the soil.
How it happens — 3 ways:
- Biological fixation (90%): Bacteria using the enzyme nitrogenase convert N₂ → NH₃.
- Rhizobium — lives in root nodules of legumes (peas, beans, lentils). Symbiotic relationship — plant gives it sugars; bacteria gives it fixed nitrogen.
- Azotobacter, Clostridium — free-living in soil. Fix nitrogen independently.
- Cyanobacteria (Blue-Green Algae) — fix nitrogen in water and flooded rice paddies.
- Atmospheric fixation by lightning: The energy of lightning breaks the N≡N triple bond, allowing nitrogen to combine with oxygen to form nitrogen oxides (NOₓ), which dissolve in rain and fall to soil as nitrates.
- Industrial fixation (Haber-Bosch Process): N₂ + H₂ → NH₃. Used to make synthetic fertilizers. Human-made nitrogen fixation now equals or exceeds natural fixation globally.
Step 2: Nitrification
Ammonia/ammonium (NH₄⁺) in the soil is converted into nitrite (NO₂⁻) and then nitrate (NO₃⁻) by nitrifying bacteria. This is a 2-step process:
- NH₄⁺ → NO₂⁻ by Nitrosomonas bacteria (aerobic — needs oxygen)
- NO₂⁻ → NO₃⁻ by Nitrobacter bacteria (aerobic — needs oxygen)
Why does this matter? Nitrate (NO₃⁻) is the primary form in which plants absorb nitrogen through their roots. So nitrification makes nitrogen plant-available.
⚠️ Both bacteria are aerobic — they need well-aerated soil. Waterlogged soils have less nitrification.
Step 3: Assimilation
Plants absorb nitrate (NO₃⁻) and ammonium (NH₄⁺) from the soil through their roots and use these inorganic forms to build organic nitrogen compounds — proteins, amino acids, nucleic acids (DNA/RNA), and chlorophyll.
When animals eat plants, they obtain this organic nitrogen and use it to build their own proteins. This is how nitrogen enters the food web and travels from producers to consumers.
🌾 Legumes (dal, peas, soybean) are especially rich in protein because they have Rhizobium fixing nitrogen right at their roots — giving them a built-in nitrogen supply.
Step 4: Ammonification (Decomposition)
When plants and animals die, their organic nitrogen (proteins, urea, uric acid) is broken down by decomposers (bacteria and fungi) back into ammonia (NH₃) or ammonium (NH₄⁺). This is also called mineralisation.
Sources of organic nitrogen that undergo ammonification:
- Dead plant and animal matter (proteins, amino acids)
- Animal waste — urea (mammals), uric acid (birds, reptiles)
- Sewage and manure
The ammonia produced is then available for nitrification (Step 2) or direct plant uptake — completing the loop.
Step 5: Denitrification
Certain bacteria convert nitrate (NO₃⁻) back into nitrous oxide (N₂O) and finally into nitrogen gas (N₂), which escapes back into the atmosphere. This closes the cycle — returning nitrogen to where it started.
- Done by Pseudomonas, Thiobacillus, Bacillus, Paracoccus denitrificans
- Occurs in anaerobic conditions — waterlogged soils, swamps, deep in soil near water table
- These bacteria use nitrate instead of oxygen for respiration when O₂ is absent
⚠️ Denitrification removes usable nitrogen from soil — this is why waterlogged or flooded soils become nitrogen-poor over time. But wetlands use this process to clean up excess nitrates from agricultural runoff.
