Cycling of matter in environment

CARBON, NITROGEN AND SULFUR IN ENVIRONMENT

In the global environment, a vast number of elements exist in a variety of chemical species and are continually transformed from one species to another. These transformations from one chemical species to another involve cycling of these elements or chemical species amongst different components of the environment i.e. amongst atmosphere, lithosphere, hydrosphere and biosphere. These cycles of elements involving different components of the environment are, therefore, considered as biogeochemical cycles. These biogeochemical cycles are highly complex and interact strongly with each other and, therefore, are of fundamental importance in maintaining global environmental balance and in understanding the dynamics of environment. Further, human activities cause increase or decrease in natural amounts of chemical species in the environment or cause addition of chemical species not found in nature resulting in disturbance in natural biogeochemical cycles. Such disturbances constitute environmental pollution, which has profound impact on the stability of global environment. The biogeochemical cycles of element carbon, nitrogen and sulfur are most important from the point of view of global environmental balance. Therefore, important features of the cycles of these three elements have been discussed below.

CARBON CYCLE

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 (CO₃^2-) 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

Reservoir	Carbon-content in Pg (1015 g)
OCEANS
1. Total dissolved CO2	37400.0
2. Dissolved CO2 in mixed layer (75 m depth)	670.0
3. Living biomass carbon	3.0
4. Dissolved organic carbon	1000.0
SEDIMENTS
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
BIOSPHERE
1. Terrestrial biomass	650.0
2. Soil organic	2000.0
3. Oceanic organic	1000.0
ATMOSPHERE (mostly as CO2)
1. Pre-industrial estimate (290 ppmv)	615.0
2. present estimate (350 ppmv)	734.0

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.

Compoud	Major sources	Production (Tg Y-1)	Background concentration	Polluted concentration	Lifetime	Sinks
CO2	Oceans, biosphere, fossil fuels	7.6 x 104	350 ppmv	380 ppmv	5 years	Oceans
CO	Biomass burning, atmospheric photochemistry	660.0	<50 ppbv	150-200 ppbv	1-2 months	Oxidation to CO2
CH4	Animals, wetlands, decay of vegetation	610.0	1650 pptv	>1800 pptv	10 years	Oxidation to CO, soils
NMHCs	Vegetation, human activities	Variable	few ppbv	Variable	Variable	Photochemical reactions
POCs	Secondary atmospheric photochemistry	Small	0.1 g m-3	>2.0 g m-3	1 week	Wet and dry deposition
Elemental carbon	Biomass burning	Small	0.2 g m-3	>1.0 g m-3	1 week	Wet and dry deposition

NITROGEN CYCLE

Nitrogen is primarily exchanged between atmosphere, biosphere and soil. Following Table-3 shows the estimated total 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-3. Nitrogen storage in various

components of global environment.

Location	Nitrogen storage in Tg (1012 g)
Lithosphere	2 to 6 x 106
Soil	85 x 103
Continental biomass	10 x 103
Atmosphere	3.8 x 103
Surface litter	1.5 x 103
marine biomass	380.0
Oceans	23.0
Human beings	5.5

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-4. Indicative characteristics of major atmospheric nitrogen compounds.

Compound	Major sources	Nitrogen produced (Tg Y-1)	Background concentration	Polluted concentration	Lifetime		Sinks
NH3	Animals, soils, biomass burning	54.0	0.1 ppbv	>6.0 ppbv	6 days	Conversion to NH4
NH4+	Conversion from NH3	65.0?	0.05 g m-3	>1.5 g m-3	5 days	Wet & dry deposition
NO3-	Secondarily from NOx	26.0	0.5 g m-3	>10.0 g m-3	5 days	Wet & dry deposition
N2O	Soil	41.0	310 ppbv	-	170 days	Strato-spheric photo-chemistry
NO, NO2	Fossil fuels, lightening, biomass burning, intercons-versions	48.0	<100 pptv	100 pptv	<2 days	Oxidation to HNO3 & NO3-, photo-dissociation

Sulfur cycle

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 13^th most abundant element in Earth’s crust (0.1%) and 9^th 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-5. Indicative characteristics of major tropospheric sulfur compounds.

