The sulphur cycle is a complex series of processes in which all of the sulphur atoms present in the earth circulate. Sulphur is one of the more than 100 known elements, and one of six main constituents of living organisms. The earth does not receive matter from outside except from occasional meteorites (and this is negligible). Thus, the sulphur atoms (as also other elements) are part of a closed system, that is they cannot be lost or replenished. These fixed number of sulphur atoms are recycled throughout all of earth’s processes that use chemicals containing sulphur. These processes form a cycle that involves the living sphere called the biosphere, and the nonliving spheres – lithosphere, atmosphere and hydrosphere.
 The Cycle
The sulphur cycle can be thought of as beginning with sulphur dioxide gas (SO2) or the particles of sulfate compounds in the air. These compounds reach earth’s surface as they fall down or are rained out of the atmosphere. From the soil, plants take up various forms of these compounds and incorporate them into their tissues. Then these organic sulphur compounds are returned to the land or water after the plants die or are consumed by animals. Bacteria can transform the organic sulphur to hydrogen sulfide gas (H2S). In the oceans, certain phytoplanktons produce a chemical that subsequently transforms to SO2. These gases can re-enter the atmosphere, water, and soil, and continue the cycle.
 Did You Know?
- An important distinction between cycling of Sulphur and Nitrogen and Carbon Cycles is that sulfur is "already fixed". That is, plenty of sulfate anions (SO42-) are available for living organisms to utilize. By contrast, the major biological reservoirs of nitrogen atoms (N2) and carbon atoms (CO2) are gases that must be pulled out of the atmosphere.
- The largest physical reservoir is the Earth's crust wherein sulfur is found in gypsum (CaSO4) and pyrite (FeS2).
- Some of the reactions that occur in the sulfur cycle open up new environments to life. They support biological communities in unlikely places such as deep sea thermal vents, areas of low pH and areas of high temperature.
The main reservoirs of sulphur are:
- Earth’s crust is the largest reservoir of sulphur. Here sulphur is found in gypsum (CaSO4) and pyrite (FeS2).
- The ocean is another large reservoir of sulphur. Here biologically useful sulphur is found as sulfate anions, dissolved hydrogen sulfide gas and elemental sulphur. Freshwater contains sulfate, hydrogen sulfide and elemental sulphur.
- Land contains sulfate.
- Atmosphere contains sulphur oxide (SO2) and methane sulfonic acid (CH3SO3).
The sulphur cycle is significant both to climate and life. Sulphur is one of the components of proteins and vitamins in animals. Proteins consist of amino acids which in turn contain sulphur atoms. Sulphur is also important for the functioning of proteins and enzymes in plants. Plants absorb sulphur which is dissolved in water. Animals consume these plants, and take up enough sulphur as required by them. In its fully oxidized state, that is as sulfate, sulphur is the major cause of acidity in both natural and polluted rainwater. This makes sulphur important to geochemical, atmospheric, and biological processes such as the natural weathering of rocks, acid precipitation, and rates of denitrification. The lifetime of most sulphur compounds in the air is relatively short, may be, a few days. Superimposed on these fast cycles of sulphur are the extremely slow sedimentary-cycle processes or erosion, sedimentation, and uplift of rocks containing sulphur. In addition, sulphur compounds from volcanoes are intermittently injected into the atmosphere, and a continual stream of these compounds is produced from industrial activities.
 Human Activity
Sulphur is also one of the main biogeochemical cycles that is significantly influenced by human activity. According to some estimates, emissions of sulphur to the atmosphere from human activity are at least equal or probably larger in magnitude than those from natural processes. Burning of fossil fuels and metal processing are the main culprits. Sulphuric acid particles contribute to the polluting smog that engulfs some industrial centers and cities where many sulphur containing fuels are burned. Sulphur dioxide is also involved in the phenomenon of acid rain. Such particles floating in air (known as sulfate aerosols) can cause respiratory diseases or cool the climate by reflecting some extra sunlight to space.
The levels of sulphur dioxide can be reduced by controlling pollution in coal-based power plants and in industrial processing units. Use of fuels like natural gas which have lower sulphur content also helps.
- Sulfur Cycle
- THE SULFUR CYCLE
- Biogeochemical Cycles
 See Also