Chapter5

The growth and life processes of living organisms require about 40. elements. Of these, some elements like hydrogen, oxygen, carbon, nitrogen,

sulphur and phosphorus are required in large quantities and are called Macro-nutrients. Several others, such as, sodium, iron, zinc, cobalt, calcium,potassium, etc. are also important but needed in very small quantities and arereferred to as Micro-nutrients. All these elements are taken up from theenvironment (i.e. lithosphere/ or atmosphere/ or hydrosphere) by the greenplants, made a part of living protoplasm and finally returned back to theenvironment. Thus the elements cycle continuously through the living organismsand environment. These cycles, therefore are called Biogeochemical cycles (Fig.5.1).

8 I-'" :- I Consumers I

'"O'9.~11nutrient I Decomposersl

pool ,r'~nutrients

The movement of nutrient elements throughan ecosystem is called Biogeochemical cycle. Theseare called cycles because nutrient elements, unlikesolar energy, may be used over again and again byliving systems. Organisms, today are using the sameatoms that were present in the primitive earth. Someof the atoms that comprise your body may havebeen a part of extinct dinosaur or the every breatheyou inhale contains several million atoms onceinhaled, say, by Aristotle.

the abiotic environment provides the principal reservoir of elementsi.e. lithosphere (solid rocks of earth); atmosphere (composed ofgases); and hydrosphere (water).

the introduction of elements into the biotic environment almost alwaysrequire primary producers (mainly plants and phytoplankton).

The biotic and abiotic components of. a biogeochemical cycle are, thus,tightly intertwined in. such a way that without life (i.e. living organisms) thebiogeochemical cycles would cease, and without biogeochemical cycles, therewould be no life.

a. Gaseous cycles: In gaseous cycle the reservoir where the elementsare stored is the 'atmosphere', and elements pass through a gaseousphase before completing the cycle. Important elements like oxygen,nitrogen, carbon, hydrogen, etc. cycle in this way.

Ill)

b. Sedimentary cycle: In this cycle, the reservoir where the elementsare present is earth's crust or rocks. Weathering and erosion into thesoil, rivers, lakes or oceans release elements, which are present inthe form of compounds. These elements are phosphorus, calcium,sodium, sulphur, etc.

All the biogeochemical cycles are closely tied to water cycle and the flowof energy through the biosphere. Following is the account of some of theimportant biogeochemical cycles.

Living organisms, atmosphere and earth maintain between them acirculation of water and moisture, which is referred to as Water Cycle. Waterforms a very significant part of an ecosystem and is vital for the existence of life.Without water, biogeochemical cycles could not function, ecosystem could notexist and the life could not be maintained. Water covers about 71% of the earth'ssuriace, occurring in lakes, rivers, glaciers, underground and oceans. The globalcycling of water is influenced by the following factors.

,Precipitation is the form in which atmospheric moisture

descends to earth. in the form of rain, snow, sleet or hail. Precipitation offreshwater that does not soak into the ground is called Surface water. Itforms rivers, lakes, streams, wetlands, reservoirs, etc.

Run off Watersheds (also called drainage basins)are areas of land that drain into the bodies of suriace water. Waterflowing off the land into the bodies of suriace water is called "surface runoff"; and water flowing in rive'rs to the ocean is called "river run off".

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Ground water Some precipitation infiltrates the ground andfills the pores in soil or rock. The subsurface area where all available soiland rock spaces are filled by water is called the "zone of saturation",and water in these pores is called "ground water". The Water table isthe upper zone of saturation. The water table falls in dry weather andrises in wet weather.

rr=:~s--'~J~ ( transportation

:J \ "'-.. of water vapours ._VI ~.II) C if' sllTIaltaneous:5.,g transpiration precipitation!5 i \ & evalPOrratlonevaporatlon

il~ ~\\_t10r_1 _soil water -

deep penetration!

ground water

Fig. 5.2 The details of Water cycle

The water vapours from oceans, lakes, reservoirs, rivers, etc., keep onevaporating in the atmosphere due to solar radiation. A smaller proportion ofwater is released by plants and animals through transpiration and respirationrespectively (it has been worked out that one acre corn crop tran'spires upto 1.5million liters of water in one growing season).

