Environment of Earth

June 2, 2011

Indian forests as carbon sink

Filed under: Air pollution,Environment,Matter cycling — gargpk @ 7:42 pm
Tags: ,

India’s Forest Cover accounts for 20.6% of the total geographical area of the country as of 2005. In addition, Tree Cover accounts for 2.8% of India’s geographical area.

Over the last two decades, progressive national forestry legislations and policies in India aimed at conservation and sustainable management of forests have reversed deforestation and have transformed India’s forests into a significant net sink of CO2.

Following link is a report of Ministry Of Environment and Forests, Government of India summarizing impact of various programmes and policies in this direction.


September 23, 2009


Filed under: Matter cycling — gargpk @ 5:50 pm

In the ecosystem, autotrophic organisms (chiefly the green plants) use the energy of solar radiation to produce organic matter, which is used by all the living organisms, including autotrophic organisms themselves, in running their life activities. The organic matter is used by the living organisms through their respiratory activity. Out the total organic matter produced by photosynthetic activity of autotrophic organisms, a certain portion is consumed by these organisms themselves and the remaining organic matter is available in the ecosystem as the net organic matter production of autotrophic organisms. A relatively very small part of this net organic matter production in the ecosystem is directly transformed into mineral substances. This takes place without the participation of any other living organisms through the processes such as forest and prairie fires during which organic matter is transformed into carbon dioxide, water vapor and certain mineral compounds. Further, a still smaller portion of organic matter is deposited in the upper layers of lithosphere and at the bottom of water bodies in the form of coal, peat and other organic compounds. The remaining organic matter is now passed on to heterotrophic organisms in the ecosystem through various food chains. All the living organisms of a particular type in the ecosystem that receive organic matter as food in a particular manner constitute a trophic level. The organic matter received by a trophic level undergoes three fates:

  1. A portion is consumed by that that trophic level itself though respiration in that trophic level

  2. A certain other portion is passed on to next higher trophic level as organic food and

  3. Remaining organic matter is stored in the trophic level as increase in the biomass of that trophic level (i.e. increase in the number of organisms of that trophic level).

From the point of view of ecosystem energetics, the organic matter that is received, passed on to next higher trophic level or stored by a trophic level represents the amount of energy received, passed on or stored by that trophic level. It is obvious that in a dynamically stable ecosystem, there can not be any storage of energy (i.e. organic matter) in any of its trophic levels. Therefore, in the dynamically stable global ecosystem, a very small portion of the net production of organic matter by autotrophs is stored in the abiotic components of the environment (i.e. lithosphere and hydrosphere) while major portion is consumed by heterotrophic organisms through their respiration.

The consumption of organic matter in a trophic level (including autotrophic organisms themselves) through the respiration in that level represents the loss of energy in that trophic level. It is a feature of global ecosystem that the flow of energy (represented by flow of organic matter as food) between trophic levels is associated with large losses of energy at each trophic level. The ratio of the amount of energy passed on from a trophic level to its next higher trophic level (n) and the amount of energy received by that trophic level from its previous trophic level (n1) is termed ecological efficiency () of that trophic level i.e.

Ecological efficiency () = n/n-1

The ecological efficiency of trophic levels, in general, is estimated to range between 10-20%. Such small general value of ecological efficiency indicates that biomass in each successively higher trophic level in the ecosystem is bound to be substantially reduced. Since ecological efficiency of a trophic level depends on the respiration of that level, smaller the value of ecological efficiency of a trophic level, greater is the consumption of organic matter through respiration (i.e. loss of energy) at that trophic level. As a result, there is greater reduction of biomass in that trophic level and in the next higher trophic level.

Nature of organisms and transformation of organic matter

Since intensity of metabolism per unit mass of a live organism usually increases with decrease in the size of organism, the biomass present at a specific trophic level in the food chain depends on the size of organisms of that trophic level. One of the causes of this relationship is that the metabolism depends substantially on the ratio of the rate of diffusion of gases through the surface and the mass of organism. This ratio increases as the size of organism decreases. Thus the rate of metabolism of a given unit weight of microorganisms is many times greater than that of macro-organisms. Further, metabolism also depends on the nature of physiological processes within the tissues of organisms. In wood of plants, the metabolism is usually much slower than in vertebrate tissue of similar size. These general principals largely determine the total biomass of various types of organisms in the global ecosystem.

