Sulfur Cycle

Sulfur is an important nutrient for organisms, being an key constituent of certain amino acids, proteins, and other biochemicals. Plants satisfy their nutritional needs for sulfur by assimilating simple mineral compounds from the environment. This mostly occurs as sulfate dissolved in soil water that is taken up by roots, or as gaseous sulfur dioxide that is absorbed by foliage in environments where the atmosphere is somewhat polluted with this gas. Animals obtain the sulfur they need by eating plants or other animals, and digesting and assimilating their organic forms of sulfur, which are then used to synthesize necessary sulfur-containing biochemicals.

In certain situations, particularly in intensively managed agriculture, the availability of biologically useful forms of sulfur can be a limiting factor to the productivity of plants, and application of a sulfate-containing fertilizer may prove to be beneficial. Sulfur compounds may also be associated with important environmental damages, as when sulfur dioxide damages vegetation, or when acidic drainages associated with sulfide minerals degrade ecosystems.

Sulfur (S) can occur in many chemical forms in the environment. These include organic and mineral forms, which can be chemically transformed by both biological and inorganic processes.

Sulfur dioxide (SO₂) and sulfate (SO₄^-2 )

Sulfur dioxide is a gas that can be toxic to plants at concentrations much smaller than one part per million in the atmosphere, and to animals at larger concentrations. There are many natural sources of emission of SO₂ to the atmosphere, such as volcanic eruptions and forest fires. Large emissions of SO₂ are also associated with human activities, especially the burning of coal and the processing of certain metal ores. In the atmosphere, SO₂ is oxidized to sulfate, an anion that occurs as a tiny particulate in which the negative charges are electrochemically balanced by the positive charges of cations, such as ammonium (NH ⁺ 4 ), calcium (Ca²⁺), or hydrogen ion (H⁺). These fine particulates can serve as condensation nuclei for the formation of ice crystals, which may settle from the atmosphere as rain or snow, delivering the sulfate to terrestrial and aquatic ecosystems. If the sulfate is mostly balanced by hydrogen ion, the precipitation will be acidic, and can damage some types of freshwater ecosystems.

Hydrogen sulfide (H₂S)

Hydrogen sulfide is another gas, with a strong smell of rotten eggs. Hydrogen sulfide is emitted from situations in which organic sulfur compounds are being decomposed in the absence of oxygen. Once H₂S is emitted The sulfur cycle. Illustration by Hans & Cassidy. Courtesy of Gale Group. to the atmosphere, it is slowly oxidized to sulfate, which behaves as described above.

Metal sulfides

Sulfur can occur in many chemically reduced mineral forms, or sulfides, in association with many metals. The most common metal sulfides in the environment are iron sulfides (such as FeS₂; these are called pyrites when they occur as cubic crystals), but all heavy metals can occur in this mineral form. Whenever metal sulfides are exposed to an oxygen-rich environment, certain bacteria begin to oxidize the sulfide, generating sulfate as a product, and tapping energy from the process which is used to sustain their own growth and reproduction. This autotrophic process is called chemosynthesis, and the bacteria involved are named Thiobacillus thiooxidans. When a large quantity of sulfide is oxidized in this way, an enormous amount of acidity is associated with the sulfate product.

Organic sulfur

Organic sulfur refers to an extremely diverse array of sulfur-containing organic molecules. These can range in weight and complexity from amino acids, proteins, and nucleic acids, to large molecules such as humic substances in soil or water. Organic sulfur is a chemically reduced form of sulfur. When it is exposed to an oxygen-rich atmosphere, bacteria oxidize the organic sulfur to derive energy, while liberating sulfate, usually in association with hydrogen ions and acidity.

Human activities influence the rates and character of certain aspects of the sulfur cycle in important ways, sometimes causing substantial environmental damages.

Acid rain is a well-known environmental problem. Acid rain is ultimately associated with large emissions of sulfur dioxide to the atmosphere by human sources, such as oil- and coal-fired power plants, metal smelters, and the burning of fuel oil to heat homes. The SO₂ is eventually oxidized in the atmosphere to sulfate, much of which is balanced by hydrogen ions, so the precipitation chemistry is acidic. In addition, the vicinity of large point-sources of SO₂ emission is generally polluted by relatively large concentrations of this gas. If its concentration is large enough, the SO₂ can cause toxicity to plants, which may be killed, resulting in severe ecological damages. In addition, atmospheric SO₂ can be directly deposited to surfaces, especially moist soil, plant, or aquatic surfaces, since SO₂ can readily dissolve in water. When this happens, the SO₂ becomes oxidized to sulfate, generating acidity. This means a direct input of sulfur dioxide is called dry deposition, and is a fundamentally different process from the so-called wet deposition of sulfate and acidity with precipitation.

Acid mine drainage is another severe environmental problem that is commonly associated with coal and metal mining, and sometimes with construction activities such as road building. In all of these cases, physical disturbance results in the exposure of large quantities of mineral sulfides to atmospheric oxygen. This causes the sulfides to be oxidized to sulfate, a process accompanied by the generation of large amounts of acidity. Surface waters exposed to acid mine drainage can become severely acidified, to a pH less than 3, resulting in severe biological damages and environmental degradation.

Sulfur is also an important mineral commodity, with many industrial uses in manufacturing. Sulfur for these purposes is largely obtained by cleaning sour natural gas of its content of H₂S, and from pollution control at some metal smelters.

In a few types of intensively managed agriculture, crops may be well fertilized with nitrogen, phosphorus, and other nutrients, and in such cases there may be a deficiency of sulfate availability. Because sulfate is an important plant nutrient, it may have to be applied in the form of a sulfate-containing fertilizer. In North America, sulfate fertilization is most common in prairie agriculture.