“A biogeochemical cycle or nutrient cycle is a pathway
by which a chemical element or molecule moves through both
biotic (biosphere) and abiotic (lithosphere, atmosphere, and
hydrosphere) compartments of Earth.”
Elements, chemical compounds, and other forms of matter are
passed from one organism to another and from one part of the
biosphere to another through the biogeochemical cycles
All chemical elements occurring in organisms are part of
Biogeochemical cycles are the processes where in different
elements on earth surface are interconverted such that
autotrophs can best utilize them.
The flow of energy takes place from primary producers to
The process involves transfer of energy in terms of food and
also involves chemical conversion.
All of the biogeochemical cycles are mainly involved proper
maintenance of ecosystem with proper energy flow across
primary producers and consumers at tertiary stage.
Carbon is the building block of life. It is the element that anchors
all organic substances, from coal and oil to DNA
(deoxyribonucleic acid, the compound that carries genetic
Carbon is one of the elements that have a gaseous phase as part of
its cycle, occurring in the Earth’s atmosphere as carbon dioxide
(CO2) and methane (CH4), both greenhouse gases.
Carbon enters the atmosphere through the respiration of living
things, through fires and by diffusion from the ocean. It is removed
from the atmosphere by photosynthesis of green plants, algae, and
“The carbon cycle is the
biogeochemical cycle by which carbon is exchanged
among the biosphere, geosphere, hydrosphere, and
atmosphere of the Earth.”
It is one of the most important cycles of the earth and allows for
carbon to be recycled and reused throughout the biosphere and
all of its organisms.
The carbon cycle was initially discovered by Joseph Priestley
and Antoine Lavoisier, and popularized by Humphry Davy
The Carbon Cycle
The major reservoirs
The following major reservoirs of carbon interconnected by
pathways of exchange:
The Terrestrial biosphere
The Sediments including fossil fuels
The Earth's interior (carbon from the Earth's mantle and crust is
released to the atmosphere and hydrosphere by volcanoes and
Carbon in the Atmosphere
Carbon exists in the Earth's atmosphere primarily as the gas
carbon dioxide (CO2). Although it is a small percentage of the
atmosphere (0.03%), it plays a vital role in supporting life.
Other gases containing carbon in the atmosphere are methane
Trees and other green plants convert carbon dioxide into
carbohydrates during photosynthesis, releasing oxygen in the
The phytoplankton, get their carbon from atmospheric carbon
dioxide that has dissolved in ocean water.
Carbon in the Biosphere
About half the dry weight of most living organisms is carbon. It
plays an important role in the structure, biochemistry, and
nutrition of all living cells.
Living biomass holds about 575 gigatonnes of carbon, most of
which is wood. Soils hold upwards of 2700 gigatonnes, mostly in
the form of organic carbon, with perhaps a third of that inorganic
forms of carbon such as calcium carbonate.
Autotrophs are organisms that produce their own organic
compounds using carbon dioxide from the air or water in which
they live. The most important autotrophs for the carbon cycle are
trees in forests on land and phytoplankton in the Earth's oceans.
Carbon in the Hydrosphere
The oceans contain around 36,000 gigatonnes of carbon.
Carbon is readily exchanged between the atmosphere and ocean.
In regions of oceanic upwelling, carbon is released to the
atmosphere. Conversely, regions of down welling transfer carbon
(CO2) from the atmosphere to the ocean. When CO2 enters the
ocean, it participates in a series of reactions which are locally in
In the oceans, dissolved carbonate can combine with dissolved
calcium to precipitate solid calcium carbonate. When these
organisms die, their shells sink and accumulate on the ocean floor.
Carbon is transferred within the biosphere as heterotrophs feed on
other organisms or their parts (e.g., fruits).
Most carbon leaves the biosphere through respiration. When
oxygen is present, aerobic respiration occurs, which releases
carbon dioxide into the surrounding air or water. Otherwise,
anaerobic respiration occurs.
Burning of biomass can also transfer substantial amounts of carbon
to the atmosphere.
Carbon may also be circulated within the biosphere when dead
organic matter (such as peat) becomes incorporated in the
The Importance of the
Carbon is a key element for life, composing almost half of the dry
mass of the earth’s plants (that is, the mass when all water is
The carbon budget is the balance of carbon among the three
reservoirs. The carbon cycle is vitally important to life on earth.
Through photosynthesis and respiration it is the way the earth
produces food and other renewable resources. Through
decomposition, it serves as the earth’s waste disposal system.
In addition, the carbon cycle is important because carbon-
containing gases in the atmosphere affect the earth’s climate.
Increased carbon dioxide (CO2) in the atmosphere has been
responsible for more than half of the climate warming observed in
The carbon cycle is inextricably linked to other chemical cycles,
including those of nitrogen, phosphorus, and sulfur, as well as to
the global hydrological cycle.
Carbon is an element found in all living substances as well as in
many inorganic materials. Both diamond and coal are nearly pure
carbon, but with different structures.
Nitrogen is essential to life because nitrogen is necessary
for proteins including DNA, the carrier of genetic
Free nitrogen makes up approximately 80% of the Earth’s
atmosphere. However, organisms cannot use nitrogen
Some, like animals, require nitrogen in an organic
compound. Others, including plants, algae, and bacteria,
can take up nitrogen either as the nitrate ion (NO3
) or the
ammonium ion (NH4
In contrast to hydrogen, oxygen, and carbon, nitrogen is a
relatively non-reactive element.