The largest proportion of forests in the overall biomass of living organisms is due to the fact that autotrophic trees are located at the first link in the food chains and also due to the large size of individual trees. Together with specific properties of the wood, this feature substantially reduces the rate of metabolism per unit biomass in forests. Though the productivity of ocean phytoplankton is comparable with forests, small size of individual plankton organisms intensifies their metabolism per unit weight so much that the total mass of plankton on Earth is negligible in comparison with that of forests.

About 95% of the total biomass on Earth belongs to plants and rest to the animals. Biomass of aquatic organisms is substantially less than that of terrestrial organisms. Therefore, the distribution global biomass is largely determined by the distribution of terrestrial plant cover i.e. by the forest cover on continents. Considering that total biomass on Earth (global biomass) is approximately 3×1012tonnes and total productivity of plants on continents is approximately 140×109tonnes, the time period of one cycle of organic matter for the plants on Earth comes to be approximately 20 years. This average figure relates to forests that constitute major portion of the biomass of plants on Earth. In other natural zones on continents, the duration of one cycle of plant organic matter is much shorter. The duration of this cycle in the oceans having phytoplankton is still shorter and appears to be only a few days.

The total biomass of animals is assumed to be approximately 1011 tonnes. Assuming that the animals assimilate about 10% of the total productivity of plants, the average duration of one cycle of animal organic matter comes to be several years. However, the actual length of life of one generation varies widely in animal kingdom and the nature of the distribution of biomass among different animal groups is still not much clear.

Invertebrates are the largest components of animal biomass and among them, most important are organisms living in soil. The zoological mass of large animals per unit area on Earth is relatively quite low. Calculations of Huxley (1962) show that while in African savannahs, the biomass of large wild animals may be 15-25 tonnes/km2, this figure is only about 1.0 ton/km2 in middle latitudes, 0.8 ton/km2 in tundra and 0.35 ton/km2 in semi-desert areas.

Man occupies topmost position in the food chain on the Earth and consumes both the primary production of autotrophic plants and the biomass produced by many herbivorous and carnivorous animals. For the present size of human population of over 4.0 billion, its biomass is approximately 0.2×109 tonnes. Assuming that each human being expends on average about 2.5×103 kcal of energy per day, the total energy consumption of human population comes to be about 1.8×1015 kcal/year. Thus, the human population consumes about 0.2% of the total production of Earth’s organic world.


Filed under: Matter cycling — gargpk @ 4:59 pm

Most of the sulfur on Earth is stored in oceans (about 1.3 x 106 Pg), sedimentary rocks (about 2.7 x 106 Pg) and evaporites (about 5 x 106 Pg). Very small percentage reaches the surface and is exchanged with atmosphere. Accuracy of the natural emissions of sulfur is about 50% only.

Sulfur in lithosphere

Sulfur is 13th most abundant element in Earth’s crust (0.1%) and 9th most abundant in sediments. Sulfur content of rocks varies considerably e.g. sedimentary rocks have about 0.38% while igneous rocks have only 0.032%. Sulfur in lithosphere is mobilized by slow weathering of rock material. Dissolved in runoff, it moves with river-water and is deposited in continental shield sediments in oceans. Eventually on geological time-scale, this uplifts to surface again thus completing the geological part of the sulfur cycle.

Sulfur in hydrosphere

Main storage of sulfur in oceans is through dissolved sulfate, averaging about 2.7 g per kg. Most volatile sulfur compound in sea water is dimethyl sulfide (DMS; (CH3)2S) which is produced by algal and bacterial decay. Its concentration in sea water is about 100 x 10-9 L-1, highest concentrations being in coastal marshes and wetlands.

Sulfur is second most abundant compound in rivers with concentrations fluctuating highly with seasons and frequency of drought, flood and normal flow. Rivers transport about 100 Tg of sulfur per year to the oceans. The storage of main sulfur mass in oceans, sedimentary and evaporite rocks establishes the base for sulfur cycle.

Sulfur in soil and biosphere

Sulfur is major essential nutrient in the biosphere and is concentrated mainly in soil from where it enters biosphere through plant uptake. From soil, sulfur is also removed in solution to groundwater and by chemical volatilization. Its main sources are deposition from atmosphere, weathering of rocks, release from decay of organic matter and anthropogenic fertilizer, pesticides and irrigation water. In soil, it is present mainly in oxidized state (e.g. SO4) with concentrations varying according to the amount of organic matter in soil. Rich organic soils may have upto 0.5% sulfur by dry weight.