Compound	Major sources	Sulfur produced (Tg Y-1)	Background concentration	Polluted concentration	Life-time	Sinks
COS	Soils, coastal marshes, biomass burning	4.7	500 pptv	?	44 years	slow photoche-mistry, stratosphere, oceans
CS2	Oceans, soils	1.6	15-30 pptv	100-200 pptv	12 days	Photoche-mical production of SO2
DMS	Oceans, algal deposition	27-56	<10 pptv	100 pptv	0.6 days	Oceans, oxidation to SO2
H2S	Bacterial reduction, soils, wetlands	Variable	30-100 pptv	330-810 pptv	4.4 days	Photoche-mistry
SO2	Anthropo-genic sources, volcanoes, oxidation of H2S	103	24-90 pptv	>5 ppbv	2-4 days	Wet & dry deposition
SO4-	Sea-sprays, oxidation of SO2	138	0.1 g m-3	>2.5 g m-3	1 week	Wet & dry deposition

ORGANIC MATTER IN THE ENVIRONMENT

In the environment, organic matter is synthesized in its biotic component i.e. biosphere. Autotrophic organisms are the only organisms that can synthesize organic matter using solar radiation and mineral matter taken from atmosphere, hydrosphere and edaphosphere. Autotrophs synthesize organic matter either by photosynthesis or by chemosynthesis. While chemosynthesis is important for cycling of nitrogen and certain other processes in the environment, photosynthesis is the major process responsible for formation of organic matter in the environment. Autotrophic green plants, particularly land plants are most important from the point of view of photosynthetic production of organic matter. In the photosynthesis, carbon dioxide and water are used and a certain portion of short-wave solar radiation is absorbed and expended within the plant cover. In considering the role of plant cover of Earth in the global energy and water balance, it is necessary to consider the amount of solar radiation and water utilised by plants in production of biomass i.e. in photosynthesis. For this, the quantities that are calculated and studied are efficiency of photosynthesis and productivity of transpiration.

1. Efficiency of photosynthesis: It is the ratio of energy expenditure on the synthesis of biomass to the total quantity of solar energy absorbed by plant cover in an area. Many experimental studies have shown that this efficiency of photosynthesis is very modest and under normal conditions, usually does not exceed 0.1 to 1.0 percent. However, under very favorable conditions, it may increase to several percent.

2. Productivity of transpiration: It is the ratio of the amount of biomass produced to the quantity of water transpired by photosynthesizing plant cover. This productivity of transpiration usually ranges from 0.5 to 0.1 percent which indicates that photosynthesizing plants use very little water and abundant transpiration in them merely circulates the water in the environment.

Thus general low values of both the above quantities indicate that under natural conditions, plant cover assimilates only a negligible part of available energy and water resources i.e. there is substantial limitation on the use of natural resources in production of biomass in the environment. It is important to establish the causes of this limitation for the study of the relationship of productivity of plant cover to climatic factors. Experimental studies of maximum possible efficiency of photosynthesis in controlled environmental conditions when carbon dioxide of the atmosphere is fully utilized indicate that under such conditions, plants can assimilate 5% or more of the solar energy received and the productivity of transpiration also increases manifold. However, in natural conditions maximum possible photosynthesis and, therefore, the production of biomass is greatly limited by various factors other than the availability of resources.

