Biogeochemical Cycles | Biology for JAMB PDF Download

What is a Biogeochemical Cycle?

Biogeochemical cycles mainly refer to the movement of nutrients and other elements between biotic and abiotic factors.

• The term biogeochemical is derived from “bio” meaning biosphere, “geo” meaning the geological components and “chemical” meaning the elements that move through a cycle.
• The matter on Earth is conserved and present in the form of atoms. Since matter can neither be created nor destroyed, it is recycled in the earth’s system in various forms.
• The earth obtains energy from the sun which is radiated back as heat, rest all other elements are present in a closed system. 
• The major elements include:

1. Carbon
2. Hydrogen
3. Nitrogen
4. Oxygen
5. Phosphorus
6. Sulphur

All the types of material required by ecosystem in addition to energy, are available continuously to system through recycling. Thus there is a constant exchange of materials between the living organisms and their abiotic environment through the recycling of materials. This phenomenon is called Bio-geo chemical cycle.

The mineral elements taken up from the environment (soil as well as air) by the green plant – the producers, are again returned to the environment through consumers and decomposers.

These elements are recycled through the biotic and abiotic components of the ecosystem. The atmosphere, hydrosphere, and lithosphere are the abiotic components of the ecosystem.

Types of Biogeochemical Cycles

The following types of cycle are found in an ecosystem:

(i) Gaseous Cycle – C, H, N, O cycles. Reservoir is in the atmosphere (air) or in Hydrosphere(water).

(ii) Sedimentary cycle – P, S, Ca cycles reservoirs are in the earth's crust (lithosphere).

Note : In these cycles, the bulk material remains in the inactive reservoir on earth's crust like sediment of sea, or water bodies.

1. Carbon Cycle

The main source of carbon is atmosphere and in hydrosphere it is rocks of carbonates. Carbon is present in lithosphere in the form of coal and petroleum. The carbon released from them is present in the atmosphere in the form of carbon dioxide. The green autotrophs utilize CO₂ from the air to synthesize food materials which is obtained by other organisms as food. Carnivores obtain their carbonic food from the herbivores. These carbonic matter produce CO₂ through the oxidation or respiration which dissolve in air or water and is again utilized by the plants.

2. Nitrogen Cycle

Atmosphere is the only source of nitrogen. 78% nitrogen is present in atmospheric air. Plants absorb nitrogen in the form of nitrate ions.

Nitrogen cycle is completed in following steps:

(i) Nitrogen Fixation

In this process first of all some bacteria and blue green algae converts atmospheric nitrogen into nitrogenous compounds viz ammonia, amino acid or nitrate salts.

e.g.

Blue green algae – Anabaena , Aulosira ,Nostoc → In flooded rice field in tropics.

Note :

(1) Azotobacter is aerobic bacteria.

(2) Clostridium is anaerobic bacteria.

(3) Symbiotic relationship where the nitrogen fixing bacteria does not penetrate deep into host tissue, is known as associative symbiosis.

(ii) Nitrification

Conversion of ammonia into nitrate is called nitrification.

Nitrification process is completed in two steps:

Note :

(a) Few quantity of nitrate comes in soil by electrochemical or photochemical process.

(b) Now few quantity of this nitrate is absorbed by plants.

(iii) Nitrogen Assimilation

Plants absorb nitrate from the soil and form protein. When consumers eat these plants, these plant proteins are transferred into consumers.

After the death of plants and consumers, litter is formed in which protein is present.

(iv) Ammonification

Protein present in litter, is converted into ammonia by some ammonifying bacteria.
e.g. Bacillus vulgaris, Bacillus mycoides, Bacillus ramosus.

(v) Denitrification

Some quantity of nitrate which is not used by plants is converted into nitrogen by denitrifying bacteria. e.g. Thiobacillus denitrificans, Pseudomonas denitrificans

Note : Some nitrate washed out from the ecosystem through seepage.

3. Phosphorus Cycle

Phosphorus is the main constituent of protoplasm, plasma membrane, bones and teeth. Main source of phosphorus is rocks. It comes from the weathering of phosphorus containing rock in the soil. Plants absorb this phosphorous from the soil and transfer this phosphate to animals and after the death of animals it is released again into the lithosphere by the action of decomposers.

Sometimes some of the elements like phosphorus and calcium reach into the sea through water, from where they transform into rocks. They separate from the cycle for a long time so it is also known as sedimentary cycle.

But when these rocks break after sometime then this phosphorus is again made available to the sea plant or sea weeds, which pass into fish and sea birds. The excretory materials of birds on the rocks of sea shore is called Guano and it is a source of phosphorus.

Note :- Plants absorb phosphate from the soil in the form of orthophosphate (Po₄^3–).

Difference between Carbon and Phosphorus Cycle:

1. Atmospheric inputs of phosphorus through rainfall are much smaller than carbon.

2. Gaseous exchanges of phosphorus between organism and environment are negligible.

4. Water Cycle

Water is indispensable for life. Sea is the main source of water on earth, water is always present in the atmosphere in the form of water vapour. Water is also evaporated from other places from where it reaches into the atmosphere. These vapours form clouds in the atmosphere, condensation of water vapour forms clouds, mist, fog and ice at high altitudes. It comes again on the earth in above forms.

Water cycle plays a significant role on the earth. Plants and animals receive water or absorb water. Water participates in various metabolic activities of the body of organisms and again it is formed. Water is converted into food through the photosynthesis in plants. Water is also present in protoplasm of all organisms.This water again comes into the atmosphere through transpiration from the plants and evaporation and sweating from the animals. Thus, this cycle goes on continuously. The oxygen cycle and hydrogen cycle is also found in the ecosystem as above cycles.

Note :
1. Water cycle is directly operated by solar radiation and it is under the control of forests.

2. The term "Cycle" is used for the movement of matter and the term "Flow" is used for the movement of energy.

5. Oxygen Cycle

This biogeochemical cycle moves through the atmosphere, the lithosphere and the biosphere. Oxygen is an abundant element on our Earth. It is found in the elemental form in the atmosphere to the extent of 21%.Oxygen is released by the plants during photosynthesis. Humans and other animals inhale the oxygen exhale carbon dioxide which the plants again take up.  They utilise this carbon dioxide in photosynthesis to produce oxygen, and the cycle continues.

6. Sulphur Cycle

This biogeochemical cycle moves through the rocks, water bodies and living systems. Sulphur is released into the atmosphere by the weathering of rocks and is converted into sulphates. These sulphates are taken up by the microorganisms and plants and converted into organic forms. Organic sulphur is consumed by animals through food. When the animals die and decompose, sulphur is returned to the soil, which the plants and microbes again obtain, and the cycle continues.