GLOBAL BIOGEOCHEMICAL CYCLES of C, N, P and S, MINERAL CYCLES

GLOBAL BIOGEOCHEMICAL CYCLES  of  C,  N,  P and S, MINERAL CYCLES (PATHWAYS, PROCESSES  BUDGETS)  IN  TERRESTRIAL  AND  AQUATIC ECOSYSTEMS

The earth system involves interactions amongst the physical climate, chemical cycles and living organisms.  In any ecosystem there is relationship between two major components.  These are abiotic and biotic components.  Biotic components represent all the living organisms, which are plants, animals and microbes, while abiotic components represent non-living and living components.  These components consist of lithosphere, hydrosphere and atmosphere.  

Hence recycling of matters takes place in all these environments.  Actually living organisms require 40 necessary minerals, which get deposited in the organic form in the body and later on, after death, microorganisms decompose them.  These type of cycles, which depend on living organism and non living matter, are called as "Biogeochemical cycle".  

Actually this word consists of Bio + Geo, where Bio word is represent living organisms, and Geo word is consist of geological forces, which may be physical or chemical.  Hence, the movement of substances in between along the ecosystem is called as biogeochemical cycle.  Biogeochemical cycle is of two types :

a. Those cycles in which nutrient is found in gaseous form and atmosphere plays an important role in

this cycle, then these are called as "atmospheric cycle".  Example - Carbon cycle.

b.  Those cycles, which complete in the hydrosphere itself only, then this type of cycle is called as edaphic cycle or sedimentary cycle. 

Carbon Cycle

Carbon is found in every living organism in the organic form, while in the environment or atmosphere; it is present in the inorganic form.  The main source of carbon is atmosphere, where it is present in the form of CO₂ in the concentration 0.345% or 345 ppm.  In the carbon cycle, producers and decomposers are two major components, which regulate carbon cycle.

In the carbon cycle, two processes are very important :

1.         Immobilization: The process in which inorganic carbon is converted into organic carbon; then it is called as immobilization.  Green plants regulate this process only, because they convert CO₂ into glucose in the presence of sunlight and chlorophyll.

2.         Minralization:  The process in which organic carbon is converted into inorganic carbon, is called as Minralization.  This process is regulated by decomposers, which are bacteria, fungi, nematodes etc.

            Carbon cycle is a gaseous cycle, because this cycle goes continuously in between atmosphere and terrestrial area (lithosphere) and it mainly depends on CO₂.  Concentration of CO₂ in the atmosphere is 345 ppm.  Green plants absorbs CO₂ from atmosphere and converts this CO₂ into glucose in the presence of chlorophyll and sunlight.  It is called as photosynthesis.

In this process, inorganic carbon gets converted into organic carbon i.e. glucose.  This glucose gets transformed into various forms as starch, cellulose, glycogen etc.  In plants it is stored in the form of starch.  From plants these substances enter the food chain and when herbivore eat plants, then organic contents gets into herbivores and these enter from herbivores to carnivores.  Thus, these remain in organic form in the whole food chain.  Although they get transferred from starch to glucose and from glucose to glycogen, yet in each tropic level these organic compounds gets oxidize during respiration due to which organic compounds converts into CO₂.

   

This CO₂ enters into the atmosphere, but a large part of organic compounds enters soil in the form of excretory substances.  Similarly after death also, this compounds enter into the soil, where different types of decomposers converts it in the form of complex organic compounds to simple organic compound like starch and cellulose get converted into glucose.  This gets decomposed in the presence of cellulosic fungus, later on, during anaerobic decomposition, and then this gets converted into alcohol and acids.

          

At last, these gets converted into CO₂ by aerobic fungus and bacteria.

This CO₂ reacts with water and forms H₂CO₃, which forms carbonates from rocks.  Along with it, carbon deposits in the form of coke, coal and petroleum, which later on are used in the form of fuel and are released in the form of CO₂ into the atmosphere which is called as combustion.

       

Carbon cycle goes on in between terrestrial zone, atmosphere and hydrosphere, in which global cycle shows 10¹⁵ gm carbon deposition.

Nitrogen Cycle

Nitrogen is an important nutrient for plants and animals.  78% nitrogen is found in atmosphere normally, but plants cannot absorb nitrogen directly from atmosphere.  They absorb it as ion from the soil.  Hence nitrogen can be divided into two forms, available and unavailable from.  Gaseous form is unavailable form like N₂, N₂O, NO₂, NO etc., but ionic forms as NO₂, NO₃^- and NH₄⁺ of nitrogen are available form.  Hence, it is necessary for nitrogen to be converted from gaseous to ionic form.  Then only, plants can absorb nitrogen.  Nitrogen cycle is also a gaseous cycle.  Following steps are important in nitrogen cycle :

1.         Nitrogen fixation:  The process in which unavailable form of nitrogen (gaseous) gets converted in to available form (ionic form), is called as nitrogen fixation.  This process takes place by two ways.  When nitrogen gets fixed due to physical factors, then it is called as physical nitrogen fixation.  In this process nitrogen converts into nitrate while if nitrogen gas is converted into fixed form ammonium nitrogen with the help of living organisms, then it is called as biological nitrogen fixation.  This process is regulated by microbes e.g.; Rhizobium, pseudomonas, cyanobacteria etc.

2.         Minralization: The process in which organic nitrogen is converted into inorganic nitrogen, is called as Minralization.  Since in this process, the first product is ammonia, so it is called as ammonification.  This process is anaerobic and is regulated by ammonifying bacteria.