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The water vapours gather in the form of clouds and move with winds overthe earth's surface. When warm air carrying the water vapours cools, thesecondense and fall as rain, snow, hail sleet or dew over earth's surface. A largepart of precipitation falls over ocean (because water covers 71 % area of earth'ssurface). Of the rain water that falls over land, some is absorbed by the soil whilerest runs off into streams, rivers and finally goes back into the sea. A very smallfraction is directly absorbed by plants or consumed by animals and humans. Theground water is returned to the surface by springs, or by activities of the plants(which absorb it through their roots) or by the activities of man (e.g. through thetube wells). It has been estimated that 25% of all the water used in USA consistsof groundwater.

Thus, the cycle is repeated again and again. The only thing required tooperate the cycle is energy, which is needed to evaporate water, and that comesfrom the sun in the form of 30lar radiations.

It has been calculated that the total volume of water on our planetamounts to 1460 million cubic kilometers. About 97.41 % of the total volume issalt water and is present in oceans and seas. About 1.984% is frozen in polaricecaps and glaciers; 0.592% is ground water. The''f~st of 0.014% of freshwaterinclude lakes (0.007%), atmospheric water vapour (0.001%), soil moisture(0.005%), rivers (0.0007%) and biota (0.0003%). In simple words, if the worldwater supply were only 100 liters, our supply of freshwater would be only about0.003 liters (i.e. one half teaspoon).

It has been roughly estimated that on average, 84% of water is lost fromoceans by evaporation while 77% is gained by it again from precipitation. Landareas lose only 16% of water by evaporation but gain 23% by precipitation.Water'run off through streams ahd rivers from land to ocean makes up 7% whichbalances the evaporation deficit of the oceans (Fig. 5.3).

Thus the water balance of earth's surface can be expressed by simplerelationship,

j/16%~ J_3~J -

ot precipitation

over· ocean

evaporation btrom ocean

~ ~-*-9-__l~_

precipitationover land

Evaporation84%

Precipit~tion + Run off77% + 7%

Evaporation + Run off=16% 7%

Precipitation23%

(i) a dry climate, (ii) drought (j.e.period in which evaporation is high andprecipitation is much lower), (Iii) desiccation (drying up of the soil because ofdeforestation or overgrazing, and (iv) water stress (i.e. -low capita availability ofwater due to increasing number of people relying on the fixed level of run off).

Humans intervene in the water cycle in two ways. First we withdraw largequantities of freshwater from lakes, streams and underground surfaces. Inheavily populated or heavily irrigated areas, withdrawals have led to groundwater depletion or intrusion of ocean salt water into the underground watersupplies.

1I-J

Second, we clear vegetation from the land for agriculture mining, roads,building structures and other activities. This increases run off and reduces theseepage of water (in other words, the recharging of ground water is reduced), italso increases the risk of flood and accelerates soil erosion and mudslides.

Earth is the only place in the solar system where free oxygen exists insignificant quantities (i.e. 21 % by volume). It also occurs in the form of oxidesand carbonates in rocks and water. The two main sources of atmospheric ormolecular (02) are:

photosynthesIs-----respiration

CO .•.•."". CO2

+ H2~

2 ~. ~- - ~ ----::=- ~ ..•.- - - - ~ ~ - - . ~ '" - .•. -. ," ~... •... -:. - ...- - - - - - - .....•••..-- ------- - - - .•. -... 'CO2 + H20 photosynthesIs ~ Organic '. I ' :'. ~ ~

, (phytopla~ktonJ compound + O2 • ~ --- ~.,.respiration ~ ' " r' ~ :...\''' •.•.

~ ''1. \ •. ,~ ,'.f_, ' ~ ~ . ~/' r- _. •

. ~ . -'-'- - -~~.---------Fig. 5.4 The Oxygen cycle. Virtually all of the oxygen present in the atmospheretoday has been placed there by the photosynthesizing plants and algae. Oxygenserves as the final acceptor of electrons in cellular,respiration.

