Sulfur Cycle

Sulfur is a constituent of the amino acids (methionine, cysteine and cystine), vitamins (biotin, thiamine, B1), and many protein enzymes that regulate photosynthesis and N-fixation. S is closely associated with nitrogen in the processes of protein and enzyme synthesis. S is also a constituent of various aromatic oils that give plants of the cabbage and onion families their characteristic odours and flavours. Hence, the cabbage, onion, and legume families require especially large amounts of S. Since S is immobile, deficiencies in plants affects younger tissues first. Symptoms of deficiency include yellowing of leaves and stunted growth.

S is also responsible for several types of air, water, and soil pollution and is consequently of increasing environmental interest. The environmental issues associated with S include acid rain, certain types of forest decline, acid mine drainage, and acid sulfate soils.

Sources

Sulfur added to the atmosphere through combustion of coal and oil

The atmosphere contains varying amounts of hydrogen sulfide (H₂S), sulfur dioxide (SO₂), carbonyl sulfide (COS), and other sulfur gases. These atmospheric forms arise from industrial plants, fires, volcanic eruptions, etc. In recent decades, the contributions from industrial sources have become the dominant sources in certain locations.

Sulfur Fertilizers

Sulfur fertilizers consist essentially of sulfur in combination with nitrogen, phosphorus, and potassium. Examples of some of the most common sulfur fertilizers include:

Elemental sulfur - S

Sulfur-bentonite (a product consisting of 90% elemental sulfur and 10% bentonite, which has been manufactured in Canada and the US since 1970)

Ammonia-sulfur solution - NH3 + S

Sulfur dioxide - SO₂

Ammonium thiosulfate - (NH₄)₂S₂O₃

Ammonium bisulfite - NH₄HSO₃

Plant Residues

Animals

Animals consume plants and in turn add various residues (excrement and remains) to the soil that then serve as a source of soil organic matter or humus.

Humans

Humans consume plants and in turn add various residues to the soil that then serve as a source of soil organic matter or humus.

Throughfall

Throughfall is the process which describes how wet leaves shed excess water onto the soil surface. In the water that drips from trees following the interception of rain there are always some sulphates that rain picked up from the atmospheric air.

Transformations

Mineralization

Mineralization is the overall process of conversion of sulfur from an organic to an inorganic form as a result of microbial decomposition. The sulfur ions released by mineralization are readily available to higher plants and to microorganisms.

Immobilization

Immobilization is the process of conversion of sulfur from the inorganic (mineral) to organic form in microbial tissues or in plant tissues, thus rendering sulfur not readily available to other organisms or plants.

Oxidation

During the microbial decomposition of organic C-bonded S compounds, sulfides are formed along with an elemental S and thiosulfates (S₂O₃^2-). These (reduced) substances are subject to oxidation carried out by the autotrophic bacteria Thiobacillus. The oxidation of hydrogen sulfide (H₂S) is an acidifying process, since for every S atom oxidized, two hydrogen ions are formed:

The acidifying effect of sulfide oxidation can bring about extremely acid soil conditions (to pH as low as 1.5).

Reduction

Losses

Volatilization

Volatile sulfur compounds are produced through microbial transformations under both aerobic and anaerobic conditions. Sulfur volatilization is minimal, particularly in soils low in organic matter, where volatilized sulfur represents < 0.05% of the total soil sulfur.

Harvesting

The increasing removal of sulfur through plant harvesting makes it essential that farmers and foresters be attentive to prevent deficiencies of sulfur. In some parts of the world (e.g., certain semi-arid grasslands) sulfur is already the next most limiting nutrient after nitrogen.

Erosion

Erosion is one of the main pathways through which sulfur is lost from the soil.

Leaching

Sulfate compounds are very soluble and the sulfate (SO₄^2-) ion can be readily leached from the soil, especially in humid regions. When sulfate ions leach from the soil they are usually accompanied by equivalent quantities of cations, including Ca and Mg.

Forms of S

Sulfates in Soil Solution

Sulfur is absorbed by plant roots mostly from the soil solution as sulfate ions (SO₄^2-). Readily soluble sulfates in many soils represents between 1 to 10% of the total sulfur content.

Sulfide Soil Minerals

Sulfur is held in several mineral forms in soils, with the sulfate and sulfide minerals being most common. Sulfides are sometimes found in humid region soils with poor drainage. When these soils are drained, oxidation of sulfides occurs, and ample available sulfur is released. However, this oxidation also releases large amount of H⁺ ions and extreme acidity results. Examples of sulfide soil minerals include:

Pyrite and marcasite - FeS₂

Sphalerite - ZnS

Pyrrhotite - Fe₁₁S₁₂

Sulfate Soil Minerals

Sulfur is held in several mineral forms in soils, with the sulfate and sulfide minerals being most common. Sulfate minerals (which are very soluble) are most common in arid and semi-arid regions. Examples of sulfate soil minerals include:

Gypsum - CaSO₄•2H₂0

Anhydrite - CaSO₂

Epsomite - MgSO₄•7H₂O

Mirabilite - Na₂SO₄•10H₂O

Sulfate Adsorbed on Clays

Adsorption of sulfate ions (SO₄^2-) occurs on positively charged soil colloids (Fe and Al coatings of clays, and to a limited extent, 1:1 type silicate clays) that characterize acid soils. Adsorption of sulfate ions increases at lower pH as positive charges (e.g., OH₂⁺) become more prominent on the particle surfaces.

Sulfur in Organic Matter

In the topsoil of temperate humid regions, 90-98% of the sulfur is present in organic forms. The exact forms of the sulfur in organic matter are not known. Some sulfur is carbon bonded (C-S fraction), mostly in amino acids and proteins. These materials are bound with organic and clay colloids, and are thereby protected from microbial decomposition. A somewhat transitory pool of organic sulfur is in the ester sulfate form (C-O-S), such as that found in glucose sulfate.

Over time, soil microorganisms break down these sulfur compounds into soluble inorganic forms (mainly sulfate).

In soils of arid and semi-arid regions, the proportion of organic sulfur is lower than in humid-region soils. However, in arid and semi-arid region soils gypsum, which supplies inorganic sulfur, is often present in the subsurface horizons.