Sulphur Cycle

What is the Sulphur cycle? What is the role of sulphur cycle in the natural processes of the earth? Read to find out more.

Sulphur (S), also spelled sulfur, is a nonmetallic chemical element and one of the most reactive of the elements. Pure sulfur is a tasteless, odorless, brittle solid, and pale yellow. It is a poor conductor of electricity and is insoluble in water.

It reacts with all metals except gold and platinum, forming sulfides. It also forms compounds with several nonmetallic elements.

The sulphur cycle is a biogeochemical cycle that describes the movement of sulfur in various forms through the Earth’s biosphere, geosphere, hydrosphere, and atmosphere. Sulfur is an essential element for living organisms, and it plays a vital role in ecosystems.

Where is sulphur found naturally?

Sulphur is found in various reservoirs, including rocks, minerals, sediments, soil, water bodies, and the atmosphere. The largest reservoir of sulfur on Earth is in rocks and minerals.

Sulphur, usually sulfide, is present in many types of meteorites.  Sulphur is also present in all kinds of proteins. Plants directly absorb sulfur-containing amino acids such as methionine, cystine, and cysteine.

Sulphur is released into the atmosphere by the burning of fossil fuels, volcanic activities, and the decomposition of organic molecules. On land, sulfur is stored in underground rocks and minerals. It is released by precipitation, weathering of rocks, and geothermal vents.

The Sulphur cycle

The sulphur cycle is a biogeochemical cycle in which sulphur moves between rocks, waterways, and living systems. It is a sedimentary cycle but hydrogen sulfide (H2S) and sulphur dioxide (SO2) are the gaseous components of the sulphur cycle.

The soil and sediments act as the sulfur reservoir where they are captured in organic (coal, oil, Peat) and inorganic (pyrite, sulfur rock). They are stored in the form of sulfates, sulfides, and organic sulfur.

1. It is released by weathering of rocks, erosional runoff, and decomposition of organic matter.
2. It is carried to terrestrial and aquatic ecosystems in the form of salt solutions.
3. The sulfides are converted to elemental sulphur by bacterial action. This is again converted to sulfates by chemolithotrophic bacteria.
4. Plants take up sulfur in the form of sulfates and convert it into sulfur-containing amino acids through a series of metabolic processes.
5. These amino acids are incorporated in the proteins of the autotroph tissues which eventually pass along through the grazing food chain.
6. The sulphur is carried back into soil, lakes, and seas from living organisms through the excretion and decomposition of dead organic matter.
7. Sulphur enters the atmosphere from sources like volcanic eruptions, burning of fossil fuels, the surface of oceans, and gases released by decomposition.
8. Atmospheric H2S gets oxidized to SO2 which is carried back to earth after being dissolved in rainwater as weak sulphuric acid (acid rain).

Economic importance of sulphur

• Millions of tons of sulfur are produced each year, mostly for the manufacture of sulfuric acid, which is widely used in industry.
• Sulfur is directly involved in the production of fossil fuels and a majority of metal deposits because of its ability to act as an oxidizing or reducing agent.
• The presence or absence of sulphur is one of the limiting factors for the concentration of precious metals in the source.

Sulphur Uptake by Plants:

• Sulphur is taken up by plants in the form of sulfate ions (SO4^2-) from the soil. Plants incorporate sulfur into their organic compounds, including amino acids, proteins, and coenzymes.

Sulfur in the Food Web:

• Sulphur moves through the food web as herbivores and carnivores consume plants or other animals. Sulfur-containing compounds are transferred from one organism to another.

Decomposition and Mineralization:

• When plants and animals die, Sulphur is released during decomposition by the action of sulfur bacteria and fungi. This process involves the conversion of organic Sulphur compounds into inorganic forms like hydrogen sulfide (H2S).

Sulfur Bacteria:

• Sulphur bacteria play a crucial role in the sulfur cycle. They can oxidize or reduce sulfur compounds, contributing to the release or consumption of sulfur in various forms.

Impact of human activities on Sulphur cycle

Human activities are having a major impact on the global sulphur cycle. The burning of coal, natural gas, and fossil fuels has greatly increased the amount of sulphur in the atmosphere and ocean. It has also depleted the sedimentary rock sink.

• Without human factors, sulphur would stay stored in rocks for millions of years until it gets uplifted through tectonic events, which is then released through erosion and weathering processes. But now it is being drilled out and burned at an increasing rate unnaturally.
• Over the most polluted areas of the world, there has been a 30-fold increase in sulfate deposition. The mining of coal and extracting of petroleum at such high rates that the global sulphur flux has become double in the last 100 years.
• Acid rain is caused by emissions of sulphur dioxide and nitrogen dioxide, which react with the water molecules in the atmosphere to produce acids.
• Acid rain has adverse impacts on forests, freshwaters, and soils, killing microbes, insects, and aquatic life-forms. It damages infrastructure by causing the paint to peel, corrosion of steel structures such as bridges, and weathering of stone buildings and statues.
• Acid rain has indirect impacts on human health also. The acid rain in rainwater is diluted and does not harm humans directly, but the particulates, sulphur, and nitrogen dioxide have adverse effects on human health.