'~H~H20 H20

~OH'''/ "'"photo-dissassociation of wEttervapour

024--0'--

115

a. photo-disassociation of water vapour, in which most of the hydrogenreleased escapes into outer space, and

b. photosynthesis by plants during which oxygen is released as a resultof photolysis of water.

The atmospheric oxygen pool serves the needs of all terrestrial respiringorganisms and as it dissolves in water, the needs of aquatic organisms as well.

For every molecular oxygen used in cellular respiration, a molecule ofC02 is liberated. Conversely, for every molecule of carbondioxide taken up inphotosynthesis, a molecule of oxygen is given off (Fig. 5.4). Because

. #photosynthesis and respiration are cyclic, involving both the release andutilization of oxygen, one would seem to balance the other. Most of the terrestrialenvironment is thus provided with a uniform and adequate supply of oxygen.

At high altitudes and. in the surface layers of soils, however theconcentration value of oxygen gradually drops. Part of the atmospheric oxygenthat reaches the higher level of the troposphere is reducl?d to ozone by ultra-violet radiations.

Carbon is the basic constituent of all organic compounds. The chiefsources of carbon are:

a. Atmospheric carbondioxide,b. Carbonates present in water an.d rocks, andc. Fossil-fuels (like petroleum, gas, coal, etc) which on combustion

release carbondioxide.

On land, plants take carbondioxide from the air, and throughphotosynthesis, they incorporate' carbon into carbohydrates. In aquaticecosystem the exchange of carbondioxide with the atmosphere is indirect. C02from the air combines with water to give bicarbonate, a source of carbon foralgae and other aquatic plants.

I Q. How many people do not have clean drinking water?

About 10% of the estimated roughly 700 billion metrictons of carbondioxide in the atmosphere is fixed annually bythe process of photosynthesis by phytoplanktons, algae andother green plants.

In addition to the above mentioned quantity ofcarbondioxide in the atmosphere, approximately 1000 billionmetric tons are dissolved in oceans. The fossil fuels, primarilyoil and coal contains more than 5000 billion metric tons ofacJditional carbon and between 600 and 1000 billion metrictons are'locked up in living organisms at any given time.

I- A carbon cycle is pictured in Fig.' 5.5. The level of carbondioxide ismaintained by animals and plants, both of which release the gas d!,Jringrespiration. Beside releasing carbondioxide, plants also have the ability to usecarbondioxide in the manufacture of carbohydrates, lipids and proteins.

The carbohydrates and other foods in plants can follow one of the severalcourses:

a. If plant is eaten by animal, the food is burned in the animal tissues toyield energy and the release of carbondioxide back into theatmosphere. Some carbo,n°may pass out in waste products such asurine and feces.

b. When animals die, the dead organic matter of the body alongwith theone present in urine and fecal material is decomposed by microbesand carbondioxide is released back to the atmosphere.

c. Or the plants may simply die before being eaten by animals and inthis case its organic matter (i.e. lipids, proteins and carbohydrates) isacted upon directly by the bacteria and fungi of decay.

d. Sometime, however, the plant material may undergo a quite differentprocess at death. When the bodies of plants are compressed in tightlypacked layers of mud under water, they are not decayed by bacteriabut undergo a series of chemical changes to form peat. then browncoal (lignite) and finally coal. The bodies of certain marine plants andanimals undergo similar changes to form petroleum. This process,however, requires a time of millions of years.

I Ans: 1.5 billion.

photosynthesis"-... ~ combustion

diffusion ""- ~espi,alion .. W b\

dJ;""O' I " ""'-, ~ '''''\'"- =-~7~-.-;-~~--~ ;~r-- ~(" "" -~)," ".,

Bicarbonates • Plankton seaphotosynthesISofI ~ decay ca,bonlficatlon

sea algae J~ 'death

\ decay / Fossil Fuels"*'organtsm

Fig. 5.5: Diagrammatic representation of the carbon cycle. Carbon from the atmosphere,in the form of C02 is incorporated into carbohydrate in the process of photosynthesis.Carbohydrate is oxidized for energy by plants, animals and microorganisms. This processreturns C02 to atmosphere. Some carbon is taken out of circulation for varying periods oftime in what can be called a "carbon sink" as coal, petroleum, gas or limestone. In time,much of the carbon locked in the carbon sink will be returned to the atmosphere as C02,through one or the another form of oxidation.