Sulfur in soil may be in bound or unbound form, as organic or inorganic compounds, organic sulfur being most prevalent. Plants take up sulfur from the soil mainly as sulfate and it is passed on with the food chain in the biosphere. It leaves biosphere on death of living organisms when aerobic decay and decomposition brings back sulfate in the soil. Finally, anaerobic decomposition in soil releases part of organic sulfur as H2S, DMS and other organic compounds into the atmosphere. About 7 Tg of sulfur per year is released from global soils, with considerable latitudinal variation. The release of sulfur is dependent upon warmer temperatures.

Sulfur in atmosphere

Several sulfur compounds are released into the atmosphere due to interaction of processes between Earth’s surface and the atmosphere. Of these, most important six compounds are discussed below.

1. Carbonyl sulfide (COS): It is the most abundant sulfur species in atmosphere and in nature is mainly produced by decomposition processes in soil, marshes and wetlands along ocean coasts and areas of ocean upwelling that are rich in nutrients. Anthropogenic combustion processes produce less than 25% of COS. Its average concentration of about 500 pptv shows enough uniformity throughout latitudes and altitudes to suggest a long lifetime and no rapid sinks of this compound. A lifetime of 44 years is suggested with only sink being stratospheric photolysis and slow photochemical reactions in troposphere. Ocean may act both as source and sink. About 80% of total atmospheric sulfur is COS, but it is relatively inert and does not add much to atmospheric sulfur pollution problem.

2. Carbon disulfide (CS2 ):It is far more reactive than COS and has similar sources though on a smaller scale. It has lifetime of 12 days only and its major sink is photochemical reactions. As a result, CS2 shows greater spatial variation across the globe, ranging from 15 pptv in clean air to 190 pptv in polluted air. Its concentration decreases rapidly with altitude. The most important source of the compound is microbial processes in warm tropical soils. Major secondary sources are marshes and wetlands along sea coasts. Small anthropogenic inputs are from fossil fuel combustion.

3. Dimethyl sulfide (DMS): It is released from oceans in much greater amounts than COS or CS2 and has extremely small lifetime and is very rapidly oxidized to sulfur dioxide or is redeposited to oceans. In the sulfur cycle, most of natural gas released from oceans is DMS. Its concentrations are high during night, particularly in areas under some influence from continental sources.

4. Hydrogen sulfide (H2 S): It is mainly produced in nature during anaerobic decay in soils, wetlands, salt marshes and other areas of stagnant water with maximum concentrations occurring over tropical forests. This highly reactive is removed by reaction with hydroxyl radical (OH) and COS. Its highest concentrations occur at night and in early morning when photochemical activity is at a minimum.

4. Sulfur dioxide (SO2 ): Its natural source is oxidation of H2S and major anthropogenic source is combustion of fossil fuels. Its atmospheric concentrations are most influenced by anthropogenic emissions. In some industrialized areas such as eastern North America, over 90% of SO2 is from anthropogenic sources. Normally about half of global SO2 originates from natural sources. The lifetime of the gas is 2-4 days indicating that loss due to photochemical conversion to sulfate is quite important. Rest of the gas (about 45%) is removed from atmosphere by wet and dry deposition.

5. Sulfate aerosol: Sulfate aerosol particles originate from sea spray that is the largest natural source of sulfur to the atmosphere. Only 3 TG per year of sulfate is added to atmosphere from anthropogenic sources directly but much greater amounts are formed through secondary reactions from various sulfur species in atmosphere. Most of the salt spray sulfate falls back to oceans but some is carried over the continents to be included in deposition processes there.

Table-1. Indicative characteristics of major tropospheric sulfur compounds.


Major sources



(Tg Y-1)

Background concentration







coastal marshes, biomass burning


500 pptv




slow photoche-mistry, stratosphere, oceans





15-30 pptv





Photoche-mical production of SO2



algal deposition


<10 pptv





Oceans, oxidation to SO2


Bacterial reduction, soils,



30-100 pptv








sources, volcanoes, oxidation

of H2S


24-90 pptv

>5 ppbv



Wet & dry deposition


Sea-sprays, oxidation

of SO2


0.1 g m-3

>2.5 g m-3



Wet & dry deposition


Filed under: Matter cycling — gargpk @ 4:51 pm

Nitrogen is primarily exchanged between atmosphere, biosphere and soil. Following Table-1 shows the estimated total nitrogen stored in the atmosphere and surface locations on a global scale.