PHOTOSYNTHESIS WITHIN TERRESTRIAL PLANT COVER

In nature, most of the photosynthesis takes place within the terrestrial plant cover in which different meteorological conditions exist at different levels. The efficiencies of photosynthesis at various levels within the plant cover are not same and are determined by particular microclimatic (meteorological) conditions prevailing at different levels. The microclimatic effects of a forest cover are explained in terms of:

1. Plant coverage characteristics: These characteristics depend upon:

1. Density of dominant forms in the forest covers.

2. Distribution of different forms in the forest covers.

1. Stratification characteristics of plant cover: These characteristics depend upon:

   1. Total vertical height of plant cover.

   2. Number of vertical strata in the plant cover.

   3. Morphological characteristics of each strata in the plant cover which are determined by branching pattern of plants, evergreen or deciduous nature of foliage, size, density, texture and orientation of leaves.

Importance of the above features can be judged from comparison of tropical and temperate forest plant covers. In tropical forests, average height of tall trees is 46-55 metres, species diversity is 40-100 species per hectare, stratification is strong with 4-5 strata, undergrowth is dense commonly with two upper foliage strata and lower strata being denser. In temperate forests, average height of tall trees is about 30 metres, species diversity is less than 20 species per hectare, stratification is poor with usually 2-3 strata which are almost continuous from low shrubs to top of trees.

In the study of photosynthesis at various levels within plant cover, averaging of the values of meteorological elements at one level along the horizontal line is appropriate and it makes it possible to exclude the influence of individual plants on the meteorological regime. By applying such averaging techniques, following conclusions have been established:

1. Microclimate within the plant cover may be represented by a series of vertically varying profiles of meteorological elements, particularly of solar radiation, water vapor pressure, air temperature, carbon dioxide concentration and wind speed.

2. The profiles of meteorological elements show diurnal and seasonal variations.

3. The average vertical flow of short wave and long-wave radiation, heat and water vapor within the layer of plant covers and the momentum of the system depend substantially on height.

Microclimatic profiles within plant cover

1. Solar radiation: Plant cover significantly changes the pattern of incoming and outgoing radiation. Short-wave reflectivity of area depends somewhat on the density and characteristics of the plant cover. The albedo of areas having coniferous forests is about 8-14 while that having deciduous forests is about 12-18. Albedo of semiarid savannas and woodlands is much higher.

Large amount of solar radiation is trapped within the foliage canopy e.g. Fagus sylvetica forest traps about 80% of incoming radiation in the top strata of canopy and less than 5% reaches the ground. Such trapping is more pronounced on sunny days.

The foliage canopy absorbs more short-wave radiation than long-wave infrared radiation e.g. in tropical forests of Nigeria, only 7.6% radiation of <0.5 m reaches the ground while 45.3% of radiation of >0.6 m reaches the forest floor.

Effect of the age of plant cover on the penetration of light into the plant cover can be judged by the observation that in Pinus sylvestris forest in Germany, percentage of light reaching the ground floor is about 50% at 1.3 year, only 7.0% at 20 years and again 35% at 130 years.

Penetration of solar radiation within the plant cover generally obeys Bougner-Lambert Law:

I = I_o e^-KL

Where, I = radiation intensity on a horizontal plane within the plant cover; I_o = radiation intensity on a horizontal plane above the plant cover; L = leaf area index; K = Extinction coefficient.

Extinction coefficient (K) is constant for a given species and is related to:

1. Amount and type of leaf chlorophyll.

2. Canopy architecture and

3. Reflectivity of leaves. Its value lies between 0.3 and 0.5 for grass-type plant cover and approaches 1.0 for nearly horizontal leaves. Value of K shows inverse relationship to chlorophyll content and reflectivity of leaves.

In general, light penetration into plant cover depends upon the type of plants (particularly trees), spacing of plants, age of plants, crown density, height of plants (particularly trees) and time of year. Percent light reaching the forest floor in some types of forests is given below:

Birch-beech forest 50-75%

Pine forest 20-40%

Spruce-fir forest 10-25%

Tropical forest 0.1-0.01%

In deciduous forests, light penetration increases during leafless conditions.

Thus the intensity of solar radiation decreases exponentially from top of plant cover towards Earth’s surface due to absorption and radiation scattering by the surface of plants. The resulting radiation balance, therefore, also decreases in the same direction due to screening effect of plants.

