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Consortium for
Educational
Communication
Module on
Biogeochemical Carbon
Cycle, Form of Carbon, Ploos
and Fluxes
By
AFREEN JAN LOLU
Research Scholar
Department of Botany
University of Kashmir
SRINAGAR-190006
9797860902
Email:[email protected]
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TEXT
Carbon cycle mainly involves the cycling of carbon between the
reservoirs in which carbon is stored (pools) and processes that
transfer it from one pool to another (fluxes). Collectively all the
major pools and fluxes of carbon on earth comprise what we refer
to as the global carbon cycle. The global carbon budget is the
balance of the exchanges (incomes and losses) of carbon between
the carbon reservoirs or between one specific loop (for example
biosphere atmosphere) of carbon cycle. An examination of the
carbon budget of a pool can provide information about whether
the pool is functioning as a source or sink for CO2.
CARBON POOLS AND FORMS OF CARBON
Carbon is found in all the four spheres or reservoirs of the earth
in different forms. There are five global carbon pools of which the
oceanic pool is the largest and the biotic pool is the smallest. The
following major reservoirs/pools of carbon are interconnected by
pathways of exchange:
• Hydrosphere/Oceanic pool: Carbon in this pool exists in the
form of H2CO3, HCO3-, and CO3--.The oceanic carbon pools
comprises the largest carbon pool of about 38140 Gt and
it is increasing at the rate of 2.3 GtCyr-1.It stores carbon
in the form of dissolved inorganic carbon stored at great
depths residing for longer periods of time. A much smaller
amount of carbon is located near the ocean surface which
is exchanged rapidly with the atmosphere through both
physical processes (CO2 dissolution into water) and biological processes(growth, death and decay of plankton).
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• Lithosphere/Geological pool: Species of carbon present
in this pool include CaCO3, Coal, oil and gas.The second
largest amount of carbon on earth is stored in geological
pool in the form of either sedimentary rocks within the
planet’s crust or in the form of fossil fuels. The carbon is
also stored in the earth’s crust as hydrocarbons formed
over millions of years from ancient living organisms under
intense temperature and pressure. These are commonly
known as fossil fuels. The geological carbon pool, comprising fossil fuels, is estimated to be more than 6000Gt,
of which 85% is coal, 5.5% is oil and 3.3% is gas.Presently coal and oil each account for approximately 40% of
global CO2 emissions(Schrag, 2007). Thus, the geological
pool is depleting through fossil fuel combustion at a rate
of 7.0 Gt C yr-1.
• Lithosphere/Pedologic pool: Carbon in this pool exists in
organic and inorganic forms.This is the third largest pool
and isestimated at 3200 Gt to 1 m depth. It consists of two
distinct components: soil organic carbon (SOC) pool estimated at 1550 Gt and soil inorganic carbon (SIC) pool at
950 Gt (Batjes, 1996). The SOC pool includes highly active
humus and relatively inert charcoal carbon.Itcomprises a
mixture of: (i) plant and animal residuesat various stages
of decomposition; (ii) substances synthesized microbiologically and/or chemically fromthe breakdown products;
and (iii) the bodies of livemicro-organisms and small animals and their decomposingproducts (Schnitzer, 1991).
The SIC poolincludes elemental C and carbonate minerals, such ascalcite, dolomite and gypsum, and comprises
primaryand secondary carbonates. The primary carbonatesare derived from the weathering of parent material.
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Incontrast, the secondary carbonates are formedthrough
the reaction of atmospheric CO2 with Ca+2and Mg+2brought
in from outside the localecosystem (e.g. calcareous dust,
irrigation water,fertilizers, manures).
• Atmosphere: In this reservoircarbon is mostly found in
the form of carbon dioxide –CO2 with smaller amounts of
methane and various other compounds.The fourth largest
pool is the atmospheric poolcomprising 780 Gt of CO2–C,
and increasing at therate of 3.5Gt C yr-1 or 0.46% yr-1. It
considerably stores less carbon than the above mentioned
reservoirs. Carbon in the atmosphere is of vital importance
because of its influence on the green house effect and climate. The relatively small size of its carbon pool makes
it more sensitive to disruptions caused by an increase in
sources or sinks of carbon from the earth’s other pools.
This is the reason for its present day higher value than that
which occurred before the onset of fossil fuel combustion
and deforestation.
• Biosphere: It includes organic matter which refers to compounds produced by living things including leaves, wood,
roots,dead plant material and the brown organic matter in
soil.The smallest among the global carbon pools is the biotic pool estimatedat 650 Gt.It stores carbon in the form of
plants, animals, soil and micro organisms of these, plants
and soils are by far the largest. Most of the carbon in biosphere exists in organic forms.
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GFX-II
Fig 1: Principle Carbon Pools and Fluxes between them. The data
on carbon pools among various reservoirs are fromBatjes(1996),
Falkowski et al.(2000) and Pacala and Socolow (2004)and the data
on fluxes are from IPCC (2007).
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Fig 2 (a): Different reservoirs of carbon (Gt C).
Source: IPCC Working Group I Report, 2007.
Fig 2 (b): Annual flux of carbon (GtC per year).
Source: IPCC Working Group I Report, 2007.
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CARBON FLUXES
Carbon flux may be defined as a transfer of carbon from one pool
to another. A single carbon pool can often have several fluxes
both adding and removing carbon simultaneously. The size of
various fluxes can vary widely, for example, the atmosphere has
inflows from decomposition, from combustion and forest fires and
outflows from plant growth and uptake by the oceans. Some of
the important carbon fluxes include:
PHOTOSYNTHESIS
During photosynthesis plants use energy from sunlight
to combine carbon dioxide from the atmosphere with water
from the soil to create carbohydrates and thus help in the
removal of CO2 from the atmosphere and its storage in the
structure of plants. Virtually all of the organic matter on
earth is formed through this process.