Nitrogen in hydrosphere

In comparison to biosphere or atmosphere, very little nitrogen is present in oceans and continental surface waters. Over 95% of nitrogen stored in oceans is present in inactive molecular form. Only nitrate (about 2.5% of total oceanic nitrogen) and organic matter (about 1.5% of total oceanic nitrogen) have some active role. Oceanic nitrogen comes through river runoff from continents and wet and dry deposition from atmosphere. Its loss occurs through deposition to sediments in the bottom of oceans and through release to atmosphere in areas of biological activity. Nitrogen content in ocean water can vary spatially; for example, ammonia in surface oceanic waters varies between 0.05 to 2.0 mmol m-3 with smallest concentrations in the open oceans where biological activity is lowest. The amount of nitrogen released from oceans to the atmosphere (about 0.5 Tg Y-1) is quite low in comparison to that from other sources.

Table-1. Nitrogen storage in various components of global environment.


Nitrogen storage

in Tg (1012 g)


2 to 6 x 106


85 x 103



10 x 103


3.8 x 103

Surface litter

1.5 x 103

marine biomass




Human beings


Nitrogen in rocks

The amount of nitrogen stored in lithosphere is much greater than the amounts stored in all other locations combined together. In lithosphere, most of the nitrogen is stored in primary igneous rocks and thus is not available to ecosystem. Weathering and other natural processes release only a very small fraction (<<1%) of this stored nitrogen into global ecosystem.

Nitrogen in soil and biosphere

Major active zone of nitrogen use and transfer occurs in the soil and biosphere on continents with very minor activity occurring in aquatic ecosystems. Inactive N2 of atmosphere is converted to form available to ecosystem through the process of nitrogen fixation, which mainly involves bacterial activities (though some nitrogen fixation also occurs during atmospheric lightening). Fixed nitrogen is made available first to plants in the ecosystem through mineralization to ammonia or through oxidation of reduced ammonia to nitrate (NO3). This process termed nitrification occurs under aerobic conditions. The oxidized nitrogen in soil is returned to atmosphere through the process termed denitrification under anaerobic conditions.

Nitrogen content of soil determines the nitrogen availability to biosphere and various soil types differ in their nitrogen content. Most of the soils contain about 0.05% to 0.2% nitrogen by weight though richest organic soils may contain upto 0.5% of total mass as nitrogen. During rains, some of the soil nitrogen is leached by runoff or infiltration and reaches groundwater or river water to be transported elsewhere.

Nitrogen entering the plants mainly as nitrate or ammonium is assimilated there into a variety of organic nitrogenous compounds, mainly the proteins and amino acids which are passed on from plants to animals as food. Nitrogen then traverses to different trophic levels in the ecosystem as different animals eat each other. Finally, nitrogen is returned back to soil or atmosphere from the biosphere after death and decay of plants and animals. In the ecosystem, aerobic processes form NO2 also while anaerobic processes produce NO, N2O and N2. Most of these products is released to atmosphere.

All the processes and pathways involved with nitrogen cycle depend on the environmental conditions such as soil pH, water content, soil type etc. Temperature is crucial factor in nitrogen cycle because biological activity is highly sensitive to temperature.

Though nitrogen fixation is the natural source of biospheric nitrogen, nitrogen fertilizers added to soil and surface deposition of nitrogenous materials that are emitted into atmosphere by human activities have also become important inputs to biospheric nitrogen.

Nitrogen in atmosphere

Nitrogenous species important in global nitrogen cycle found in atmosphere are:

1. Molecular nitrogen: The N2 gas constitutes about 79% of air by volume and it provides the main source of nitrogen to biosphere through nitrogen fixation as discussed above.

2. Ammonia and ammonium: Ammonia is very important component of nitrogen cycle as it is the only water-soluble gaseous nitrogen species. It can directly act as plant nutrient being converted to ammonium (NH4+) which forms the atmospheric nitrogen aerosol component. About 54 Tg nitrogen is emitted to atmosphere per year and ammonia released from animal urea makes up about half of this. Nitrogen inputs through biomass burning depend on the nitrogen content of the biomass which differs in different ecosystems. Average nitrogen content of tropical forest wood is 0.45%, of tropical litter is 0.85%, of coniferous and deciduous forest wood is 0.32%, of fuel wood is 0.2% and of tropical grasses is 0.2% to 0.6%. Other minor sources include coal combustion, human excreta and fertilizers.

