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 Sulphur cycle : The cycling of sulphur between biotic and abiotic systems is called sulphur cycle. It is a sedimentary cycle. Sulphur is an important component of proteins and amino acids.

Sulphur exists in a number of states. Of these, three are important. They are elemental sulphur, sulphides and sulphates. Sulphur is present in rocks. It is made available for plants in the form of inorganic sulphate by weathering and erosion. Sulphur passes into the animals through food chain. By the death of plants and animals, the decomposers again bring the sulphur to the soil for the use of plants.

Some sulphur in dead bodies is released into the air as hydrogen sulphide 

(H2S)

 by the bacteria called Escherichia coli under anaerobic combustion. Similarly incomplete combustion of fossil fuel releases sulphur dioxide  (SO2)

 into the air.

Certain bacteria (green and purple photosynthetic bacteria) oxidise 

H2S

of air to sulphate which can be used by plants.

H2S+2O2→SO4+ 2H+

Certain amount of sulphur is lost in the sediments. If iron is present in the sediments, sulphur combines with it to form iron sulphide.                     

Fe+S→FeS