Sulfur Cycle: Stages and Importance

He sulfur cycle is the set of processes by which sulfur is transported through nature in various molecules. Sulfur moves through air, soil, water and living things. This biogeochemical cycle includes the mineralization of organic sulfur sulfur, the oxidation of this to sulphate and its reduction to sulfur.

Sulfur is incorporated by microbes and forms different organic compounds. Sulfur is a very abundant element in the universe; It is considered a non-metal, its color is yellow and has no smell. Sulfur is released into the atmosphere by burning fossil fuels, such as coal.

Sulfur cycle

In the atmosphere, sulfur is found in the form of sulfur dioxide (SO2) and can enter it in three ways: from the decomposition of organic molecules, from volcanic activity and geothermal vents, and from the burning of fossil fuels By humans.

Sulfur atoms are an important part of the structure of proteins. Sulfur is found in the amino acid cysteine ​​and is involved in the formation of a type of bond called a disulfide bridge. These links are essential in the determination of the three-dimensional structure of proteins.


  • 1 Stages
  • 2 Sulfur flow
    • 2.1 The sulfur that is forming compounds
    • 2.2 Sulfur that enters the soil
    • 2.3 Sulfur that is coming out of the ground
  • 3 Importance
    • 3.1 Main component in chemical compounds
    • 3.2 Associated with the productivity of plants
    • 3.3 Necessary to build proteins
    • 3.4 Commercial uses
    • 3.5 Associated with environmental damage
  • 4 Impact of the human being on the sulfur cycle
  • 5 References


The sulfur cycle involves the movement of this element in many directions through the atmosphere, the hydrosphere, the lithosphere and the biosphere. In the lithosphere, the erosion processes of the rocks that release the stored sulfur occur.

Sulfur undergoes a series of chemical transformations as it is transported through different means. Throughout its journey, sulfur goes through four fundamental chemical stages:

- Mineralization of organic sulfur to inorganic form, such as hydrogen sulfide, elemental sulfur and other sulfur-based minerals.

- Oxidation of hydrogen sulphide, elemental sulfur and sulphate-related minerals.

- Sulphate reduction to sulfur.

- Microbial immobilization of sulfur compounds and subsequent incorporation into the organic form of sulfur.

Sulfur flow

Despite its complexity, the flow of sulfur can be summarized in three major groups:

The sulfur that is forming compounds

This group includes atmospheric sulfur, organic sulfur, inorganic sulfur (minerals), reduced sulfur and sulfur that forms sulphates.

The sulfate is absorbed by plants and microorganisms, which incorporate them into their organic molecules. The animals then consume these organic forms through the food they eat, moving the sulfur along the food chain.

Sulfur that enters the soil

Sulfur is incorporated into the soil in different ways; for example, by atmospheric deposition, through the use of fertilizers of animal origin, by the waste of the plants, by the use of mineral fertilizers and by the wear of the rocks.

Sulfur that is coming out of the ground

Sulfur is removed from the soil in several ways. For example, when plants absorb sulfates through their roots, when crops are harvested and when some reduced compounds are volatilized.

Another part of the sulfur in the soil is lost through filtration, runoff and erosion. Volcanoes and some gases resulting from organic decomposition are another source of sulfur that is transferred directly to the atmosphere.

However, most of the Earth's sulfur is stored in rocks, minerals and sulfate salts buried deep in the ocean sediments.


Main component in chemical compounds

Sulfur is an important nutrient for organisms because it is a fundamental component of the amino acids cysteine ​​and methionine, as well as other biochemical compounds. The plants satisfy their nutritional needs of sulfur by assimilating mineral compounds from the environment.

Associated with the productivity of plants

In certain situations, particularly in intensive agriculture, the availability of biologically useful forms of sulfur can be a limiting factor for plant productivity; consequently, the application of fertilizers based on sulfates is necessary.

The recognition of the importance of sulphate for the growth and vigor of plants, as well as the nutritional importance of sulfur for human and animal diets, has led to a greater emphasis on research on the processes of absorption, transport and assimilation of sulphate .

Necessary to build proteins

After entering the plant, sulphate is the main form of sulfur transported and stored. Sulfur is necessary for the construction of proteins, enzymes and vitamins, and is a key ingredient in the formation of chlorophyll.

Crops that are deficient in sulfur typically show limitations in their development. Thus, plants with a lack of sulfur are observed thinner and smaller, their younger leaves turn yellow and the amount of seeds is reduced.

Commercial uses

Apart from the production of fertilizers, sulfur has other commercial uses, for example: in gunpowder, matches, insecticides and fungicides. Additionally, sulfur is involved in the production of fossil fuels due to its ability to act as an oxidizing or reducing agent.

Associated with environmental damage

Sulfur compounds can also be associated with significant environmental damage, such as sulfur dioxide that damages vegetation, or acid drainages associated with sulfides that degrade ecosystems.

Impact of the human being on the sulfur cycle

Human activities have played an important role in altering the balance of the global sulfur cycle. The burning of large quantities of fossil fuels, especially coal, releases large amounts of hydrogen sulphide gases into the atmosphere.

When this gas is crossed by rain, acid rain occurs, which is a corrosive precipitation caused by rainwater that falls to the ground through sulfur dioxide, converting it into weak sulfuric acid that causes damage to aquatic ecosystems.

Acid rain damages the environment by reducing the pH of the lakes, which kills a large part of the fauna that lives there. It also affects unnatural structures created by man, such as the chemical degradation of buildings and statues.

Many marble monuments, such as the Lincoln Memorial in Washington, DC, have suffered significant damage from acid rain over the years. These examples show the far-reaching effects of human activities in our environment and the challenges that remain for our future.


  1. Butcher, S., Charlson, R., Orians, G. & Wolfe, G. (1992). Global biogeochemical cycles. Academic Press.
  2. Cunningham, W. & Cunningham, M. (2009). Environmental Science: A Global Concern (11th ed.). McGraw-Hill.
  3. Jackson, A. & Jackson, J. (1996). Environmental Science: The Natural Environment and Human Impact.
  4. Loka Bharathi, P. A. (1987). Sulfur Cycle Global Ecology , (1899), 3424-3431.
  5. Meyer, B. (2013). Sulfur, Energy and Environment.
  6. O'Neill, P. (1998). Environmental Chemistry (3rd ed.). CRC Press.

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