Most of the fossil fuels (which we are, using today) were formed during CarboniferffJusperiod, between 285 and 375 million years ago,when shallow seas repeatedly covered vastforests, preventing decomposition.

As we burn these fossil fuels today, wereturn this carbon to atmosphere.

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In addition to fossil fuels, there is a second very large reservoir for carbonthat recycles very slowly. Calcium carbonate (limestone) is precipitatedchemically in tropical oceans and hardwater lakes and streams. Calciumcarbonate is also produced biologically by reef building corals, foraminifera andmolluscs.

On a global scale, the return of carbondioxide to the atmosphere. byrespiration closely balances its removal by photosynthesi~.

However, since the Industrial Revolution, the increased burning of woodand fossil fuels has increased the level of carbondioxide in the atmosphere. Thishas created a number of problems for man who is drawing on the atmosphericoxygen reserves to burn these fossil fuels.

The total emission of carbondioxide in [heatmosphere as a result of burning offossil fuels (i.e.petroleum, coal and natural gas) and wood wasestimated ((J be 1.5 million tons in 1950. But today wespew about 8 billion tones of carbondioxide everyyear.

The property of thi~ gas is such that it preventsthe escape of infra red heat radiations from earth toouter space and thus causes a "greenhouse effect" i.e.rise in global temperature. it has been calculated thaton average, the gloh,altemperature is rising by reinevery 50 years time. It is feared that gradual rise inthe global temperature will create hazardousproblems. For example, the rise in temperature willmelt glaciers and ice-caps and·there will.be rise in sealevel by several meters. RiSe in sea-level would resultin the drowning of coastal countries like Indonesia,Philippines, Holland and Maldive·s. The rise intemperature will not only alter the patter/y"of globalclimate but will adversely affect the pleintalion andcrops.

Nitrogen is absolutely necessary for the existence of life, as it is essentialcomponent of proteins and nucleic acids, which are vital to all living organisms.Luckily, the earth's atmosphere has 78.9% nitrogen by volume. Although nitrogenis so important for organisms and present in huge quantity in atmosphere, it cannot b~ used by plants and animals in gaseous or elemental (N2) form. Plants canonly use nitrogen if provided in the form of nitrate (N03), nitrite (N02) orammonium ions (NH4). The plants absorb nitrate compounds or ammonium ionsfrom ·the soil and make prot~ins and nucleic acids. Whereas, animals get theirnitrogen already fixed in plant tissues they eat.

The movement of nitrogen .from the atmospheric reservoir, then back tothe atmosphere involves following processes. Fig. 5.6 shows the Nitrogen cycle(the numbers in circle corresponds to the numbers in the following text).

,1. RESERVOIR: The major reservoir of nitrogen is the atmosphere,

where nitrogen exists.

2. NITROGEN FIXATION: The conversion of free or gaseous nitrogeninto nitrate compounds or ammonia is called Nitrogen Fixation. Theseare three principal ways in which nitrogen fixation can occur:

i. Atmospheric fixation: When lightning or thunderstormsoccur, greater quantities of atmospheric nitrogen are turnedinto nitrates or nitric acid. These are dissolved in rain andcarried to earth. Plants acquire these when they absorb waterand other minerals through their roots. It has been estimatedthat about 10% of nitrates are added to soil in manner.

ii. Biological fixation: The second method. which contributes90% of fixed nitrogen to earth, is biological. It is accomplishedby either,

a. Free living bacteria in the soil: Among the free livingbacteria responsible for nitrogen fixation are Azobacter,Clostridium, Derxia and so on. These bacteria take up

,blue-green

algae

(!Itr.:1t~.:,d)SI:lrb8din sc'11

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nitrqgen from atmosphere and form ammonia. They use itIn their own growth and maintenance. When the nitrogenfixing bacteria die. the ammonia IS released in the soil.

