Sulphur Cycle and Human Impact on It
Environment & Ecology › Nutrient Cycling (Biogeochemical Cycles) · Topic 5.7
The sulphur cycle is the movement of sulphur between rocks, soil, water, the atmosphere and living things. It is a sedimentary cycle with a gaseous phase. The main reservoir is the Earth's crust, in sulphide and sulphate minerals such as pyrite and gypsum, and in ocean sulphate. Weathering of rocks releases sulphate, which plants absorb and use in amino acids (cysteine, methionine), vitamins and proteins. Animals get sulphur by eating plants. Decomposers release hydrogen sulphide from dead matter, and sulphur bacteria oxidise it to sulphate or sulphur. Natural gaseous inputs are volcanoes, ocean dimethyl sulphide from plankton and decaying matter. Humans add large amounts of sulphur dioxide by burning coal and oil and smelting ores, causing acid rain, smog and damage to health and buildings.
5 min read · UPSC Prelims · Environment & Ecology · Nutrient Cycling (Biogeochemical Cycles)
At a Glance
- Type Sedimentary with gas phase
- Reservoir Rocks and ocean sulphate
- Key gas Sulphur dioxide
- Natural gas Dimethyl sulphide
- Pollution effect Acid rain
- Exam link UPSC Prelims, Environment & Ecology: Nutrient Cycling (Biogeochemical Cycles)
Where Does It Fit?
This topic follows the oxygen cycle. Sulphur is a vital element for proteins and a major air pollutant, so its cycle links to acid rain and the pollution topics. Previous topic: Oxygen Cycle. Next topic: Phosphorus Cycle and Human Influence on It.
What Are the Steps of the Sulphur Cycle?
| Step | Process |
|---|---|
| Weathering and erosion | Rocks with sulphides and gypsum release sulphate into soil and water |
| Uptake by plants | Plants and microbes absorb sulphate and build sulphur-containing amino acids |
| Food chain transfer | Herbivores and carnivores obtain sulphur from plants and prey |
| Decomposition | Bacteria break down dead proteins and release hydrogen sulphide, a gas with a rotten-egg smell |
| Oxidation by bacteria | Sulphur bacteria such as Thiobacillus and Beggiatoa convert hydrogen sulphide and sulphur into sulphate |
| Reduction | In oxygen-poor mud, Desulfovibrio reduce sulphate to sulphide, which may form pyrite with iron |
| Atmospheric phase | Volcanoes, ocean spray, dimethyl sulphide from plankton and human sources add sulphur gases, which return as sulphate in rain |
How Do Humans Affect the Sulphur Cycle?
- Fossil fuel burning: coal and oil contain sulphur, which burns to sulphur dioxide
- Smelting: extraction of metals such as copper and zinc from sulphide ores releases sulphur dioxide
- Acid rain: sulphur dioxide forms sulphuric acid in the air, falling as acid rain that harms lakes, forests, soils and monuments
- Health effects: sulphur dioxide irritates the lungs and forms fine particles
- Climate effect: sulphate aerosols reflect sunlight and cool the climate, partly masking warming
- Fertiliser: sulphur deficiency is now spreading in soils, so sulphur-containing fertilisers are used
- Regulation: controls on sulphur in fuel, flue gas desulphurisation and cleaner fuels cut emissions
- Marine fuel: the International Maritime Organization capped sulphur in ship fuel from 2020
What Else Should You Remember?
- Pyrite: iron sulphide, called fool’s gold
- Gypsum: calcium sulphate, used in cement and as a soil amendment
- Sulphur in proteins: cysteine and methionine contain sulphur and form disulphide bonds
- Sulphur in oils: mustard and other oilseeds have high sulphur needs and show deficiency in Indian soils
- Dimethyl sulphide: released by marine plankton, forms cloud condensation nuclei and may influence climate
- CLAW hypothesis: a proposed climate feedback through dimethyl sulphide, still debated
- Volcanoes: release large pulses of sulphur dioxide, cooling the climate for a year or two, as Mount Pinatubo did in 1991
- Sulphate aerosols: reflect sunlight, so reductions in sulphur pollution can reveal more warming
- Solar geoengineering: a proposal to inject sulphate aerosols to cool Earth, controversial
- Sulphur dioxide standards: India sets limits in its National Ambient Air Quality Standards; check the latest values
- Coal power plants: Indian rules require flue gas desulphurisation, with timelines revised several times; check the latest status
- Fuel standards: Bharat Stage VI fuels cut sulphur to about 10 parts per million
- Taj Mahal: acid rain and pollution concerns led to rules for the Taj Trapezium Zone
- Marble cancer: acid rain eats away marble and limestone
- Lakes: acid deposition lowers pH and kills fish in sensitive lakes
- Forests: acid rain leaches calcium and releases aluminium harmful to roots
- Hydrogen sulphide: toxic gas in swamps, sewers and volcanic areas
- Sulphur bacteria: include photosynthetic purple and green sulphur bacteria
- Black smokers: deep sea vents with sulphur-based chemosynthesis
- Link to previous: see Oxygen Cycle: Sources, Sinks and Human Impact
- Link to pollution: see Air, Marine, Noise and Other Pollution macro
- Link to vents: see Marine Ecosystem and Marine Organisms
- Caution: acid rain also involves nitrogen oxides, not just sulphur
- Natural versus human: humans now emit about as much sulphur gas as natural sources or more
- Exam hint: remember sulphur dioxide as the main pollutant and Thiobacillus as the oxidiser
Why Does It Matter for the Exam?
