Sulphur Cycle

  • Sulphur cycle, circulation of sulfur in various forms through nature. Sulphur is key to protein structure and is released to the atmosphere by the burning of fossil fuels. Sulphur occurs in all living matter as a component of certain amino acids. It is abundant in the soil in proteins and, through a series of microbial transformations, ends up as sulphates usable by plants.
  • Sulphur-containing proteins are degraded into their constituent amino acids by the action of a variety of soil organisms. The sulphur of the amino acids is converted to hydrogen sulphide (H2S) by another series of soil microbes. In the presence of oxygen, H2S is converted to sulfur and then to sulphate by sulfur bacteria. Eventually the sulfate becomes H2S.
  • Hydrogen sulphide rapidly oxidizes to gases that dissolve in water to form sulphurous and sulphuric acids. These compounds contribute in large part to the “acid rain” that can kill sensitive aquatic organisms and damage marble monuments and stone buildings.

Thus cycle can be divided as:

· Sulphur Cycle in Soils

  • Sulphur enters the trophic cycle in terrestrial plants via root adsorption in the form of inorganic sulphates (e.g., calcium sulphate, sodium sulphate) or by direct assimilation of amino acids released in the decomposition of dead or excreted organic matter. Bacterial and fungal (Aspergillus and Neurospora) mineralization of the organic sulphhydryl in amino acids followed by oxidation results in sulphate; this adds to the sulphate pool for root adsorption.

· Sulphur Cycle in Atmosphere

  • Sulphur in the atmosphere comes from several different sources: decomposition and/or combustion of organic matter, combustion of fossil fuels, and ocean surfaces and volcanic eruptions. The most prevalent form of sulphur entering the atmosphere is sulphur dioxide (SO2). It, along with other atmospheric forms such as elemental sulphur and hydrogen sulphide, is oxidized to sulphur trioxide (SO3), which combines with water to form sulphuric acid (H2SO4), leading to acid rain.
  • Atmospheric sulphur, largely in the form of sulphuric acid, is removed by two general processes: rainout, which includes all processes within clouds that result in removal; and washout, which is the removal by precipitation below the clouds. Depending on the amount of the various sulphur compounds available to form the sulphuric acid, the degree of acidity can be strong enough to ap-proximate that of battery acid. Atmospheric inputs of sulphuric acid provide the dominant source of both hydrogen ions (H+) for cation replacement.

· Sulphur in Sediments

The sedimentary aspect of the cycle involves the precipitation of sulphur in the presence of such cations as iron (Fe) and calcium (Ca) as highly insoluble ferrous sulphide (FeS) and ferric sulphide (Fe2S3, pyrite) or relatively insoluble calcium sulphate (CaSO4).

The oxidation of sulphides in marine sediments is a key process, though poorly understood.

Human Impact on the Sulpher cycle:

  • The sulphur cycles are increasingly being affected by industrial air pollution. The gaseous oxides of nitrogen and sulphur are toxic to varying degrees. Normally, they are only transitory steps in their respective cycles; in most environments, they are present in very low concentrations.
  • The combustion of fossil fuels, however, has greatly increased the concentrations of these volatile oxides in the air, especially in urban areas and in the vicinity of power plants, to the point where they adversely affect important biotic components and processes of ecosystems. When plants, fish, birds, or microbes are poisoned, humans eventually are also adversely affected.
  • Coal-burning emissions and automobile exhaust are major sources of SO2 and SO4 production and, along with other industrial combustion, a major source of poisonous forms of nitrogen. Sulphur dioxide is damaging photosynthesis, as was discovered in the early 1950s when leafy vegetables, fruit trees, and forests showed signs of stress in the Los Angeles Basin. The destruction of vegetation around copper smelters is largely caused by SO2.
  • Furthermore, both sulphur and nitric oxides interact with water vapour to produce droplets of dilute sulphuric and nitric acid (H2SO4 and H2NO3) that fall on Earth as acid rain, a truly alarming development.
  • Acid rain has the greatest impact on soft-water lakes or streams and already acidic soils that lack pH buffers (such as carbonates, calcium, salts, and other bases). Acid rain damages building materials. Our heritage monuments (such as Taj Mahal at Agra) are threatened by the corrosive action of acid deposition. Acid rain adversely affects terrestrial and aquatic vegetation. Most planktons, mollusks and fry young fish cannot tolerate water having pH below 5.0. Low pH conditions also damage soil microbial community.