b. Free living blue green algae: In aquatic systemsCyanobactena (commonly called as blue green algae) areImporiant nitrogen fixers The important species areNos/oc, Anabaena, Gleolnchia, Trichodesmium, etc.

c. Symbiotic bacteria: The symbiotic bacteria fix far greateramount of atmospheric nitrogen than do any oillerorganisms The symbiotic bacterium 'Rhizobium is amongthe most Important nitrogen fixers. Rhizobium bacteria iivein the roots at leguminous plants (such as, peas, be ,:lII S ,

clovers) where they form rounded swellings called nodules.The bacteria obtain carbohydrates and other substancesfrom the juices of plants and in turn fix nitrogen in the formof nitrates which is utilized by plants. Because at trlepresence of Rhizobium bacteria. leguminous plants notonly fix enough nitrogen for themselves. but also releaseexcess fixed nitrogen intu the soil where it can be used oyother plants.

iii. Industrial fixation: Today large quantities of fixed nitrogenare artificially added to agricultural soil rn the form of fertilizer,;.The production of nitrogen fertilizers is achieved in largefactories by combining nitrogen and hydrogen in the presenceof heat (about 600" C) and high pressure. This process h8Sincreased the fertility of soil but requires large amounts ofenergy.

3. BIOSYNTHESIS: Nitrate compounds are absorbed by plants trom tilesoil through their roots and synthesized Into the aminoacids anaproteins of plant cells. The plants may be eaten by animals that in turnuse the aminoacids from the plant proteins In synthesizing their ownprotoplasm. .~ .

I o. How much air pollution is produced by motor vehicles? I

It may be pointed out that plants can synthesize al/ the20 kinds of i:Jminoacids whereas animals can syn!hesize only 8.For rest of the aminoacids; animals depend on plants.

4. AMMONIFICATION: When animals aFld plants die, the decay bacteriaconvert the nitrogenous compounds of the body into ammonia.Animals also excrete several kinds of waste products containingnitrogenous compounds such as, urea, uric acid and ammonia. Thedecay bacteria convert these wastes also to ammonia. This processof ammonia formation is called Ammonification. The bacteriaresponsible for ammonification are mostly actinomycetes and speciesof Bacillus. The ammonia so formed is either released to atmosphereor retained in the soii to be absorbed by plants or converted intonitrate compounds.

5. NITRIFICATION: Most of the ammonia is converted into nitratecompounds by a process called Nitrification. The nitrification iscompleted into two steps, firstly the ammonia is converted into nitrites(N02), and later the nitrites are converted into nitrates (N03). The firstphase is completed by bacteria like Nitrosomonas, Nitrococcus,Nitrospira (collectively known as Nitrosofying bacteria), and thesecond phase is completed by bacteria like Nitrocystis, Nitrobacter(co!lecti\/ely known as Nitrifying bacteria). Some nitrates arereabsorbed by the plants while others are carried in strea.ms andrivers to the oceans, where they become part of the deep seasediments.

6. DE-NITRIFICATION: Most nitrate or ammonia compounds in the soilare redu",ad again to free molecular nitrogen (N2) which is releasedback to the atmosphere. Reduction of nitrates into free molecularnitrogen is called De-nitrification. Mostly, in the anaerobic conditions(when oxygen concentration is low) the nitrate is often converted tonitrogen gas and nitrous oxide, both of which return to atmosphere.Important bacteria responsible for de-nitrification are Bacillus species,Pseudomonas, Micrococcus, Thibacillus, etc.

7. The gas from volcanoes is rich in nitrogen, hence 30nle newnitrogen is added to the atmosphere.

All organisms require large quantities of phosphorus to construct DNA,RNA, cellular membrane and ATP. Many animals also need phosphorus to makebones, teeth and shells.