2. Air temperature: During day time, heating of foliage canopy causes a convectional transfer of sensible heat and so air temperature within upper canopy may be higher than above the canopy or below. At night, the relationship is reversed as upper canopy layer of air is cooled by contact which are both losing heat by radiation and also transpiring slowly.

Modification of thermal environment is due to shelter from sun, blanketing at night, heat loss by evotranspiration, reduction in wind speed and obstruction to vertical airflow.

Blanketing causes lower maximum and higher minimum temperatures and causes lower mean monthly temperatures in tropical and temperate forests.

At sea level, mean monthly differences in air temperature in temperate forest may reach 2.2^OC in summer but only 0.1^OC in winters. In hot summers, this difference can be more than 2.8^OC.

In forests, which do not transpire greatly in summers e.g. forteto oak maquis of Mediterranean area, day temperatures in woods may cause mean monthly temperatures to be higher than in open.

Altitude in the same climatic zone may affect the degree of temperature decrease in temperate forests. At 1000 meters altitude, lowering of temperatures may be twice that at sea level.

Vertical stratification in plant cover modifies the thermal profile within it in complex ways. In tropical forests, dense foliage canopy heats up greatly during daytime and cools rapidly during night. It shows a much greater diurnal temperature range in denser canopy than in the lower strata. Whereas daily temperatures of second story are intermediate between those of the tree tops and undergrowth, the nocturnal minima are higher than either tree tops or undergrowth because the second story is insulated by trapped air both below and above.

3. Saturation water vapour pressure: This profile within plant cover shows close correspondence with temperature profile both during day and night. Forest temperatures differ strikingly from those in open and the forest water vapour pressures were found to be higher within an oak stand than outside it for every month except December.

At night, actual water vapor pressure almost reaches saturation as the air and canopies are cooled by radiation and convection. Some water vapor is transferred through transpiration from the canopy. During day, upper canopy is air heated by convection and water vapor pressure curve shifts much from saturation curve. Deficit between the two increases the downward and at quite lower level, actual water vapor curve inflects. Towards the bottom of canopy, it reapproaches saturation curve due to transpiration coupled with low air movement and low temperature towards base of plant cover.

The flow of water vapor within the plant cover increases with height because of the influence of transpiration by plants and the momentum of the system declines downwards from the plant cover’s upper boundary as a result of the inhibiting effect of plants on the movement of air. This effect is associated with the reduction in turbulent exchange within the layer of plant cover compared with higher air layers. The coefficient of turbulent exchange within the layer also declines towards Earth’s surface.

Humidity conditions within the plant cover are very much different from those outside it due to evotranspiration characteristics of the cover. It generally depends upon the type of plant cover, density of plant cover, structure of vertical stratification and temperature effects. Time of the day and season also affect evotranspiration and, therefore, humidity within the plant cover.

Evotranspiration generally increases with density of vegetation and within the plant cover, relative humidity may be 3-10% higher than outside. This effect is more pronounced in summers.

Rainforests have high transpiration and so have high humidity inside their plant cover. Mean annual relative humidity excess is reported to be 9.4% in beech, 8.6% in Pinus abies forest, 7.9% in larch forest and 3.9% in Pinus sylvestris forests.

In tropical forests, night exhibits complete saturation while in daytime, the humidity decreases with height.

4. Carbon dioxide: The profile of carbon dioxide concentration within plant cover shows much diurnal variation due to photosynthetic uptake of carbon dioxide during daytime and respiratory addition of this gas during night. Carbon dioxide concentration in soil is very low and its use by plants is spatially and temporally very inhomogeneous.

During daytime, CO₂ concentration decreases from upper canopy towards ground. It reaches a minimum point near middle of canopy. Below this point, CO₂ concentration rapidly increases towards ground and becomes equal to CO₂ concentration outside the canopy at a point that roughly corresponds to compensation light intensity. It reaches fairly high level at soil surface. This profile is due to photosynthetic depletion of carbon dioxide in upper canopy, equilibrium corresponding to compensation point lower in the canopy and respiratory addition of carbon dioxide from lowest shaded leaves and soil microorganisms.