⭐ Nitrogen Cycle — Memory Trick (FANNAD)
- Fixation — N₂ → NH₃ (Rhizobium, Azotobacter, lightning)
- Assimilation — Plants absorb NO₃⁻ / NH₄⁺ → make proteins
- Nitrification — NH₄⁺ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter) [aerobic]
- Nitrification … done ✓
- Ammonification — Dead matter / urea → NH₃ (bacteria & fungi)
- Denitrification — NO₃⁻ → N₂O → N₂ (Pseudomonas) [anaerobic]
Bacteria cheat sheet: Rhizobium (root nodules, legumes) · Azotobacter (free in soil) · Nitrosomonas (NH₄⁺ → NO₂⁻) · Nitrobacter (NO₂⁻ → NO₃⁻) · Pseudomonas (NO₃⁻ → N₂)
When farmers grow dal, peas, or soybean, the Rhizobium bacteria in their root nodules fix atmospheric nitrogen into the soil. After harvest, the roots are left in the field — they decompose and release this fixed nitrogen, enriching the soil for the next crop. This is why traditional Indian farming always rotated between cereal crops and legume crops — it was a natural fertilization system. UPSC has connected this to sustainable agriculture and the Green Revolution.
Excess fertilizer use: Synthetic nitrogen fertilizers (urea, ammonium nitrate) from the Haber-Bosch process add far more fixed nitrogen to soil than natural cycles. This nitrogen washes into rivers and lakes → causing eutrophication (algal blooms, oxygen depletion, dead zones).
Acid rain: Burning fossil fuels releases nitrogen oxides (NOₓ) into the atmosphere. These react with water to form nitric acid (HNO₃) — causing acid rain, which damages forests, kills aquatic life, and corrodes buildings.
N₂O as greenhouse gas: Denitrification in agricultural soils releases nitrous oxide (N₂O), which is ~300 times more potent as a greenhouse gas than CO₂ over 100 years.
The nitrogen cycle is among the most tested ecology topics in UPSC. Know: (1) Rhizobium fixes nitrogen in root nodules of legumes — tested directly. (2) Nitrification requires aerobic (oxygenated) conditions; denitrification requires anaerobic conditions. (3) Nitrosomonas converts NH₄⁺ → NO₂⁻; Nitrobacter converts NO₂⁻ → NO₃⁻. (4) Excess nitrogen → eutrophication. (5) Burning fossil fuels → NOₓ → acid rain. (6) N₂O is a potent greenhouse gas. (7) The legume-Rhizobium relationship is a classic example of mutualism.
Methane (CH₄) Cycle
Methane (CH₄) is the simplest hydrocarbon and the second most important greenhouse gas after CO₂ in terms of its contribution to climate change. It is produced when organic matter decomposes in the absence of oxygen (anaerobic decomposition) by methanogenic archaea (microbes). Methane is about 80 times more potent than CO₂ as a greenhouse gas over a 20-year period (and ~25-28 times over 100 years).
Any place that is wet, warm, and oxygen-free produces methane — because anaerobic decomposition happens there. Cow stomachs, rice paddies, swamps, landfills — all wet and oxygen-poor inside. Think: “Wet + Airless = Methane.”
🌿 Natural Sources of Methane
- Wetlands — the largest natural source. Anaerobic decomposition in waterlogged soil by methanogens.
- Termites — gut bacteria of termites produce methane while digesting wood.
- Ocean sediments — methane hydrates (frozen methane) in deep ocean and permafrost.
- Wild ruminants — wild buffaloes, wildebeest (enteric fermentation).
- Wildfires — incomplete combustion of biomass.
- Volcanic activity — minor source from degassing.
🏭 Human (Anthropogenic) Sources
- Rice paddies (paddy cultivation) — flooded fields create anaerobic conditions; among the largest agricultural sources.
- Livestock — cattle, buffalo, sheep, goats produce methane through enteric fermentation (digestion in stomach). Also from manure.
- Landfills — decomposing organic waste in garbage dumps produces methane (biogas).
- Coal mining — methane trapped in coal seams is released when coal is mined.
- Natural gas and oil production — leakage during extraction, processing, and transport.
- Biomass burning — incomplete burning of wood, crop residues, forests.
- Sewage treatment plants — anaerobic digestion of organic waste.
🔑 Methane Sinks — Where Methane Goes
- Reaction with hydroxyl radicals (OH⁻) in troposphere (main sink): About 90% of methane is destroyed by reacting with OH radicals in the lower atmosphere. CH₄ + OH → CO₂ + H₂O. This is the primary removal mechanism.
