Biogeochemical Cycles

Biogeochemical cycles refer to the cyclic flow of nutrients between non-living components (soil, rock, air and water) and living organisms, which make the biosphere dynamic, but a stable system.  

A biogeochemical cycle is a pathway by which a chemical substance moves through biotic (living world) and abiotic (lithosphere, atmosphere, and hydrosphere – non-living) components of Earth. Such cyclic flow involves the transfer of matter and energy between the different components of the biosphere.  We cover (i) Water Cycle, (ii) Nitrogen Cycle, (iii) Carbon Cycle and (iv) Oxygen Cycle.


There is a constant exchange of water between air, land, ocean and the living organisms. The whole process by which water evaporates and falls on the land as rain and later flows back into the sea via rivers is called the water or hydrological cycle.

All the water that falls on the land does not immediately flow back into the sea. Some of it seeps into the soil and becomes a part of the underground reservoir of freshwater. Some of this underground water comes up to the surface through springs. Humans bring it to the surface through wells or tube wells. Water is also used by terrestrial animals and plants for various life processes.

Water, during the water cycle, flows through or over rocks containing soluble minerals.   Some of these minerals get dissolved in the water. Thus, rivers carry many nutrients from the land to the sea. These nutrients are then used by marine organisms.

Water Cycle in Nature
Water Cycle
Water Cycle
Water Cycle


It is the cyclic process by which nitrogen passes from its elemental form present in the atmosphere into simple nitrogenous compound present in the soil and water that can enter the living beings and forms complex molecules in them.  These complex molecules are then broken down again to release nitrogen back into the atmosphere.

Our atmosphere contains 78% of nitrogen gas.  It is present in all living organisms as structural components in the form of proteins, amino acids, nucleic acids (DNA and RNA) and some vitamins.  It is also found as a constituent of other compounds, such as alkaloids and urea. Nitrogen is thus, an essential nutrient for all life-forms.

Nitrogen cycle involves the following important steps
(i) Nitrogen-Fixation; The first step of the nitrogen cycle involves nitrogen-fixation. In this process, inert nitrogen molecules are converted to nitrates or nitrites by nitrogen-fixing bacteria.   These bacteria may be free-living or associated with some dicot plants. Most common nitrogen-fixing bacteria are found in the roots of legumes, in their root nodules. Atmospheric nitrogen can also be converted into soluble nitrates and nitrites by physical processes such as lightning. During lightning, high temperatures and pressures created m the air convert nitrogen into oxides of nitrogen. These get dissolved in water to give nitric and nitrous acids. They fall on land along with rain. These · are then utilized by various life forms.

(ii) Nitrogen Assimilation: It is carried out by plants. Plants absorb nitrate and nitrites and convert them to amino acids. They are used to make proteins. Other complex compounds containing nitrogen are formed in other biochemical pathways. Animals can take organic nitrogen from plants directly or indirectly.

(iii) Ammonification:   It is the process of production of ammonia (compound of nitrogen).  It occurs by the decomposition of dead plants and animals

(iv) Nitrification: It is the process of conversion   of ammonia into nitrites and then into nitrates,  by nitrifying bacteria (e.g. Nitrosomonas and Nitrobacter)

(v) Denitrification: It is the process of reducing nitrates or ammonia, present in the soil to molecular nitrogen (N2) that goes back into the atmosphere.

This is done by microorganisms such as Pseudomonas.

Nitrogen Cycle
Nitrogen Cycle
Nitrogen Cycle
Nitrogen Cycle


Carbon is found in various forms on the Earth. It occurs in the elemental form as diamond and graphite.  In the combined state, carbon is found as
(i) Carbon dioxide in the atmosphere.
(ii) Carbonate and hydrogen-carbonate salts in various minerals.
(iii) Fossil fuels like coal, petroleum and natural gas.
(iv) Carbon-containing molecules like proteins, fats, nucleic acids, vitamins and carbohydrates.
(v) Endoskeletons and exoskeletons of various animals (carbonate salts).
It is incorporated in life forms through plants by photosynthesis. Green plants perform photosynthesis by utilizing CO2 and H2O in the presence of sunlight and chlorophyll.

In this process, CO2 is transformed into simple carbohydrate (glucose). O2    is liberated as a byproduct. These glucose molecules provide energy for the synthesis of other biologically important molecules or get converted into other substances. Respiration is the process of obtaining energy from glucose. It releases C02 back into the atmosphere. Oxygen may or may not be used in this process.

The other process that gives out CO2 into the atmosphere are:
(i) Decomposition of dead bodies and organic wastes by decomposers.
(ii) Combustion (burning) of fossil fuels like coal and petroleum on large scale.
(iii) Weathering of rocks and volcanic eruptions.

Note: The percentage  of CO2 in the atmosphere has doubled, since the industrial revolution

The cyclic flow of carbon occurs through different forms like water by various physical and biological activities, as given below

Carbon Cycle
Carbon Cycle
Carbon Cycle
Carbon Cycle


Oxygen is an abundant element forming about 21 % of the atmospheric gases. It is an essential component of most biological molecules like carbohydrates, proteins, nucleic acids and fats.

In Earth’s crust, it is found as oxides of many metals and also as carbonate, sulphate, nitrate and other minerals.  It is present in combined form also as in carbon dioxide and in water.

Oxygen cycle maintains the level of oxygen in the atmosphere. Oxygen from the atmosphere is used up in the processes of combustion, respiration and in the formation of oxides of nitrogen.

Photosynthesis is the only major process, by which oxygen is returned to the atmosphere. Therefore, green plants are the major source of oxygen in the atmosphere.

Note: Some forms of life especially bacteria are poisoned by elemental oxygen.  Even the process of nitrogen-fixation by bacteria does not take place in the presence of oxygen.

Greenhouse Effect and Global Warming
Some gases such as carbon dioxide (CO2), methane  (marsh gas), water vapour, chlorofluorocarbons (CFCs). etc., prevent the escape of heat from the Earth. These gases are present in the atmosphere in right concentration are responsible for the heating of Earth ‘s surface. This is called the greenhouse effect. An increase in the percentage of greenhouse gases leads to global warming (the enhanced greenhouse effect).

Note: Carbon dioxide is one of the greenhouse gases. Increase in the C02 content in the atmosphere would cause more heat to be retained by the atmosphere.  It leads to global warming.

Oxygen Cycle in Nature
Oxygen Cycle in Nature


Ozone is a triatomic molecule of oxygen with the formula O3. It is found in the upper region of the atmosphere, above the region where elemental oxygen (diatomic molecule) is present.  Ozone is poisonous and luckily, it is not stable near Earth’s surface.

Importance of Ozone
Ozone forms a thick layer (like a shield) in the upper region of atmosphere and absorbs harmful ultraviolet (UV) radiations from the Sun.

Thus, preventing these radiations from reaching the surface of the Earth. It protects life forms the damaging effects of UV-rays.

The threat to Ozone Layer
Various man-made compounds like chlorofluorocarbons or CFCs (carbon compounds having both fluorine and chlorine), which are very stable and are not degraded by any biological process, reach the ozone layer. They react with the ozone layer. It results in the reduction or thinning of the ozone layer. It has already resulted in an ozone hole above Antarctica in 1985.

This is the End

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