It is difficult to establish the global representative concentrations of ammonia and ammonium. Ammonia concentration is lowest over remote oceans (about 0.1 ppbv); while in continental background air it is 6-10 ppbv. The ammonia concentrations are higher in summers than in winters and during daytime than in night due to higher temperatures influencing the activities of soil-based microbial sources. The lifetime of ammonia is only about 6 days and so it is rapidly converted to ammonium, which is the major component of two most prevalent atmospheric aerosols, ammonium sulfate and ammonium nitrate. Concentrations of both these aerosols and the gas decrease exponentially with altitude. Major sink of these aerosols is wet and dry deposition that removes about 49 Tg of nitrogen per year from atmosphere.

3. Nitrous oxides: Apart from N2, nitrous oxide (N2O) is the other inert gas in the atmosphere. Its lifetime is about 179 years and its major sink is photochemical reactions in stratosphere. It is also a greenhouse gas. Major sources of N2O emission are soil and oceans through microbial processes. Highest concentrations of the gas over oceans occur in areas where strong upwelling brings deep-water nutrients to the surface waters. Emissions due to human activities are adding about 8% of the natural input. N2O emissions increase with higher temperature and moisture and, therefore, reach a daily maximum around noon and seasonal maximum in summers. Emissions can be greatly increased on a local scale by irrigation practices. The gas shows very little variation in global distribution due to its long lifetime and major natural sources. Depending on the photochemical activity, the concentration of gas decreases slightly with altitude in the troposphere.

4. Nitrogen oxide species: NO and NO2 are major part of a series of highly active primary and secondary compounds (including HCN and N2O5). Primary emission occurs mainly of NO which is rapidly converted to NO2, which thus becomes dominant in the atmosphere. Both these are quite short-lived species and are rapidly oxidized to nitrate aerosol or sulfuric acid. Both the gases are crucial in tropospheric and stratospheric ozone chemistry and in the chemistry of photochemical smog.

NO and NO2 are strongly influenced by anthropogenic emissions. Over 60% of nitrogen oxides come from combustion of fossil fuels and biomass. The amount of gases released from fossil-fuel combustion depends on the temperature of combustion process and nitrogen content of the fuel. Nitrogen content of coal is 1-2%, of crude oil is <1% and of natural gas is 5-10%. Concentrations of nitrogen oxides show high spatial variability during their short lifetime indicating that local and regional sources are highly important to their global budget. Natural sources of these oxides are soil and thermal dissociation of atmospheric N2 during lightening. Global emission of nitrogen oxides is about 50 Tg Y-1, which forms about 33% of total nitrogen, input into the atmosphere. About 43 Tg nitrogen is removed from atmosphere per year. This removal involves almost entirely the wet and dry deposition with a very small quantity lost to photochemical reactions. Concentrations of nitrogen oxides in clean ocean air in the troposphere are <100 pptv. Concentrations in rural air over the continents are 200-300 ppbv and in air influenced by human activities may be >10 ppbv reaching upto 500 ppbv in urban air. Highest concentrations are found in Northern Hemisphere around 400 N latitude where major anthropogenic sources of these oxides are located. Concentrations rapidly decrease with altitude to a background value of 10 pptv in the upper troposphere. Higher concentrations occur in winters, particularly in the mid-latitude areas under urban influence since temperature inversions are more prevalent and photochemical activity is at a minimum.

Table-2. Indicative characteristics of major atmospheric nitrogen compounds.


Major sources



(Tg Y-1)

Background concentration







soils, biomass burning


0.1 ppbv

>6.0 ppbv

6 days

Conversion to NH4


Conversion from NH3


0.05 g m-3

>1.5 g m-3

5 days

Wet & dry deposition


Secondarily from NOx


0.5 g m-3

>10.0 g m-3

5 days

Wet & dry deposition




310 ppbv

170 days

Strato-spheric photo-chemistry


Fossil fuels, lightening, biomass burning, intercons-versions


<100 pptv

100 pptv

<2 days

Oxidation to HNO3 & NO3, photo-dissociation


Filed under: Matter cycling — gargpk @ 4:45 pm

The carbon cycle is mainly associated with living matter, although inorganic carbon provides important segments to complete the cycle. The cycling of carbon is strongly controlled by its storage in natural reservoirs. The time period of such storage may range from millennia in rocks, through decades in deep ocean layers to seasons in active biota. Relevant time periods of such storage suggested by Warneck (1988) are:

1. Geological activity involving rocks: 2,400 to 30,000 years

2. Soil humus: 200 years

3. Long-term biosphere storage: 75 years

4. Short-term biosphere storage: 15 years

5. Ocean mixed layers: 4 to 10 years

Estimates of mass content of carbon in various global reservoirs are given in the Table-1.