The reservoir of phosphorus is not air but earth's crust. In sedimentarycycles, the minerals (such as, phosphorus, calcium) move from land, 10sediments in ocean and then back to the land. Phosphate deposits in the rocksare eroded and transported to the soil by wind and water. Phosphate in the soilare absorbed by the plants and converted into a variety of phosphatecompounds. Animals get their phosphorus in water they drink or as organicphosphates from the plants they eat. When plants and animals die.phosphatizing bacteria breakdown the tissues of the dead animals and plants,and return phosphorus to the soil. Phosphates are also returned to the soil byanimal wastes. Much of the phosphates in soil is washed away by run off waterand eventually reaches the ocean via rivers and is deposited in the sediments.From ocean, small amounts of phosphates are returned to land surface by fisheating sea birds. The marine birds bring back phosphates to the earth in the formof fecal material called "guano" which is deposited along the marine coasts inlarge quantities.

The phosphate cycle is not completely balanced, for phosphates arebeing carried into the sediments at the bottom of sea at a faster rate than theyare being returned back to earth by the action of marine birds.

It has been estimated that the world'srivers carry about 17 million metric tons ofphosphorus to the sea every year but. only60,000 metric tons of phosphorus per year isreturned back to the land by fish eating birds inthe form of guano. Peru, a South Americancountry, is the largest exporter of phosphatefertilizers in the form of guano today.

Thus, the means of returning phosphorus to land surface are inadequateand does not compensate the loss to the deep sea. This is why, for CUltivation,these days artificial fertilizers are used in agriculture. Only some geologicalupheaval can bring deep sea deposits of phosphates back to the surface in theform of mountains. This process, however, would take millions of years.

Sulphur is essential part of proteins and aminoacids. It exists in number offorms such as, elemental sulphur (S), sulphide sulphur monoxide, sulphite andsulphates. Of these, three are important ir: nature: sulphurdioxide, sulphide dndsulphates.

The sulphur cycle (Fig. 5.8) is both sed:mentary and gaseous (i.e itincludes gaseous phase and a sedilnentary phase). The gQseous phase is tuosmall and less pronounced, whereas the major reservoir of sulphur is the soil inaquatic environment, sulphate is the main form of sulphur that is available 10

autotrophic organisms.

Sulphur enters the atmosphere in the form of hydrogen suipllide (a higfllypoisonous gas with a "rotten egg" smell) from burning of fO::isil fuel, volcaniCeruption and the gases released when animals and piants decay

I

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The hydrogen sulphide quickly oXidizes into another water-soluble form,the sulphurdioxide (802). Atmospheric sulphurdioxide with rain is carried back toearttl as sulphates. From the soil, sulphates are absorbed by plant roots and areincorporated in the form of proteins. The sulphur in protein is transferred toanimals when they eat plants, with excess being excreted :n wastes or feces.Particles of sulphate salts enter atmosphere from sea spray as well.

Excretion of waste and death of plants and animals carry sulphur back tothe soil and bottom of ponds, lakes and oceans where the organic material isacted upon by bacteria. The bacteria and fungi responsible for decay fall into twogroups:

a. Under aerobic conditions, the sulphur is mineralized by species ofAspergillus, Neurospora, etc., and

b. Under anaerobic conditions, however, some sulphur may be reduceddirectly to sulphates by bacteria like Proteus. Eucherichia, etc.8ulphates are also reduced under anaerobic conditions to elementalsulphur or to sulphide by such bacteria as species of Oesulfa-vibrio,Aerobacter, etc.

Anot~ler step in the cycle is the oxidation of elemental sulphur andsulphides into sulphates by bacteria like Beggiota and Thio-bacillus.

The sedimentary part of sulphur cycle involves precipitation of sulphur ,nthe presence of iron under anaerobic conditions. It comb!nes with iron to formferrous sulphide, which is highly. insoluble in neutral or alkaline water and IS fiuY"ljheld in mud or wet soil. However, iron sulphide facilitates the conversion 'J!insoluole phosphorus to soluble phosphorus.

The geochemical cycles of different chemicalelements are thus closed cycles; the atoms are used overagain and again. To keep the cycles going does not requirenew matter but it does require energy. The energy cycle. onthe other hand is not a closed one. Thus, the pattern offlow, both of energy and chemical substances is different.the flow of energy is uni-directional whereas those ofchemical substances are cyclic. .