A Diagrammatic Representation of the Oxygen Cycle

Oxygen cycle, along with the carbon cycle and nitrogen cycle plays an essential role in the existence of
life on the earth. The oxygen cycle is a biological process which helps in maintaining the oxygen level by
moving through three main spheres of the earth which are:

 Atmosphere
 Lithosphere
 Biosphere.
 This biogeochemical cycle explains the movement of oxygen gas within the atmosphere, the ecosystem,
biosphere and the lithosphere. The oxygen cycle is interconnected with the carbon cycle.
The atmosphere is the layer of gases presents above the earth’s surface. The sum of Earth’s ecosystems
makes a biosphere. Lithosphere, is the solid outer section along with the Earth’s crust and it is the largest
reservoir of oxygen.
Stages of the Oxygen Cycle
The steps involved in the oxygen cycle are:
Stage-1: All green plants during the process of photosynthesis, release oxygen back into the atmosphere
as a by-product.
Stage-2: All aerobic organisms use free oxygen for respiration.
Stage-3: Animals exhale Carbon dioxide back into the atmosphere which is again used by the plants
during photosynthesis. Now oxygen is balanced within the atmosphere.

Uses of Oxygen
The  four main processes that use Atmospheric oxygen are:
Breathing – It is the physical process, through which all living organisms, including plants, animals, and
humans inhale oxygen from the outside environment into the cells of an organism and exhale carbon
dioxide back into the atmosphere.
Decomposition: It is one of the natural and most important processes in the oxygen cycle and occurs
when an organism dies. The dead animal or plants decay into the ground, and the organic matter along
with the carbon, oxygen, water and other components are returned into the soil and air. This process is
carried out by the invertebrates, including fungi, bacteria and some insects which are collectively called as
the decomposers. The entire process requires oxygen and releases carbon dioxide.
Read more: Decomposition
Combustion: It is also one of the most important processes which occur when any of the organic
materials, including fossil fuels, plastics and wood, are burned in the presence of oxygen and releases
carbon dioxide into the atmosphere.
Rusting: This process also requires oxygen. It is the formation of oxides which is also called oxidation. In
this process, metals like iron or alloy rust when they are exposed to moisture and oxygen for an extended
period of time and new compounds of oxides are formed by the combination of oxygen with the metal.

Production of Oxygen
Plants: The leading creators of oxygen are plants by the process of photosynthesis. Photosynthesis is a
biological process by which all green plants synthesize their food in the presence of sunlight. During
photosynthesis, plants use sunlight, water, carbon dioxide to create energy and oxygen gas is liberated
as a by-product of this process.
Sunlight: Sunlight also produces oxygen. Some oxygen gas is produced when the sunlight reacts with
water vapour in the atmosphere.
Also Refer: Exchange Of Gases
Some Interesting Facts about Oxygen

 Phytoplankton is one of the most significant producers of oxygen, followed by terrestrial

plants and trees.
 Oxygen is also produced when the sunlight reacts with water vapour present in the atmosphere.
 A large amount of oxygen is stored in the earth’s crust in the form of oxides, which cannot be
used for the respiration process as it is available in the combined state.

Importance of Oxygen Cycle

As we all know, Oxygen is one of the most essential components of the Earth’s atmosphere. It is mainly
required for:

 Breathing
 Combustion
 Supporting aquatic life
 Decomposition of organic waste.
Oxygen is an important element required for life, however, it can be toxic to some anaerobic bacteria
(especially obligate anaerobes).

Carbon Cycle
Carbon cycle is the process where carbon compounds  are interchanged among the biosphere,
geosphere, pedosphere, hydrosphere, and atmosphere of the earth.

Carbon Cycle Steps

Following are the major steps involved in the process of the carbon cycle:

1. Carbon present in the atmosphere is absorbed by plants for photosynthesis.

2. These plants are then consumed by animals, and carbon gets bioaccumulated into their bodies.
 3. These animals and plants eventually die, and upon decomposing, carbon is released back into
the atmosphere.
4. Some of the carbon that is not released back into the atmosphere eventually become fossil fuels.
5. These fossil fuels are then used for man-made activities, which pumps more carbon back into the
atmosphere.