Carbon in oceans

Major storage of carbon in oceans occurs in the intermediate and deep water below the thermocline. The deep layers of oceans have a very slow mixing period and carbon remains in situ for atleast 20 years in these layers. Far above in oceans, in the mixed layer, which provides the main medium of interchange with the atmosphere, carbon storage is about 1.5 orders of magnitude lower. Ninety percent of the carbon in the oceans is stored as bicarbonate (CO32-) and about 9% as carbonate (CO3). About 3% of carbon is present in organic matter in environment.

The mixing layer in oceans, broadly the layer above the thermocline, is assumed to be at depth of 75 meters. The average concentration of carbon dioxide in the oceanic surface layer (above the mixing layer) is 2.05 mmol m-3. This concentration rises rapidly with depth to about 2.29 mmol m-3 at the depth of about one-kilometer and remains fairly constant thereafter. The average oceanic carbon dioxide concentration is calculated to be about 2.25 mmol m-3. Since colder ocean water is able to hold more carbon dioxide, variations in its concentration occur with temperature of ocean water. The mass of carbon dioxide in the mixed layer is about the same as that in the atmosphere, with a total exchange between the two estimated to occur over a period of about seven years.

Table-1. Mass content of carbon in global reservoirs



in Pg (1015 g)


1. Total dissolved CO2


2. Dissolved CO2 in mixed layer (75 m depth)


3. Living biomass carbon


4. Dissolved organic carbon



1. Continental and shelf carbonates

270 x 105

2. Carbonates in oceans

230 x 105

3. Continental & shelf organic carbon

100 x 105

4. Organic carbon in oceans

200 x 104


1. Terrestrial biomass


2. Soil organic


3. Oceanic organic


ATMOSPHERE (mostly as CO2)

1. Pre-industrial estimate (290 ppmv)


2. present estimate (350 ppmv)


Organic carbon in oceans comes from precipitated remains of living organisms. About 80% of the precipitated material may be redissolved in the deep ocean layers. Dissolved organic carbon content of ocean waters is roughly estimated to be about 0.7 g m-3. Rest of the carbon in the ocean is particulate, mainly as calcium carbonate and this portion of oceanic carbon has a concentration of about 20 mg m-3. Living organisms contribute a total of only 3 Pg to the oceanic carbon storage.

Carbon in sediments and rocks

Carbon makes up only 0.032% of the Earth’s crust by mass. In terrestrial rocks, it is dissolved by rains or surface water over long periods of time and is carried by the surface runoff water to be deposited on the continental shelf sediments. In deeper oceans, deposits from organisms are built up on the ocean floor over millennia. Exchange of carbon from these locations occurs over thousands of years and is associated with activity of Earth’s crust. About two third of this carbon is inorganic carbon and rest is organic in form. Only about 1% of carbon in the form of oil and coal present in Earth’s crust can be used economically. It is estimated that if all the carbon stored in sediments is released suddenly, the atmospheric pressure will rise by 38 bars and the Earth’s atmosphere will become similar to that of planet Venus.

Carbon in biosphere

In the biosphere carbon is exchanged through:

  1. Photosynthetic activity of photosynthetic living organisms, mainly the green plants

  2. Release of carbon on decay and decomposition of dead living organisms

  3. Respiratory activity of all the aerobic living organisms including both plants and animals

  4. Release of carbon from soil humus

The mass of carbon is about three times higher than in living biosphere. The biospheric exchange processes are relatively inactive and the carbon storage may occur for 200 years. Long-lived species, particularly the plants store about 75% of the carbon present in the living biota. The major impacts on global carbon content present in the active biosphere occur in the forests, which store over 80% of the world’s biomass. Though estimates are uncertain because global distribution of different ecosystems is not known accurately, it is quite clear that tropical rain-forests, boreal forests and temperate forests are the most important ecosystems regarding storage and exchange of carbon.