- Soil uptake (methanotrophs): Certain aerobic bacteria in well-drained soils (methanotrophs) consume methane as their energy source, converting it to CO₂. Well-aerated forest soils are important methane sinks.
- Stratospheric oxidation: A small amount of methane that reaches the stratosphere is oxidized by photochemical reactions.
⭐ Methane — UPSC Must-Know Facts
- Methane is produced by methanogenic archaea under anaerobic conditions.
- Wetlands = largest natural methane source. Rice paddies = largest agricultural methane source.
- Methane is 25–28× more potent than CO₂ as a GHG over 100 years; ~80× over 20 years.
- Main methane sink = reaction with OH radicals in troposphere.
- Melting permafrost is releasing massive stored methane — a climate tipping point.
- Biogas (from cattle dung, kitchen waste) is mostly methane — used as a renewable fuel.
Methane sources are tested regularly in UPSC — especially distinguishing natural from anthropogenic sources. Rice paddies as a methane source is a classic UPSC question. Permafrost methane release as a climate feedback loop is relevant to current affairs. Also know: Wetlands are both methane sources (anaerobic decomposition) AND carbon sinks (peat formation) — this paradox is a favourite UPSC trap. Methane from landfills can be captured and used as fuel — relevant to Swachh Bharat and waste management questions.
Phosphorus Cycle
The phosphorus cycle is a sedimentary biogeochemical cycle. Unlike carbon and nitrogen, phosphorus has no significant atmospheric phase — it does not exist as a gas under normal conditions. Its reservoir is phosphate rocks (apatite minerals) in the Earth’s crust. It moves slowly through soil, water, and living organisms. The phosphorus cycle is the slowest of all major nutrient cycles.
Phosphorus is essential for DNA, RNA, ATP (energy currency of cells), and cell membranes (phospholipids). Without phosphorus, no cell can store or use energy. It is also a building block of bones and teeth. Yet it is scarce — which is why it limits plant growth in many ecosystems. Phosphorus in fertilizers is mined from ancient phosphate rock — a non-renewable resource.
🔑 Steps in the Phosphorus Cycle
- Weathering of rocks: Phosphate rocks (mainly apatite — Ca₅(PO₄)₃) are slowly broken down by weathering (rain, temperature, acids). This releases phosphate ions (PO₄³⁻) into the soil and water. This is the entry point — extremely slow (millions of years).
- Soil uptake by plants: Plants absorb inorganic phosphate ions (PO₄³⁻ and HPO₄²⁻) through their roots from soil water. Mycorrhizal fungi help plants access phosphorus from the soil more efficiently.
- Movement through food chain: Animals eat plants and obtain phosphorus. It is incorporated into bones, teeth, DNA, ATP, and cell membranes at each level.
- Decomposition (return to soil): When plants and animals die, decomposers (phosphate-solubilising bacteria and fungi) break down organic phosphorus back into inorganic phosphate ions, returning it to the soil.
- Runoff to oceans: Phosphate ions dissolved in water are carried by rivers into the oceans. Marine organisms use them; when they die, phosphorus settles on the ocean floor as sediment.
- Geological uplift: Over millions of years, tectonic activity lifts ocean sediments containing phosphate onto land, where weathering begins the cycle again. This is the slowest step — taking tens of millions of years.
Marine birds (seabirds like cormorants, gannets) eat fish from the ocean, absorb phosphorus from them, and deposit it on land as guano (bird droppings). This is one of the fastest pathways for returning ocean phosphorus to land. Guano deposits were historically one of the world’s most valuable fertilizers — wars were fought over guano islands in the 19th century!
Excess phosphorus from fertilizer runoff and sewage enters lakes and rivers. This triggers explosive growth of algae and phytoplankton — called eutrophication. The algal bloom blocks sunlight, kills underwater plants, and when algae die, their decomposition consumes all dissolved oxygen, creating dead zones where no aquatic life can survive. This is why phosphorus in detergents was banned in many countries — even a little phosphorus causes a lot of damage in water bodies.