Carbon Cycle diagram showing the flow of carbon, its sources and paths.

Carbon Cycle on Land

Carbon in the atmosphere is present in the form of carbon dioxide. Carbon enters the atmosphere
through natural processes such as respiration and industrial applications such as burning fossil fuels.
The process of photosynthesis involves the absorption of CO2 by plants to produce carbohydrates. The
equation is as follows:
CO2 + H2O + energy → (CH2O)n +O2
Carbon compounds are passed along the food chain from the producers to consumers. The majority of
the carbon exists in the body in the form of carbon dioxide through respiration. The role of decomposers
is to eat the dead organism and return the carbon from their body back into the atmosphere. The equation
for this process is:
(CH2O)n +O2 → CO2 + H2O

Oceanic Carbon Cycle

This is essentially a carbon cycle but in the sea. Ecologically, oceans take in more carbon than it gives
out. Hence, it is called a “carbon sink.” Marine animals convert carbon to calcium carbonate and this
forms the raw building materials require to create hard shells, similar to the ones found in clams and
oysters.
When organisms with calcium carbonate shells die, their body decomposes, leaving behind their hard
shells. These accumulate on the seafloor and are eventually broken down by the waves and compacted
under enormous pressure, forming limestone.
When these limestone rocks are exposed to air, they get weathered and the carbon is released back into
the atmosphere as carbon dioxide.
Importance of Carbon Cycle
Even though carbon dioxide is found in small traces in the atmosphere, it plays a vital role in balancing
the energy and traps the long-wave radiations from the sun. Therefore, it acts like a blanket over the
planet. If the carbon cycle is disturbed it will result in serious consequences such as climatic changes
and global warming.
Carbon is an integral component of every life form on earth. From proteins and lipids to even our DNA.
Furthermore, all known life on earth is based on carbon. Hence, the carbon cycle, along with the nitrogen
cycle and oxygen cycle, plays a vital role in the existence of life on earth.

Nitrogen Cycle
The series of processes by which nitrogen and its compounds are
interconverted in the environment and in living organisms, including
nitrogen fixation and decomposition.

Nitrogen cycle diagram

Nitrogen Cycle Definition

“Nitrogen Cycle is a biogeochemical process which transforms the inert nitrogen present in the
atmosphere to a more usable form for living organisms.”
Furthermore, nitrogen is a key nutrient element for plants. However, the abundant nitrogen in the
atmosphere cannot be used directly by plants or animals. Read on to explore how the Nitrogen cycle
makes usable nitrogen available to plants and other living organisms.

What is Nitrogen Cycle?

Nitrogen Cycle is a biogeochemical process through which nitrogen is converted into many forms,
consecutively passing from the atmosphere to the soil to organism and back into the atmosphere.
It involves several processes such as nitrogen fixation, nitrification, denitrification, decay and putrefaction.
The nitrogen gas exists in both organic and inorganic forms. Organic nitrogen exists in living organisms,
and they get passed through the food chain by the consumption of other living organisms.
Inorganic forms of nitrogen are found in abundance in the atmosphere. This nitrogen is made available to
plants by symbiotic bacteria which can convert the inert nitrogen into a usable form – such as nitrites and
nitrates.
Nitrogen undergoes various types of transformation to maintain a balance in the ecosystem. Furthermore,
this process extends to various biomes, with the marine nitrogen cycle being one of the most complicated
biogeochemical cycles.

Stages of Nitrogen Cycle

Process of Nitrogen Cycle consists of the following steps – Nitrogen fixation, Nitrification, Assimilation,
Ammonification, and Denitrification. These processes take place in several stages and are explained
below:

atmosphere. The fertilisers containing nitrogen

are washed away in lakes and rivers and results in eutrophication Nitrogen fixation
It is the initial step of the nitrogen cycle. Here, Atmospheric nitrogen (N2) which is primarily available in an
inert form, is converted into the usable form -ammonia (NH3).
During the process of Nitrogen fixation, the inert form of nitrogen gas is deposited into soils from the
atmosphere and surface waters, mainly through precipitation. Later, the nitrogen undergoes a set of
changes, in which two nitrogen atoms get separated and combines with hydrogen to form ammonia
(NH4+).
The entire process of Nitrogen fixation is completed by symbiotic bacteria which are known as
Diazotrophs. Azotobacter and Rhizobium also have a major role in this process. These bacteria consist of
a nitrogenase enzyme which has the capability to combine gaseous nitrogen with hydrogen to form
ammonia.
Nitrogen fixation can occur either by the atmospheric fixation- which involves lightening or industrial
fixation by manufacturing ammonia under high temperature and pressure condition. This can also be fixed
through man-made processes, primarily industrial processes that create ammonia and nitrogen-rich
fertilisers.
Types of Nitrogen Fixation

1. Atmospheric fixation: A natural phenomenon where the energy of lightning breaks the nitrogen
into nitrogen oxides and is then used by plants.
2. Industrial nitrogen fixation: Is a man-made alternative that aids in nitrogen fixation by the use
of ammonia. Ammonia is produced by the direct combination of nitrogen and hydrogen, and later,
it is converted into various fertilisers such as urea.
3. Biological nitrogen fixation: We already know that nitrogen is not usable directly from the air for
plants and animals. Bacteria like Rhizobium and blue-green algae transform the unusable form of
nitrogen into other compounds that are more readily usable. These nitrogen compounds get fixed
in the soil by these microbes.

Nitrification
In this process, the ammonia is converted into nitrate by the presence of bacteria in the soil. Nitrites are
formed by the oxidation of Ammonia with the help of Nitrosomonas bacterium species. Later, the
produced nitrites are converted into nitrates by Nitrobacter. This conversion is very important as ammonia
gas is toxic for plants.
The reaction involved in the process of Nitrification is as follows:
2NH4+ + 3O2 → 2NO2– + 4H+ + 2H2O
 2NO2– + O2 → 2NO3–

Assimilation
Primary producers – plants take in the nitrogen compounds from the soil with the help of their roots, which
are available in the form of ammonia, nitrite ions, nitrate ions or ammonium ions and are used in the
formation of the plant and animal proteins. This way, it enters the food web when the primary consumers
eat the plants.

Ammonification
When plants or animal die, the nitrogen present in the organic matter is released back into the soil. The
decomposers, namely bacteria or fungi present in the soil, convert the organic matter back into
ammonium. This process of decomposition produces ammonia, which is further used for other biological
processes.

Denitrification
Denitrification is the process in which the nitrogen compounds makes its way back into the atmosphere
by converting nitrate (NO3-)  into gaseous nitrogen (N). This process of the nitrogen cycle is the final
stage and occurs in the absence of oxygen. Denitrification is carried out by the denitrifying bacterial
species- Clostridium and Pseudomonas, which will process nitrate to gain oxygen and gives out free
nitrogen gas as a byproduct.

Nitrogen Cycle in Marine Ecosystem

The process of the nitrogen cycle occurs in the same manner in the marine ecosystem as in the terrestrial
ecosystem. The only difference is that it is carried out by marine bacteria.
The nitrogen-containing compounds that fall into the ocean as sediments get compressed over long
periods and form sedimentary rock. Due to the geological uplift, these sedimentary rocks move to land.
Initially, it was not known that these nitrogen-containing sedimentary rocks are an essential source of
nitrogen. But, recent researches have proved that the nitrogen from these rocks is released into the plants
due to the weathering of rocks.

Importance of Nitrogen Cycle

Importance of the nitrogen cycle are as follows:

1. Helps plants to synthesise chlorophyll from the nitrogen compounds.

2. Helps in converting inert nitrogen gas into a usable form for the plants through the biochemical
process.
3. In the process of ammonification, the bacteria help in decomposing the animal and plant matter,
which indirectly helps to clean up the environment.
4. Nitrates and nitrites are released into the soil, which helps in enriching the soil with necessary
nutrients required for cultivation.
5. Nitrogen is an integral component of the cell, and it forms many crucial compounds and important
biomolecules.

Nitrogen is also cycled by human activities such as combustion of fuels and the use of nitrogen fertilisers.
These processes, increase the levels of nitrogen-containing compounds.