In the night, concentration of CO₂ gradually increases towards ground level due to its respiratory addition.

5. Wind velocity: The profile of wind speed shows no strong change in day and night but overall wind speed is higher during daytime due to convectional effects. Wind profile within the canopy develops due to steady state boundary layer flow. The profile is logarithmic above the canopy and becomes exponential within the canopy. The zero plane displacement (D) depends on the height of plants. The roughness height (z_o) is a measure of community roughness and it is effectively the thickness of a laminar sublayer through which individual elements project. Value of z_o is related to height variation and spacing of individual elements which in the plant cover are plants. In extrapolation of logarithmic curve downwards, the zero velocity intercept is found to lie at the height D+z_o. If the canopy were rigid, it would have a constant value but variation of surface roughness depends on leaf flutter, movement of branches and leaf streamlining. These variations cause variations in value of z_o with wind speed. Surface frictional characteristics are entirely specified by D and z_o. The wind profile is major factor in establishment of profiles of saturation water pressure and carbon dioxide within the plant cover.

Lateral air movement is generally lesser within the plant cover than outside it. Even large variations in outside wind velocities do not affect airflow inside forest cover. Vertical stratification structure, leaf canopy architecture, density of stand and season have marked influence on wind velocity profile within a plant cover. For this reason, reduction in wind velocity within the forest cover is different in temperate and tropical forests. Reduction in wind speed from outer edge towards deep inside a forest is greater in tropical rainforests. In temperate European forests, wind velocity at outer edge of forest is reduced to 60-80%, 50% and 7% at points 30 m, 60 m and 120 m respectively deep inside the forest. In Brazilian evergreen forest, wind velocity of 2.2 m/second at the outer edge of forest is reduced to 0.5 m/second at 100 m deep inside forest while at 1000 m inside forest the wind velocity becomes negligible. In this forest, outside storm velocity of 28 m/second was reduced to 2 m/second at 11 km deep inside the forest.

6. Flow of water vapor and momentum of system: The flow of water vapor within the layer of plant cover increases with height because of the influence of transpiration by plants. The momentum of the system decreases downwards from upper boundary of plant cover towards ground level as a result of the inhibiting effect of plants on the movement of air. This effect is associated with the reduction in the turbulent exchange within the layer of plant cover compared with higher air layers. The coefficient of turbulent exchange within the layer also decreases towards ground level.

The theory of photosynthesis within plant cover and numerical models of this process developed in recent years are based on the general idea of a transition from photosynthesis within a single leaf to photosynthesis within a layer that is homogeneous horizontally but possesses different physical conditions at various heights. Application of the theory of photosynthesis within a layer of plant cover indicates following general conclusions:

1. When assimilation process is not very sensitive to different meteorological elements, total assimilation within the plant cover strongly depends on the radiation flux for low levels of radiation. For large values of radiation, the total assimilation is independent of radiation flux and becomes dependent on other factors particularly temperature.

2. Within he plant cover, increase in total assimilation with increase in inflow of CO₂ from soil is much slower than would occur if all the inflow of CO₂ from soil were to be expended on assimilation. This is because the inflow of CO₂ from soil first encounters the leaves located in shade, which are not able to photosynthesize intensively due to insufficient radiation. The general increase in CO₂ concentration produced by its inflow from below is compensated by a reduced contribution of CO₂ from above. Thus assimilation within plant cover is influenced very little by the upward inflow of CO2 from soil and is largely influenced by flow of CO₂ coming downwards from the atmosphere.

