Among the gaseous cycles, the following are very important biogeochemical cycles:

(i) Carbon cycle

(ii) Oxygen cycle

(iii) Nitrogen cycle

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(iv) Phosphorus cycle

(v) Sulphur cycle

(i) Carbon cycle:

The element of carbon is the main constituent of all organic compounds in the biosphere. It is a basic element in the fixation of energy by photo­synthesis. The greatest reserve of carbon is in the ocean, about 90% of earth’s total carbon.

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All this carbon is bound chemically in carbon dioxide, calcium carbonate and other organic compounds. The ocean absorbs car-bon di­oxide through photosynthesis by small phytoplankton.

Carbon is found in plants and animal bodies, and also in coal and petroleum deposits as organic compounds. It is also found as inorganic carbonates in water, rocks and shells. Respiration and decomposition of organic matter add carbon to the atmosphere. However, the input of carbon may be increased in the atmosphere by the activities of man.

(ii) Oxygen cycle:

As we all know, oxygen is one of the most abun­dant elements in the earth’s crust. The oxygen cycle is a complicated one because of the innumerable combina­tions into which this element occurs on the earth.

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However, atmosphere is the principal observe of available oxygen. This element present in the earth’s crust being chemically bound to other ele­ments is not available. The two impor­tant sources of atmospheric oxygen are photosynthesis and photo dissociation.

Oxygen is liberated in the process of photosynthesis, and a major part of it is absorbed through respiration. One recent factor affecting (rather adversely) the oxygen cycle and the oxygen budget of the earth is man and his activities.

It is estimated that nearly 6 billion men in­hale oxygen, exhale carbon dioxide, burn fossil fuels and reduce photo-synthesis by destroying forests and replacing them with human settlements.

(iii) Nitrogen cycle:

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The most characteristic feature of nitrogen cycle is that atmospheric nitrogen cannot be utilized directly by most plants. In fact, the biosphere requires about 1050 million metric tons of nitrogen annually to run the nitrogen cycle.

Actually the nitrogen fixing bacteria which live in the soil and are associated with the roots of certain plants, particularly the legumes, are mainly responsible for making the nitrogen available for use by plants.

Bacteria colonies reside in nodules on the legume roots and chemically combine the nitrogen from the atmosphere in the form of nitrates and ammonia. Plants use these chemically bound forms of nitrogen to produce their own organic matter.

It is noteworthy that circulation of nitrogen and its compounds between organisms and environment goes on. The inorganic nitrogen compounds in soil are taken up by plants and combined with other elements to form nucleic and proteins.

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Such proteins are consumed by herbivores (plant eating animals), part of which is again excreted as urine and returned to soil. Certain bacteria present in the soil decompose urea to form ammonia which is further oxidized into nitrates and taken up by plants.

A large amount of atmospheric nitrogen is converted to proteins by plants. Besides, non-symbiotic agents who contribute nitrogen are blue-green algae, aerobic bacteria and anaerobic bacteria.

(iv) Phosphorus cycle:

Phosphorus is one of the nutrients of considerable importance to biological systems. The main source of phosphorus is the rocks or other deposits formed in the remote past. The fish and sea birds are also important contributors in the phosphorus cycle.

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The manure of sea birds along the west coast of South America is called guano, and it is a rich source of phosphorus. However, Phosphorus is more abundant in the living organisms than in the abiotic component of the eco-system.

Phosphorus is an important constituent of protoplasm and it tends to circulate the organic compounds being bro-ken down finally to phosphates. These are then taken by plants to be passed on to food chain.

The secretions of plants and animals and decomposition of dead organic material bring Phosphorus back to environment. Forest fire as well as grassland fire also contributes a major part of phosphate.

Dead bodies of plants and animals are decomposed by phosphatising bacteria and soluble phosphates are released through this process.

The natural phosphorus cycle is adversely affected by various activities of man.

(v) Sulphur cycle:

Sulphur is a very important element of animal pro­tein and amino acid. Sulphur is avai­lable in various forms such as elemen­tal sulphur, sulphur monoxide, sul­phides, and sulphate etc. It is found in the biotic as well as abiotic compo­nents of the eco-system.

Sulphur enters the atmosphere from such sources as combustion of coal and petroleum, volcanic erup­tion, ocean surface, and gases relea­sed by decomposition. In the atmos­phere sulphur becomes hydrogen sulphide which oxidizes to S02.

This SO2 is soluble in water and because of this quality it is carried back to the earth’s surface by rain water where it is eventually absorbed first by plants and then is taken by primary consumers (herbivores).

Thus, this cycle may be taken as an excellent example of the interaction that exists between various mineral cycles. Sulphur cycle is a good example to indicate linkage between air, water and soil. Like the nitrogen cycle, it also illustrates the important role played by microorganisms.