Definition of biogeochemical cycles
Types of biogeochemical cycles
Major biogeochemical cycles
The pathway in which the chemical elements
circulate in the biosphere through a
characteristic paths from environment to
organisms and back to the environment are
called biogeochemical cycles.These cycles
involves the back and forth movement of
materials between biotic and abiotic factors.
The term "biogeochemical" tells us
that biological, geological and
chemical factors are all involved.
The circulation of chemical
nutrients like carbon, oxygen,
nitrogen, phosphorus, calcium, and
water etc. through the biological
and physical world are known as
biogeochemical cycles. In effect,
the element is recycled, although in
some cycles there may be places
(called reservoirs) where the
element is accumulated or held for
a long period of time (such as an
ocean or lake for water).
The cycle in which the
reservoir is in the atmosphere
or hydrosphere are termed as
The main pool of nutrients
possessing gaseous cycle are
the atmosphere and ocean.
Gaseous cycles circulate
many basic elements of life
such as oxygen , carbon
dioxide , nitrogen and
The cycles in which the
reservoir is in the earth’s
crust are called sedimentary
The main reservoirs of
sedimentary cycles are the
rocks of earth crust.
Sedimentary cycles circulate
elements like sulphur ,
phosphorous and calcium.
The nitrogen cycle is the process by which nitrogen
is converted between its various chemical forms.
This transformation can be carried out through both
biological and physical processes. Important
processes in the nitrogen cycle include fixation,
ammonification, nitrification, and denitrification.
Nitrogen is necessary for all known forms of life on
Earth. It is a component in all amino acids, as it is
incorporated into proteins, and is present in the bases
that make up nucleic acids such as RNA and DNA
Nitrogen gas (N2) is the largest constituent of the
Earth's atmosphere , but this form is relatively nonreactive and
unusable by plants. Chemical processing or natural fixation
(through processes such as bacterial conservation) are
necessary to convert gaseous nitrogen into compounds such
as nitrate or ammonia which can be used by plants.The
abundance or scarcity of this "fixed" nitrogen (also known as
reactive nitrogen) frequently limits plant growth in both
managed and wild environments.The nitrogen cycle, like the
carbon cycle, is an important part of every ecosystem.
The processes of the nitrogen cycle transform
nitrogen from one form to another. Many of
those processes are carried out by microbes,
either in their effort to harvest energy or to
accumulate nitrogen in a form needed for
The connversion of gaseous nitrogen to
ammonia and nitrates is called nitrogen
fixation. It may biological fixation or
Nitrates are the product of non biological fixation during lightning ,
electrical discharges in the clouds or occasionally by cosmic radiations.The
atmospheric nitrogen combines with oxygen9as ozone and produces
nitrogen oxides.These nitrogen oxides get dissolved in rain water and
reached the earth as nitric acid(H2NO3) and react with mineral
compounds to form nitrates and other nitrogenous compounds.
N2 +O2 2NO
2NO + 2O 2NO2
2NO2 + O N2O5
N2O5 + H2O 2HNO3
2HNO3 + Ca(NO3) Ca(NO3)2+CO2+H2O
In this method gaseous nitrogen is split into two
atoms .The free nitrogen then combines with
hydrogen to form ammonia with release of about
13kcal of energy.
It is brought about by following ways
NODULATED LEGUMES AND NON LEGUMINOUS
It is the major step of nitrogen cycle.In this process
proteins in dead plant and animal material are broken
down by bacteria and fungi to amino acids.The amino
acids are oxidized to carbon dioxide, water and
ammonia with a yeild of energy.
CH2NH2COOH + 1/2 O2 2CO2 + H2O + NH3 +
Ammonium or the ammonia
ion is absorbed directly by
plant roots incorporated into
amino acids and passed
through the food chain. Some
of the ammonia is dissolved
in water, part of it is trapped
in the soil, and some is fixed
in clay minerals
It is the process in which ammonia is oxidized into nitrate and nitrite
yeilding energies.It is common in warm,moist soils with a pH of 7.The
oxidation benefits the bacteria as it helps in maintaining there energies for
metabolic activities. .Two groups of bacteria are involved in nitrification.
