Note on Carbon Cycle, Sulphur Cycle and phosphorus Cycle

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  • Things to remember

Carbon Cycle:

Carbon is the basic unit of all organic compounds. The main source of nearly all carbon found in the living organisms in atmospheric CO2 and is also found in dissolved state in the water on the earth. In this way, the carbon cycle is a concern with the atmospheric CO2 incorporation into organic matter by photosynthesis and its subsequent released by the respiration of all living organisms. Carbon constituent is about forty-nine percent of dry weight of organisms. Carbon is found in three major forms in nature i.e. carbon dioxide, carbon molecule and carbonate in the earth crust. The carbon cycle is small but very active. Carbon cycles rapidly between the atmosphere and hydrosphere and living organisms. The carbon cycle is also important for climate and weather. Atmospheric CO2 dissolve with rain water in the form of acids rich to the soil. Green plants get their carbon by absorbing from the soil. Carnivores feed on plants, omnivorous feeds on carnivores. Here the carbon cycle is cycling from one level to other and when they die the carbon reaches to the atmosphere and in soil in the different process. Carbon is released into the atmosphere directly as carbon dioxide during the respiration of plants and animals when they burned then carbon dioxide is again released into the atmosphere. Bacteria and fungi attack the dead remains of animals and plants. They degrade the complex carbon compounds into simple carbon which are then available for other cycles. Some organic carbon becomes incorporated into the earth crust as coal, gas petroleum and is deposited which release after a long period of time.

Fig: Carbon Cycle


Fig: Carbon Cycle

Phosphorus Cycle:

Animals and plants obtain phosphorus which occurs in the soil as different forms and phosphate. Phosphorus is a component of nucleic acid, ATP, ADP, NADP, etc. Phosphorus occurs in soil in various forms and entry of phosphorus in the green plants occurs both from an organic and inorganic pool.

The dissolved phosphate is absorbed by plants and converted to organic form. From plants, it travels to various trophic levels in the form of organic phosphates. When animals and plants die, the decomposers attack them and liberate phosphorus to the environment. The bones and teeth of animals are composed of a good proportion of phosphorus is lost to the deep sediments. The volcanic apatite process also gives the phosphate which by erosion remains as dissolved phosphate. In this way, both physical and biological process occurs during the phosphorus cycle.

Fig: Phosphorus Cycle

Fig: Phosphorus Cycle

Phosphorus cycle includes mainly two steps:

  1. Conversion of organic phosphates into insoluble inorganic phosphates: Many soil microorganisms produce enzymes (Phosphatases) that decompose different organic phosphorus compounds (lecithins and Nucleoproteins) in the soil. In this decomposition, organic phosphorus is converted into phosphoric acid, which combines with the soil bases to produce salts of calcium, magnesium and iron. These salts are less soluble and thus less available to plants.
  2. Conversion of insoluble inorganic phosphates into soluble inorganic phosphates: Actually solubility of phosphorus is mobilisation by phosphoric acids. The micro-organisms produce acids like sulphuric acids and nitric acid which at least help in mobilising phosphorus.

The action of an acid to convert insoluble phosphates into soluble ones is generally called ‘Solublization'.

Sulphur Cycle:

Sulphur occurs in the soil and rocks as sulphides (ZnS, FeS etc.) and sulphates which are crystalline. In the atmosphere Sulphur occurs in the form of SO2 and H2S, SO2 gas is released to the atmosphere from waterlogged soils, lakes and springs. The organic and inorganic Sulphur and SO2 are formed through oxidation of H2S in an atmosphere. A small amount of Sulphur occurs in a dissolved state in rain water and it reaches the earth surface. Most of the organisms take Sulphur as inorganic sulphates. Some need an organic form of Sulphur. Most of the Sulphur which is biologically incorporated is produced in the soil from the aerobic breakdown of proteins by bacteria and fungi.

2H2S + O2 → 2S + 2H2O

2S + 2H2O + 3O2 → 2H2SO4

Figure: Sulphur Cycle
Figure: Sulphur Cycle

Some bacteria use hydrogen of H2S as the oxygen acceptor in reducing CO2. In the ecosystem, Sulphur is transferred from autotrophs to animal, then to decompose and finally, it returns to the environment through death and decay of dead organic materials as shown in the figure.

Significances of biogeochemical cycles:

The significance of biogeochemical cycle is to show that the nutrient is flowing from one level to the other and as a result ecosystem is functioning. It continues its cycle in different stages. The detailed significant is shown in the cycle that takes place in the natural environment.


E.p., Odum. Fundamentals of Ecology. USA: W.B Saunters Company, n.d.

Jr., Miller G.T. Living in the Environment. Belmont, California, USA: Wadsworth Publishing Company, 2003.

  • Some need an organic form of Sulphur. Most of the sulphur which is biologically incorporated is produced in the soil from the aerobic breakdown of proteins by bacteria and fungi.

         2H2S + O2 → 2S + 2H2O

      an   2S + 2H2O + 3O2 → 2H2SO4

  •  Phosphorus occurs in soil in various forms and entry of phosphorus in the green plants occurs both from an organic and inorganic pool.

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