PRODUCTIVITY OF PLANT COVER

The productivity of plant cover () is the difference between total assimilation and the expenditure of organic matter on respiration within a plant cover. Thus the productivity of a particular plant cover depends on photosynthesis and respiration in it. The leaves of plants are the major organs association with both photosynthesis and respiration. Therefore, the productivity of plant cover substantially depends on the value of the index of leaf surface (leaf index) and decreases for both very small and very large values of this index. In view of this, the value of productivity of a plant cover is calculated for an optimal value of leaf index i.e. the value of this index that corresponds to the highest value of productivity. From various studies, it has been established that the parameters and factors that affect the photosynthesis within a plant cover also influence the productivity of plant cover. Thus the productivity of plant cover is mainly determined by parameters characterizing the properties of plant cover itself and the climate.

In general, following important points can be observed in relation to productivity of plant cover in nature:

1. The structure of plants in the plant cover continuously changes throughout their life cycles and photosynthetic activity of leaves is never optimal throughout the entire vegetative period of any plant.

2. Availability of mineral nutrients in nature is always less than optimally required for maximum possible photosynthesis.

3. Under natural conditions, water regime of soil is also not constantly maintained at optimally required level.

Thus the productivity of plant cover in real natural conditions is always less than theoretically possible maximum level due to complex interactions between a variety of biological, climatic and soil factors.

Climatic factors and productivity of plant cover

In conditions of sufficient moisture, two climatic factors i.e. photosynthetically active radiation and temperature are particularly important in relation to productivity of plant cover.

The influence of radiation and temperature on productivity of plant cover is quite complex. In real natural situations, radiation is always a factor whose value is a ‘minimum’ because radiation available to leaves in lower layers of canopy is always insufficient. Therefore, increase in radiation flux always results in increased productivity of plant cover.

With increase in temperature, the productivity of plant cover increases initially. After attaining a certain maximum value that depends on the value of radiation flux, productivity begins to decrease with further increase in temperature. Thus productivity of plant cover substantially decreases above a certain threshold value of temperature which is determined by the radiation flux.

PRODUCTIVITY OF GLOBAL PLANT COVERS

Average values of the productivity of natural plant covers of Earth have been derived by using various theoretical and numerical models and data from a variety of studies including empirical determinations of productivity in individual biogeographical zones.

Terrestrial plant covers

Yefimova (1979) has made use of quite precise relationships between productivity of natural plant cover and meteorological factors in calculating values of the productivity and the coefficient of utilisation of photosynthetically active radiation for each continent. Results of her calculations are given in the Table-6. The data shows that the average productivity per unit area for the five continents of Earth does not differ very much. In each of these continents, magnitude of productivity over large part of continental territory is greatly limited by insufficient moisture or heat. The continent of South America is exception to this general condition since climatic conditions over large part of its territory are favourable for plant life.

Table 6: Productivity and coefficients of utilization of photosynthetically active radiation in various continents of Earth. (Yefimova 1979)

Continent	Productivity (x109 tonnes)	Productivity (center per hectare)	Coefficient of utilization of photosynthetically active radiation (as %age of total over vegetative period)
Europe Asia Africa North America South America Australia (including islands of Oceania)	8.9 38.3 31.0 18.1 37.2 7.6	85 98 103 82 209 86	1.26 0.88 0.59 0.94 1.13 0.44

In Australia and Africa, coefficients of utilization of photosynthetically active radiation are lower than average. This can be attributed to insufficient moisture over large parts of these continents, which inhibits the complete utilization of available radiation by plant covers.

In Europe and South America, most favorable conditions for the development of plant life are found. In Europe, located at higher latitudes and exposed to less solar radiation, its utilization is relatively greater.