One group belonging to the genus Nitrosomones use the ammonia in the
soil as their sole source of energy.They oxidize it first to nitrous acid and
water and then to nitrite.
NH3 + H2O2 HNO2 + H2 + 165 Kcal
HNO2 H + NO2
The other group belonging to genus nitrobacter exploits the energy left in
nitrite ion and oxidizes it to nitrate. A small amount of energy is released.
NO + 1/O2 NO3
The process by which the nitrates are reduced to
gaseous nitrogen by certain organisms is called
The denitrifiers are fungi and bacteria
They are anaerobes and prefer an oxygenated
environment but if oxygen is limited they can use
NO3ions instead of oxygen as the hydrogen
acceptor. In doing so they release N2 in the gaseous
state as a byproduct.
As a result of extensive cultivation of legumes
(particularly soy, alfalfa, and clover) and pollution
emitted by vehicles and industrial plants, human
beings have more than doubled the annual
transfer of nitrogen into biologically available
Nitrous oxide (N2O) has risen in the
atmosphere as a result of agricultural
fertilization, biomass burning, cattle
and feedlots, and industrial sources.
N2O has deleterious effects in the
stratosphere, where it breaks down and
acts as a catalyst in the destruction of
atmospheric ozone. Nitrous oxide is
also a greenhouse gas and is currently
the third largest contributor to
global warming, after carbon dioxide
and methane. While not as abundant in
the atmosphere as carbon dioxide, it is,
for an equivalent mass, nearly 300
times more potent in its ability to warm
Ammonia (NH3) in the atmosphere has tripled as the result of
human activities. It is a reactant in the atmosphere, decreasing
air quality and clinging to water droplets, eventually resulting in
nitric acid (HNO3) that produces acid rain. Atmospheric ammonia
and nitric acid also damage respiratory systems.
Ammonia and nitrous compounds contributes to smog and
acid rain, damages plants and increases nitrogen inputs to
Decreases in biodiversity can also result if higher nitrogen
availability increases nitrogen-demanding grasses, causing a
degradation of nitrogen-poor, species diverse heathlands.
The phosphorus cycle is the biogeochemical cycle that
describes the movement of phosphorus through the
lithosphere, hydrosphere, and biosphere. Unlike many
other biogeochemical cycles, the atmosphere does not
play a significant role in the movement of phosphorus,
because phosphorus and phosphorus-based
compounds are usually solids at the typical ranges of
temperature and pressure found on Earth.
Phosphorus is an essential nutrient for plants and
animals. Phosphorus is a limiting nutrient for aquatic
organisms. Phosphorus forms parts of important life-
sustaining molecules that are very common in the
biosphere. Phosphorus does not enter the
atmosphere, remaining mostly on land and in rock and
soil minerals. Eighty percent of the mined phosphorus
is used to make fertilizers.
The primary biological importance of phosphates is as a
component of nucleotides, which serve as energy storage within
cells (ATP) or when linked together, form the nucleic acids DNA
and RNA.The double helix of our DNA is only possible because of
the phosphate ester bridge that binds the helix. Besides making
biomolecules, phosphorus is also found in bone and the enamel of
mammalian teeth, whose strength is derived from calcium
phosphate. It is also found in the exoskeleton of insects, and
phospholipids (found in all biological membranes).It also
functions as a buffering agent in maintaining acid base
homeostasis in the human body.
Phosphates move quickly through plants and animals; however, the processes
that move them through the soil or ocean are very slow, making the
phosphorus cycle overall one of the slowest biogeochemical cycles.
Initially, phosphate weathers from rocks and minerals, the most common
mineral being apatite. Overall small losses occur in terrestrial environments by
leaching and erosion, through the action of rain. In soil, phosphate is absorbed
on iron oxides, aluminum hydroxides, clay surfaces, and organic matter
particles, and becomes incorporated (immobilized or fixed). Plants and fungi
can also be active in making P soluble.
Unlike other cycles, P is not normally found in the air as a gas; it only occurs
under highly reducing conditions as the gas phosphine PH3
Human interference in the phosphorus cycle
occurs by overuse or careless use of phosphorus
fertilizers.This results in increased amounts of
phosphorus as pollutants in bodies of water
resulting in eutrophication. Eutrophication
devastates water ecosystems.