3.         Nitrification:  The process in which ammonia is converted in to nitrate, it is called as nitrification.  This is an aerobic process, hence takes place in the presence of oxygen.  NH₃, first of all, converts into nitrite, Nitrosomonas.  Later on it regulates this process, and these nitrites get converted into nitrates.  Nitrobactor regulates this process.

4.         Denitrification:  The process in which nitrate, nitrogen gets converted into nitrogen gas, is called as denitrification.  Denitrifying bacteria like pseudomonas denitrificans controls this process.  It is an anaerobic process.  The nitrogen present in the atmosphere converts into ammonia or nitrate by physical or biological nitrogen fixation and enters into the soil.

In this form NH₄⁺ or NO₃^--N  is absorbed by plants and plants convert it into organic nitrogen by Immobilization.  This organic nitrogen is in the form of amino acid and proteins in the plants, which enter into animals through food chains.  In the form of different animals and plants, it enters into the soil, or after death, it enters into the soil.  Here, Ammonifying bacteria degrade it and change it into NH₃.  This NH₃ gets oxidized and forms NO₃^--N.

This NO₃^--N changes into N₂ by denitrifying bacteria, which enters into the atmosphere in the form of gas.  Or NO₃^--N enters into the underground water by the process of leaching.

Sulphur Cycle

Sulphur is an important compound for plants and animals.  It is found in some amino acids like cytosine, methionine etc.  It is also an important constituent of proteins, hormones and vitamins.  Sulphur cycle is partially a sedimentary cycle, whose most of the parts runs in the form of sediments, while SO₂ and SO₃ are found in the atmosphere in the form of H₂S gas.  Hence, in the soil and sediments, its large reservoir pool is found and in small reservoir, it is in the form of sediments.  Following steps are involved in this :

1.         Immobilization:  In this process, inorganic Sulphur gets converted into organic Sulphur, which is called as immobilization.  Green plants regulate this process.

2.         Minralization:  In this process organic Sulphur gets converted into inorganic Sulphur.  This process takes place in the presence of microbes.

3.         Reduction-Oxidation:  In this process, SO₂ or SO₃ gets reduced in the form of H₂S or H₂S gets oxidized in the form of SO₂ or SO₃.

In Sulphur cycle, sediments play the major role.  Due to microbial activity, organic Sulphur gets converted into H₂S and SO₂ or SO₃, which being water-soluble represents upward movement, which can be absorbed by plants.  This process is called as microbial recovery.  This recovery is taking place mainly in the form of SO₂ or SO₃.

Similary, SO₂ and SO₃ are produced due to combustion of fossil fuels.  Volcano activation is the other source of SO₂.  This SO₂ form SO₃ in the atmosphere by oxidation, which mixes with rainy water to form H₂SO₄.

This H₂SO₄ gives SO₄^-- ions, which later on enters the soil and form the salts in the soil.  Thus, Sulphur again reaches back into the soil in the form of SO₄^-- from the atmosphere.

            Organic S                  →        H₂S

            2H₂S + 3O₂                →        2H₂O + 2SO₂

            O₂ + 2SO₂                 →        2SO₃

            H₂O + SO₃                 →        H₂SO₄

            H₂SO₄                        →        2H + SO₄^-

            Ca⁺⁺ + SO₄^-               →        CaSO₄

Plants in the ionic form as S^- or SO₄^-, which is known as fixed Sulphur form, while SO₂ and SO₃ are gaseous form, which cannot be absorbed by plants, also absorb Sulphur.  Mainly, Sulphur cycle depends on erosion, sedimentation, leaching, rain adsorption like physical process and production and decomposition like biological process.

Phosphorus Cycle

It is the simplest biogeochemical cycle.  Mainly, it is related with lithosphere and hydrosphere, and atmosphere plays a negligible role in this cycle.  Actually, phosphorous is present in the form of PO₄^-3.  It is called inorganic form.  A large amount of phosphorous is found in the form of sedimentary deposit, which is 1000 times more than the soil and ocean.  Mainly, the flow of phosphorous takes place in between the soil and ocean.

Mainly living organisms take the inorganic form present inside the soil and after it is converted into organic phosphorous by the process of biosynthesis.  But after the death of organism or after the excretion, dead organic matter enters into the soil, where it is converted into inorganic phosphorous by microbial activity.  During rain, this organic or inorganic phosphorous reaches in the water and it enters into the ocean by the flow of river.  In ocean, dead organic phosphorous decomposes due to microbial activity, and when this inorganic phosphorous is present in upper part of ocean, then it gets absorbed by living organisms, but when it enters into the deep ocean, then its sedimentation takes place, and then it forms the phosphate rocks.

Hence, it is clear that very small amount of phosphorous takes part in this cycle.  Thus, its larger amount is present in ocean or in soil.  Its quality is very less in fresh water. Similarly, amount of phosphorous in the different biomass is very less.  Although more amount of phosphorous is present in aquatic biomass as compared to terrestrial biomass.

It means that maximum part of phosphorous is found in lithosphere and major part between or among the available P is soluble in the ocean, which is absorbed by marine plants and animals, excreted in the ocean itself.  But this phosphorous is taken out in the form of ocean plant and animal by the human activity, which are used as weeds.  These are also used as fertilizers and on the land, if phosphorous enters into the plants and animals or fertilizers are made from phosphate rocks and these fertilizers enter from insects into the soil, among which is maximum part gets deposited. Thus, phosphorous cycle is related only with lithosphere and hydrosphere.