Carbon in atmosphere

Exchange of carbon with the atmosphere occurs mainly through the biosphere with oceanic mixed layer being an important secondary source. Most important atmospheric form of carbon is CO2 gas and global estimates of its exchange between atmosphere and biosphere are:

1. Assimilation of CO2 into plants: 113 Pg Y-1

2. Re-release into atmosphere from:

  1. Respiration of living organisms: 55 Pg Y-1

  2. Microbial decay: 42 Pg Y-1

  3. Soil humus: 10 Pg Y-1

  4. Forest fires and agricultural burning: 1 Pg Y-1

3. Herbivore consumption: 5 Pg Y-1

In addition to CO2, other minor gases in the carbon chain are carbon monoxide (CO), methane (CH4) and non-methane hydrocarbons (NMHCs e.g. HCHO). Carbon dioxide gas is relatively inert while others are quite active in global atmospheric chemistry. Important features of atmospheric carbon species are discussed below.

1. Carbon dioxide: Though CO2 is a minor gas in the atmosphere in comparison with oxygen and nitrogen, it has major impact on global heat balance because of its high capacity of absorbing infra-red radiation. Continuously rising concentration of atmospheric CO2 due to various human activities, particularly the fossil-fuel burning, is major factor in global greenhouse warming. Anthropogenic carbon contributes about 3% of annual carbon loading. Further, its importance in relation to biosphere is supreme since it is required for photosynthesis and existence of biosphere depends on photosynthesis.

2. Carbon monoxide: About 90% of CO originates during photochemical production of methane in atmosphere. Some CO is produced during biomass burning and some during atmospheric oxidation of organic gases that are emitted from vegetation. Highest concentrations of CO are found in middle and high latitudes of Northern Hemisphere, which may reach 150 – 200 ppbv. The concentrations of atmospheric CO show a definite seasonal rhythm and are higher in summers than in winters. In Southern Hemisphere, CO concentrations are lower than in Northern Hemisphere by a factor of upto three. CO is removed from the atmosphere mainly by being oxidized to CO2.

3. Methane: This is a trace gas in atmosphere and is released mainly from rice paddies, wetland areas, enteric fermentation from animals and biomass burning. It has a uniform latitudinal distribution with an average concentration of about 1.6 ppmv. Major sinks of methane are temperate and tropical soils and oxidation to carbon monoxide.

4. NMHCs: This group includes a complex set of hydrocarbons with highly varying characteristics. Most of these are chemically active and have short lifetimes. The usual concentrations in the atmosphere are only few ppbv with localized peaks occurring near the sources. These compounds are removed from atmosphere usually by atmospheric photochemical reactions.

5. Particulate organic carbon (POCs): These complex mixtures of hydrocarbons, alcohols, esters and organics in particulate form. These are usually produced from secondary reactions (gas to particle conversions) and are important in cloud and precipitation processes. The concentrations of POCs in marine air may be around 0.1 to 0.5 g m-3 and in background continental air may be around 1.0 g m-3. In general, the composition of POCs has about 60% neutral compounds, 30% acids and 10% bases.

6. Elemental carbon: This comes into the atmosphere exclusively form biomass and fossil-fuel combustion. Its typical atmospheric concentration over continents is 0.02 g m-3. It is present as fine black powder and can be used as excellent tracer substance for studying long-range transport phenomena in atmosphere.

In addition to above forms, carbon is also present in the atmosphere as carbonyl sulfide, carbon disulfide and dimethyl sulfide. These compounds are important in sulfur-loading of atmosphere and have been discussed with atmospheric sulfur.

Table-2: Indicative characteristics of primary carbon compounds in atmosphere.


Major sources


(Tg Y-1)








Oceans, biosphere,

fossil fuels

7.6 x 104

350 ppmv

380 ppmv

5 years



Biomass burning,




<50 ppbv

150-200 ppbv




to CO2


Animals, wetlands,

decay of vegetation


1650 pptv

>1800 pptv

10 years

Oxidation to

CO, soils


Vegetation, human



few ppbv






Secondary atmospheric



0.1 g m-3

>2.0 g m-3

1 week

Wet and dry




Biomass burning


0.2 g m-3

>1.0 g m-3

1 week

Wet and dry