The Nitrogen Cycle
“The nitrogen cycle is the process by
which nitrogen is converted between its various chemical
forms. This transformation can be carried out via both
biological and non-biological processes.”
Important processes in the nitrogen cycle include:
1. Nitrogen fixation
“Nitrogen fixation is the natural process,
either biological or abiotic, by which nitrogen (N2) in the
atmosphere is converted into ammonia (NH3).”
This process is essential for life because fixed nitrogen is
required to biosynthesize the basic building blocks of life,
e.g., nucleotides for DNA and RNA and amino acids for
Nitrogen fixation is accomplished by
3.Blue green algae in waterand soil
“Denitrification is a microbially facilitated
process of nitrate reduction that may ultimately produce
molecular nitrogen (N2) through a series of intermediate
gaseous nitrogen oxide products.”
Harmful effects of
Nitric oxide (NO) is less toxic than nitrogen peroxide (NO2
NO attaches to hemoglobin and reduces oxygen transport
efficiency. It’s effect is much less than carbon monoxide due
to lower concentration in the atmosphere.
Acute exposure to nitrogen peroxide can be quite harmful to
human health. For exposures ranges from several minutes
to one hour, a level of 50 – 100 ppm of nitrogen peroxide
causes inflammation of lung tissue for period of 6 – 8 weeks,
after which time the subject normally recovers.
Continue………………… Exposure of the subject to 150 – 200 ppm of nitrogen
peroxide causes bro nchio litis fibro sa o blite rans , a condition
fatal within 3 –5 weeks after exposure. Death generally
results within 2 – 10 days after exposure to 500 ppm or more
of nitrogen peroxide.Exposure of plants to several ppm of
nitrogen peroxide causes leaf spotting and break-down of
Exposure to 10 ppm of nitric oxide causes a reversible
decrease in the rate of photosynthesis.
Nitrogen oxides are known to cause fading of dyes and inks
used in some textiles.
Much of the damage to materials caused by NOx comes
from secondary nitrates and nitric acid.
Control of Nitrogen Oxides
Low excess air firing is effective in reducing NOx emissions
during the combustion of fossils. Incomplete fuel burnout with
the emission of hydrocarbon, soot, and carbon monoxide
(CO) is an obvious problem with low-excess-air firing. This
may be overcome by a two-stage combustion process.
Sulfur is one of the components that make up proteins
Sulfur is important for the functioning of proteins and
enzymes in plants, and in animals that depend upon
plants for sulphur.
Plants absorb sulfur when it is dissolved in water.
Animals consume these plants, so that they take up
enough sulfur to maintain their health.
Approximately 100 million metric tons of sulfur per
year enters the global atmosphere through
anthropogenic activities, primarily as sulfur dioxide
(SO2) from the combustion of coal and residual fuel oil.
The Sulfur Cycle
“The sulfur cycle are the collection of processes by
which sulfur moves to and from minerals (including the
waterways) and living systems.”
Among the significant species involved in the sulfur cycle are gaseous
hydrogen sulfide (H2S); mineral sulfides, such as lead sulfide (PbS),
sulfuric acid (H2SO4), the main constituent of acid rain; and
biologically bound sulfur in sulfur-containing proteins.
The essential steps of the sulfur cycle are:
• Mineralization of organic sulfur
• Oxidation of sulfide and elemental sulfur (S)
• Reduction of sulfate to sulfide
• Microbial immobilization of the sulfur compounds
Effects of atmospheric
Its primary effect is upon the respiratory tract,
producing irritation and increasing airway resistance,
especially to people with respiratory weakness and
sensitized asthmatics. Sulfur dioxide causes death in
humans at 500 ppm.
Atmospheric sulfur dioxide is harmful to plants, some
species of which are affected more than others.
Acute exposure to high levels of the gas kills leaf
tissue, a condition called leaf necrosis. The edges of
the leaves and the areas between the leaf veins show
Chronic exposure of plants to sulfur dioxide causes
chlorosis, a bleaching or yellowing of the normally
green portion of the leaf.
Long-term, low-level exposure to sulfur dioxide can
reduce the yields of grain crops such as wheat or
barely. Sulfur dioxide in the atmosphere is converted
to sulfuric acid, so that in areas with high levels of
sulfur dioxide pollution, plants may be damaged by
sulfuric acid aerosols.
One of the more costly effects of sulfur dioxide
pollution is deterioration of building materials.
Limestone, marble and dolomite are attacked by
atmospheric sulfur dioxide.
Sulfur dioxide Removal
A number of processes are being used to remove
sulfur and sulfur oxides from fuel before combustion
and from stack gas after combustion.
Most of these efforts concentrate on coal, since it is
the major source of sulfur oxides pollution.
Physical separation techniques may be used to remove
discrete particles of pyretic sulfur from coal.
Chemical methods may also be employed for removal
of sulfur from coal.
1) Manahan, Stanley E. 2000 “Environmental
Chemistry”, Lewis Publishers, Boca Raton: CRC
Press LLC, New York. 7th Edition.
2) Miller, G. T, 2004 “Environmental Sciences”,
Thomson Asia Pte. Ltd., Singapore. 10th Edition.
3) Textbook of Environmental Studies by Erach
Bharucha, Universities Press, India, 2005.