Biogeochemical Cycles: Oxygen, Water, Nitrogen & Carbon Cycle

Biogeochemical Cycles Definition:

The movement of nutrients and essential elements between the living and nonliving components of the biosphere or any other ecosystem is called the biogeochemical cycle or cycle of matter.

The term Biogeochemical Cycle indicates that chemicals circulate through life means bio and through earth means geo, again and again, means cycle.

On the basis of medium of reservoir, biogeochemical cycles can be classified into gaseous cycle, as we will study in the following text and sedimentary cycle, which incudes circulation of iron, calcium, phosphorus, and other more earthbound elements.

Different Types of Biogeochemical Cycles:

The four important biogeochemical cycles which we are going to learn here are the oxygen cycle, the water cycle, the nitrogen cycle, and the carbon cycle.

Let’s learn all the biogeochemical cycles one by one, starting with the Oxygen cycle.

Biogeochemical Cycles: The Oxygen Cycle

Oxygen is a very abundant element on our Earth. It is found in the elemental form in the atmosphere to the extent of 21%. It also occurs extensively in the combined form in the Earth’s crust as well as also in the air in the form of carbon dioxide. In the crust, it is found as the oxides of most metals and silicon, and also as carbonate, sulfate, nitrate and other minerals.

It is also an essential component of most biological molecules like carbohydrates, proteins, nucleic acids and fats (or lipids). But when we talk of the oxygen-cycle, we are mainly referring to the cycle that maintains the levels of oxygen in the atmosphere.

Oxygen from the atmosphere is used up in three processes, namely Combustion, respiration and in the formation of oxides of nitrogen. Oxygen is returned to the atmosphere in only one major process, that is, photosynthesis and this forms the broad outline of the oxygen cycle in nature. Though we usually think of oxygen as being necessary to life in the process of Respiration, it might be of interest to you to learn that some forms of life, especially bacteria, are poisoned by elemental oxygen.In fact, even the process of nitrogen-fixing by bacteria does not take place in the presence of oxygen.

Biogeochemical Cycles: Oxygen Cycle

Biogeochemical Cycle: The Water Cycle

With the Sun’s heat water evaporates from the water bodies and forms clouds. The clouds are blown over the lands, where they are cooled enough to drop the water as rain, snow, hail and sleet. Rain may fall directly into the ocean or on the ground and then flow in rivers or get absorbed by the ground.

The groundwater comes to the surface again by springs and pumps.It is used in agriculture, industry and homes. The water finally evaporates into the atmosphere and the water cycle gets completed.

Nitrogen Cycle: Nitrogen is an essential nutrient for all life forms. It is present in the atmosphere- 78 % of the atmosphere and in molecules essential for life such as Proteins, DNA, RNA and Vitamins and in compounds like Alkaloids and Urea etc

The circulation of nitrogen in the biotic and abiotic components of the biosphere is called Nitrogen Cycle.

The elemental nitrogen of air is converted into simple molecules in the soil and water by the nitrogen-fixing bacteria and the process is called Nitrogen Fixation. These simple molecules later enter into plants and then into animals forming complex molecules such as protein.

Animals excrete nitrogenous waste materials such as urea, uric acid, ammonia which are converted into ammonium compounds by the bacteria and the process is called Ammonification. The ammonia is further converted into nitrites & nitrates.

Moreover, after their death, the proteins and other nitrogen compounds are decomposed by decomposers to form ammonia and nitrates which become available to plants for assimilation.

Also, different types of bacteria convert the nitrates and nitrites into elemental nitrogen and the process is called Denitrification.

The other way of nitrogen fixation, i.e. the conversion of nitrogen molecules to nitrates is the physical process. During lightning, the high temperature and pressure created in the air convert nitrogen into oxides of nitrogen and the process is called atmospheric Nitrogen Fixation. These oxides like Nitrogen Dioxide dissolve in the water to give nitric and nitrous acids and fall on land along with rain, which are then utilized by plants.

The Chemical fertilizer factories produce ammonia by combining hydrogen and atmospheric nitrogen under high pressure and temperature and the process is called Industrial nitrogen fixation. The ammonia is further converted into soluble nitrates which are further converted into nitrates and then to elemental nitrogen. Thus, there is a nitrogen cycle in nature in which nitrogen passes from its elemental form in the atmosphere into simple molecules in the soil and water, which get converted to more complex molecules in living beings and back again to the simple nitrogen molecule in the atmosphere.

Biogeochemical Cycle: Carbon Cycle

Carbon is found in various forms on the earth such as Diamond, Graphite, Carbon dioxide in the atmosphere, Carbonate and Hydrogen Carbonate salts in various minerals and in endoskeletons and exoskeletons of various Animals and in molecules essential for life such as Proteins, Salt, Nucleic Acids, Vitamins

How this Carbon is incorporated into life-forms and how its balance is maintained?

Carbon is incorporated into life forms through the basic process of Photosynthesis.

In photosynthesis, Carbon dioxide from the atmosphere or dissolved in water is converted into glucose molecules by the Plants and it remains in plants.

When Animals consume Plants directly or indirectly the Carbon compounds or the Plants become part of animal tissues. Glucose is then utilized by the living organisms in respiration and is converted back to Carbon dioxide and Carbon dioxide then goes back into the atmosphere. Plants buried under the earth for millions of years, get converted into Fossil Fuels such as Coal. Similarly, Animal Plants buried under the seabed are converted into fossils.


When these fuels are burnt, they add Carbon dioxide in the atmosphere. We know that Carbon also occurs in the rocks as Carbonates such as Limestone and Marble. These Rocks are gradually worn down and their Carbonates are, in time, converted into Carbon dioxide by the acids produced and excreted by soil Microorganisms and Plant roots. Carbon in this way is cycled repeatedly through different forms by the various physical and biological activities from the cycle side by side.

Read more about: Fossil Fuels: Coal and Petroleum, Formation, Uses & Conservation

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