Sulphur Cycle

Sulphur is an important component of most proteins, few vitamins and enzymes.

Though sulphur occurs in gaseous form, as hydrogen sulphide (H₂S) and sulphur dioxide (SO₂), the residence time of sulphur in atmosphere is very small (about 4 x 10¹⁰ kg) and its main reservoir pool is the soil (contains about 4 x 10¹⁸ kg in the form of sulphates, sulphides and organic sulphur).

So, sulphur cycle is classified under sedimentary cycles.

Sources of Sulphur

a) Soil, water and rocks containing sulphates, sulphides and organic sulphur, and also body of living organisms.

b) Oxides of sulphur in the atmosphere due to the burning of fossil fuels and volcanic emissions.

c) Sulphur occurs as elemental sulphur also.

Sulphur Utilisation

i) Green plants (producers) require sulphur in the form of sulphates (SO₄^-2), which they absorb from soil and incorporate sulphur in their proteins.

ii) Animals take in sulphur from plants as food through food chain and few animals can get from water also.

Sulphur Production

1) Decomposers like Aspergillus (aerobic fungi), Neurospora (aerobic fungi) and Escherichia (anaerobic bacteria) act on the dead and decaying organic matter of plants and animals, releasing hydrogen sulphide (H₂S) to the environment.

2) Heterotrophic bacteria like Desulfavibrio and Acetobacter reduce sulphates to elemental sulphur or sulphides under anaerobic conditions. Though sulphides are harmful to most organisms, sulphur bacteria oxidise them to sulphate and bring back the element to cycle.

3) Oxides of sulphur (SO₂ and H₂S) in the atmosphere, gets dissolved in rain water and return to soil as sulphates and sulphuric acid.

4) From the reservoir pool in deep sediments in the sea, sulphur reaches the land through food chains, sea sprays and geological upheavals.

5) Hydrogen sulphide is released into atmosphere from lakes, marshes and water logged soils, which is oxidised to sulphur dioxide in the atmosphere.

Note:

Elemental sulphur or gaseous compounds of sulphur (H₂S and SO₂) in the atmosphere, is oxidised to SO₃, which combines with water to form sulphuric acid and comes down on land as acid rain.