⭐ Phosphorus — UPSC Must-Know Facts
- No atmospheric phase — purely sedimentary. Phosphorus does NOT exist as a gas. ✅
- Reservoir = phosphate rocks (apatite minerals) in Earth’s crust.
- Essential for DNA, RNA, ATP, cell membranes, bones, teeth.
- Phosphorus cycle is the slowest of all biogeochemical cycles.
- Excess phosphorus in water → eutrophication → algal blooms → dead zones.
- Mycorrhizal fungi help plants absorb phosphorus from soil.
- Phosphate rock is a non-renewable resource — being mined faster than it forms.
The key UPSC fact: Phosphorus cycle has no atmospheric phase — this is the most tested statement about this cycle. Also: excess phosphorus → eutrophication (linked to water pollution questions). Mycorrhizae and phosphorus absorption is relevant to soil ecology. “Soil does not play any role in the phosphorus cycle” — this is a FALSE statement that has appeared in UPSC as a trap.
Sulphur Cycle
The sulphur cycle is the biogeochemical cycle that describes the movement of sulphur through the Earth’s lithosphere, atmosphere, hydrosphere, and biosphere. Sulphur is essential for life — it is a component of amino acids (cysteine, methionine), proteins, vitamins (biotin, thiamine), and enzymes. The sulphur cycle is mostly sedimentary, but unlike phosphorus, it has an important gaseous phase through two compounds: hydrogen sulphide (H₂S) and sulphur dioxide (SO₂).
Sulphur is what gives rotten eggs their smell (H₂S), what causes volcanoes to produce choking fumes (SO₂), and what makes acid rain kill forests and corrode buildings. Understanding the sulphur cycle is essential for understanding acid rain — one of the most tested UPSC topics in environment.
🔑 Reservoir of Sulphur
- The main reservoir is soil and sediments — sulphur is stored here as sulphates, sulphides, and organic sulphur.
- In organic deposits: coal, oil, peat (as organic sulphur).
- In inorganic deposits: pyrite rock (FeS₂) and sulphur rock — a major geological reservoir.
- Sulphate (SO₄²⁻) dissolved in seawater is also a major reservoir.
🔑 Steps in the Sulphur Cycle
- Weathering of rocks (release to soil): Sulphur-containing rocks (pyrite — FeS₂, gypsum — CaSO₄) are broken down by weathering, releasing sulphate ions (SO₄²⁻) into the soil and water. Erosional runoff carries dissolved sulphates to rivers and oceans.
- Absorption by plants: Plants absorb sulphate (SO₄²⁻) from the soil through their roots. Sulphate is reduced inside the plant and incorporated into organic molecules — especially amino acids like cysteine and methionine.
- Movement through food chain: Animals obtain sulphur by eating plants. Sulphur is found in the proteins of all animals — in hair, hooves, feathers, and muscles.
- Decomposition (release as H₂S): When organisms die, bacteria decompose the organic sulphur in proteins, releasing hydrogen sulphide (H₂S) gas — the “rotten egg” smell. Under anaerobic conditions, sulphate-reducing bacteria convert sulphates to H₂S.
- Atmospheric release (SO₂ and H₂S):
- Volcanic eruptions — inject large amounts of SO₂ into the atmosphere.
- Burning fossil fuels (coal, diesel, petroleum) — coal contains sulphur; burning releases SO₂. This is the primary human source of atmospheric sulphur.
- Ocean surface — marine organisms release dimethyl sulphide (DMS), a sulphur compound that contributes to cloud formation.
- Acid rain formation: SO₂ in the atmosphere reacts with water vapour and oxygen to form sulphuric acid (H₂SO₄). This falls as acid rain (pH < 5.6), damaging forests, aquatic ecosystems, soil, and buildings.