Smil (1985) gave estimates of the productivity and storage of biomass in major biomes of the Earth. These estimates are given in Table-7. Data in this table shows that there is not much difference in the area occupied by different types of ecosystems except wetlands that occupy smallest area on the Earth. However, productivity is highest in cultivated lands where one ton of biomass is produced per one ton of phytomass, followed by tropical and temperate grasslands where 0.5 ton of biomass is produced by each ton of phytomass. Next in productivity are tundra, deserts-semi-deserts and wetlands where 0.2 tonnes of biomass is produced per hectare from one ton of phytomass per hectare. These areas are followed by wetlands and shrub-lands where productivity is 0.13 tonnes per hectare. Tropical, temperate and boreal forest, though occupy almost same area on Earth, produce 0.067, 0.04 and 0.02 ton of biomass per tone of phytomass per hectare respectively. Despite these facts, most important on Earth are tropical, temperate and boreal forests that have the highest concentration of biomass on Earth (totaling about 750 tonnes per hectare). These ecosystems also have the highest total storage of biomass on Earth totaling about 850 x 10⁹ tonnes. Further, it may be noted that contribution to total biomass production is equal for tropical rainforests and tropical grasslands (20 x 10⁹ t/yr), followed by boreal forests and tropical grasslands (15 x 10⁹ t/yr) and temperate forests, woodlands-shrub-lands and temperate grasslands (10 x 10⁹ t/yr). Tundra and deserts have quite high average of net biomass production per unit area and also quite high weight of phytomass per unit area. Despite this they contribute very little to total global biomass production (1.0 – 2.0 x10⁹ t/yr). However, if total biomass storage in different types of ecosystems on Earth is considered, tropical rainforests, temperate forests and boreal forests are the most important storehouses of organic matter on Earth having 850x10⁹ tonnes of biomass. Woodland and shrub-lands having 75x109 tonnes and then tropical and temperate grasslands having 60x109 tonnes of biomass storage follow these.

Table 7: Area, productivity and storage of major global ecosystems. (Smil, 1985)

Ecosystem	Total area (x106 km2)	Average net production (tonnes/ha)	Average phytomass (tonnes/ha)	Total production (x109 tonnes/year)	Total storage (x109 tonnes/year)
Tropical rainforest Temperate forests Boreal forests Woodland and shrub-land Tropical grasslands Temperate grasslands Cultivation Tundra Deserts and semi-deserts Wetlands Settlements and transport	10.0 10.0 15.0 10.0 10.0 10.0 15.0 10.0 20.0 5.0 5.0	20.0 10.0 10.0 10.0 10.0 10.0 10.0 1.0 1.0 15.0 5.0	300.0 250.0 200.0 75.0 20.0 20.0 10.0 5.0 5.0 75.0 5.0	20.0 10.0 15.0 10.0 20.0 10.0 15.0 1.0 2.0 8.0 3.0	300.0 250.0 300.0 75.0 40.0 20.0 15.0 5.0 10.0 40.0 3.0
Total				114.0	1058.0

Aquatic plant covers

There is much less data about productivity of autotrophic plant covers in water bodies as compared to that about terrestrial plant covers. However, the available data indicates that the seas and oceans have the greatest volume of organic matter produced by phytoplankton located in the 30-40 meters deep layer of hydrosphere. At greater depths, quantity of solar radiation is insufficient for active development of photosynthesis.

In general, the productivity of shelf zones is substantially less than open ocean. It may attain maximum values in small bodies of water possessing large quantities of minerals required by the plants. The overall value of productivity for the oceans is estimated to be about 55 billion tonnes per year i.e. approximately 15 centner per hectare. This last figure is less than 1/6^th of the average productivity per unit area on continents.

Thus the estimates show that the yearly volume of productivity for the Earth as a whole is approximately 200 billion tonnes i.e. about 40 calories per hectare. This corresponds to an energy expenditure of approximately 0.15 kcal/cm² per year. This is about 0.1% of the solar radiation reaching the Earth’s surface.

TRANSFORMATIONS OF ORGANIC MATTER

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 (_n1) 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 3x10¹² tonnes and total productivity of plants on continents is approximately 140x10⁹ tonnes, 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 0.2x10⁹ tonnes. Assuming that each human being expends on average about 2.5x10³ kcal of energy per day, the total energy consumption of human population comes to be about 1.8x10¹⁵ kcal/year. Thus, the human population consumes about 0.2% of the total production of Earth’s organic world.