Depending on how long a plant survives the carbon gets
sequestered for that much period of time.Organic carbon in
plant tissues can remain sequestered for millions of years if
it is buried in soils or deep ocean sediments, but it returns
to the atmosphere quickly from the materials such as leaf
litter through the process of decomposition.
PLANT RESPIRATION
Plants also release CO2 back to the atmosphere through the
process of respiration. The outputs of respiration are the
inputs of photosynthesis and vice versa. Photosynthesis takes
energy from the sun and stores it in carbon-carbon bonds
of carbohydrate while respiration releases that energy. Plant
respiration represents approximately half of the CO2 that is
returned to the atmosphere in the terrestrial portion of the
carbon cycle.
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CO2 + H2OSUNLIGHT
﴾CH2O﴿ + O2PHOTOSYNTHESIS
﴾CH2O﴿ +O2CO2 + H2O + Energy.
RESPIRATION
Fig 3: Photosynthesis and respiration: two complimentary
processes.
LITTERFALL
Apart from the death of whole plant, a considerable portion
of their leaves, roots, and branches are shed every year. This
dead plant material is often referred to as litter and once on
the ground it starts undergoing the process of decomposition
which results in the release of CO2 as well.
SOIL RESPIRATION
The process of respiration is unique to all organisms including
the microscopic organisms in soil. The dead organic matter in
soil is broken down by the soil micro-organisms (bacteria and
fungi) and the CO2 is released into the atmosphere. Since it
can take years for this plant material to decompose, hence
carbon is temporarily stored in the organic matter of soil.
OCEAN/ATMOSPHERE EXCHANGE
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Inorganic carbon is absorbed and released at the interface of ocean’s
surface and surrounding air through the process of diffusion. This
is the first step in the uptake of carbon by oceans. It involves the
formation of H2CO3 (carbonic acid, the anion of which is called
carbonate) from the reaction of CO2 with H2O. The formation of
carbonate in sea water allows oceans to take up and store a much
larger amount of carbon which is important to a vast number of
marine organisms that use it for building shells.CO2 dissolved in
the ocean will stay a long time if sequestered in deep water, but
will escape more readily back into the atmosphere if ocean mixing
brings it to the surface.Carbon is also cycled through the oceans
by the biological processes of photosynthesis, respiration and
decomposition of aquatic plants.Biological uptake in the oceans
occurs when the phytoplankton in surface waters use CO2 during
photosynthesis to make organic matter. The organic carbon stored
in these organisms is transferred up the food chain, where most
is turned back into CO2. However, some ultimately falls to lower
depths and is stored in deep ocean waters or in ocean sediments.
Fig 4: Dissolution of CO2 at ocean water surface.
FOSSIL FUEL COMBUSTIONand LAND COVER CHANGE
The modern day carbon cycle includes some fluxes that
stem from human activities. The most important of these
is fossil fuel combustion (coal, oil and natural gas). These
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materials contain carbon that was captured by living organisms
over millions of years and has been stored in various places
within the earth’s crust. However, with the onset of industrial
revolution these fuels have been extensively exploited. Since
the main by-product of fossil fuel combustion is CO2, it can be
viewed as a rapid flux to the atmosphere of large amounts of
carbon.
Fig 5(a): Fossil fuel combustion.
Fig 5(b):
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Release of storedcarbon into
the atmosphere by fossil fuel
combustion.
Another human activity that has caused a flux of carbon
to the atmosphere is land cover change which can be seen
largely in the forms of deforestation. Natural forests have
been cleared to meet the requirements of growing human
population including the need of human settlement. Since
forests contain more carbon (in both plant tissues and soil)
than the cover types they have been replaced with, these
changes have resulted in a net flux to the atmosphere.
Fig 6: Land cover changes as a result of extensive deforestation.
GEOLOGICAL PROCESSES
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The geological processes include the formation of sedimentary
rocks and their recycling via plate tectonics, weathering
and volcanic eruption. The rocks on land are broken down
into smaller particles and dissolved materials by a process
known as weathering. These materials are combined with
plant and soil particles resulting from decomposition and
surface erosion and are later carried to the ocean where the
larger particles are deposited near shore. These sediments
accumulate slowly burying older sediments below. The
pressure created by layering and burial of sediment converts
the deeper sediments into rocks. The ocean water mixes with
the dissolved materials and is used by marine life to make
calcium carbonates, skeletons and shells and these sink to
the bottom of the ocean after the death of these organisms.
In shallow waters carbonate collects and eventually forms
another type of sedimentary rock called limestone.
Collectively, these processes slowly convert carbon that
was initially contained in living organisms into sedimentary
rocks within the earth’s crust. These materials continue to
be moved and transformed through the process of plate
tectonics, uplift of rocks in the lighter plates and melting of
rocks in the heavier plates as they are pushed deep under
the surface. These melted materials can eventually result
in the emission of gaseous carbon back to the atmosphere
through volcanic eruptions thereby completing the cycle. The
recycling of carbon through sedimentary rocks is an important
part of our planets long term ability to sustain life. However,
because this cycle works so slowly, these fluxes are small on
an annual basis and have little effect on a human time scale.
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Fig 7: Geological processes resulting in the formation
of sedimentary rocks.