- Return to soil: Acid rain and dry deposition return sulphate to the soil and water bodies, completing the cycle.
When a coal-fired power plant burns coal, it releases SO₂ into the atmosphere. SO₂ + H₂O + O₂ → H₂SO₄ (sulphuric acid). This dissolves in rain, lowering its pH to 4 or below. When this acid rain falls on forests, it leaches calcium and magnesium from the soil (nutrients trees need), damages leaf coatings, and kills fish in lakes by acidifying the water. The Black Forest in Germany was severely damaged by acid rain from industrial Europe in the 1970s–80s. In India, coal-rich Jharkhand and Odisha have been affected. Reducing SO₂ emissions from power plants is one of the key goals of the Clean Air Action Plan.
⭐ Sulphur Cycle — UPSC Must-Know Facts
- Sulphur cycle is mostly sedimentary, but has gaseous compounds: H₂S (rotten egg gas) and SO₂ (from volcanoes & fossil fuel burning).
- Main reservoir = soil and sediments (pyrite, gypsum, coal, organic matter).
- Burning coal and diesel = biggest human source of atmospheric SO₂ → acid rain.
- SO₂ + H₂O + O₂ → H₂SO₄ (sulphuric acid) = acid rain.
- Sulphur is essential in amino acids cysteine and methionine.
- Marine organisms (phytoplankton) release dimethyl sulphide (DMS) — important for cloud formation.
- “Soil does not play any role in the sulphur cycle” — FALSE (soil IS a major reservoir). UPSC has tested this exact statement.
The sulphur cycle is most frequently tested through the lens of acid rain. Know: SO₂ from coal burning → H₂SO₄ → acid rain. Also: NOₓ from vehicles and power plants → HNO₃ → also contributes to acid rain. The statement “Burning coal releases CO₂, SO₂, and oxides of nitrogen” has been asked directly in UPSC (2014). Also remember: Irrigation over a long period can contribute to soil salinization by depositing sulphates from groundwater — a tested UPSC connection between the sulphur cycle and agriculture.
Master Comparison Table
| Feature | Carbon | Nitrogen | Methane | Phosphorus | Sulphur |
|---|---|---|---|---|---|
| Type | Gaseous | Gaseous | Gaseous (compound) | Sedimentary | Mostly Sedimentary (with gaseous phase) |
| Reservoir | Atmosphere + Ocean | Atmosphere (78% N₂) | Atmosphere + Wetlands | Phosphate rocks (Earth’s crust) | Soil & sediments (pyrite, coal) |
| Atmospheric form | CO₂, CH₄ | N₂, N₂O, NOₓ | CH₄ | None (no gas phase) | H₂S, SO₂ (minor phase) |
| Key process in | Photosynthesis, Respiration, Combustion | Fixation, Nitrification, Denitrification | Anaerobic decomposition | Weathering, Decomposition, Runoff | Weathering, Burning fossil fuels, Decomposition |
| Key bacteria | Decomposers (general) | Rhizobium, Nitrosomonas, Nitrobacter, Pseudomonas | Methanogenic archaea | Phosphate-solubilising bacteria, Mycorrhizae | Sulphate-reducing bacteria, Desulfovibrio |
| Human impact | Fossil fuel burning → CO₂ → climate change | Fertilizers → eutrophication, N₂O (GHG), acid rain (NOₓ) | Rice paddies, cattle → methane → climate change | Fertilizer runoff → eutrophication | Coal burning → SO₂ → acid rain |
| Essential for life? | Yes — backbone of all organic molecules | Yes — proteins, DNA, chlorophyll | Greenhouse gas (not essential for life) | Yes — DNA, ATP, bones, cell membranes | Yes — amino acids (cysteine, methionine) |
| Speed of cycle | Fast (atmosphere days–years) | Fast (days–years) | Fast (10–12 years atmospheric life) | Very slow (millions of years for geological phase) | Slow (sedimentary) but fast (atmospheric SO₂) |