- Reservoir: rocks
- Pollutant: sulphur dioxide
- Effect: acid rain
- Current affairs link: desulphurisation rules and ship fuel limits
- Common errors: calling sulphur a purely gaseous cycle or blaming acid rain on carbon dioxide
As an analytical point, the sulphur cycle shows how a nutrient becomes a pollutant when released by industry. Cleaner fuels and scrubbers have reduced acid rain in many regions, but also cut the cooling effect of sulphate aerosols, which has to be considered in climate planning.
Why Is This Topic Important?
- Reservoir Rocks
- Gas SO2, DMS
- Problem Acid rain
What Came Next?
The next topic covers the phosphorus cycle and human influence on it. Next topic: Phosphorus Cycle and Human Influence on It.
Key People and Terms
- Sulphate Usable form of sulphur
- Hydrogen sulphide Rotten-egg gas
- Dimethyl sulphide Plankton gas
- Acid rain Acidic deposition
- Pyrite Iron sulphide
- Desulphurisation Removal of sulphur from flue gas
Exam Corner
Points to Remember
- Sedimentary cycle with gas phase.
- SO2 forms sulphuric acid.
- Plankton emit DMS.
- Thiobacillus oxidises sulphur.
- Sulphate aerosols cool climate.
Do Not Confuse With
- Sulphur dioxide and hydrogen sulphide Combustion gas versus decay gas
- Sulphate and sulphide Oxidised versus reduced forms
- Acid rain and acid deposition Wet versus wet and dry
Memory Hook
Rock gives sulphate, plants make proteins, decay gives a rotten-egg gas, coal gives acid rain.
Mains Angle
Questions ask for sources and effects. These points help.
- Cycle Steps
- Human Burning
- Control Scrubbers
A Question You May Face
An original practice question, not a past paper question.
“Describe the sulphur cycle and examine how human activities have altered it, with reference to acid rain.”
How to Answer
- Cycle Steps.
- Human Fuels.
- Acid rain Effects.
- Control Rules.
GS Relevance
Frequently Asked Questions
What is the sulphur cycle?
The sulphur cycle is the movement of sulphur through rocks, soil, water, air and organisms. Weathering releases sulphate from rocks, plants absorb it to make proteins, decomposers release hydrogen sulphide, bacteria convert it back to sulphate, and volcanic and human emissions add sulphur dioxide to the air.
Is the sulphur cycle gaseous or sedimentary?
The sulphur cycle is mainly a sedimentary cycle, because the largest reservoir is the Earth's crust and ocean sulphate, but it also has an important gaseous phase involving hydrogen sulphide, sulphur dioxide and dimethyl sulphide. Some books therefore call it a sedimentary cycle with a gas phase.
How does sulphur dioxide cause acid rain?
Sulphur dioxide released by burning coal and oil reacts with oxygen and water in the atmosphere to form sulphuric acid. This falls as acid rain or dry deposition, lowering the pH of soils and water bodies, damaging forests, fish and crops, and corroding buildings and monuments made of marble and limestone.
What is dimethyl sulphide?
Dimethyl sulphide is a sulphur gas produced by marine phytoplankton. It is released in large amounts from the ocean and is oxidised in the air to form tiny particles that help clouds to form. It is the most abundant natural sulphur compound entering the atmosphere from the sea.
Which bacteria are important in the sulphur cycle?
Sulphur-oxidising bacteria such as Thiobacillus and Beggiatoa convert hydrogen sulphide and elemental sulphur into sulphate, while sulphate-reducing bacteria such as Desulfovibrio convert sulphate into sulphide in oxygen-poor mud. Purple and green sulphur bacteria use sulphur compounds for photosynthesis.
PYQ Practice — Statement Analysis
1 Sulphur dioxide is a major cause of acid rain.
Forming sulphuric acid.
2 The main reservoir of sulphur is the atmosphere.
The Earth's crust.
3 Marine plankton release dimethyl sulphide.
Which seeds clouds.
4 Hydrogen sulphide smells like rotten eggs.
From decay.
Past Mains Questions on This Topic
This topic comes under Nutrient Cycling (Biogeochemical Cycles). UPSC Mains questions asked on the matching syllabus topic, year-wise with marks:
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