paper : 12 module 32 ecosystem: ecosystem processes -i

ZOOLOGY Principles of Ecology

Ecosystem: Ecosystem Processes-I (Part-4)

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Paper : 12 Principles of Ecology

Module : 32 Ecosystem: Ecosystem Processes-I (Part-4)

Development Team

Paper Coordinator: Prof. D.K. Singh Department of Zoology, University of Delhi

Principal Investigator: Prof. Neeta Sehgal Department of Zoology, University of Delhi

Content Writer: Dr. Kapinder Kirori Mal College, University of Delhi

Content Reviewer: Prof. K.S. Rao Department of Botany, University of Delhi

Co-Principal Investigator: Prof. D.K. Singh Department of Zoology, University of Delhi



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Description of Module

Subject Name ZOOLOGY

Paper Name Principles of Ecology: Zool 012

Module Name/Title Ecosystem

Module Id M32: Ecosystem: Ecosystem Processes-I (Part-IV)

Keywords Biogeochemical cycles, Nutrient cycles, Gaseous cycle,

sedimentary cycle, Sulfur cycle, Phosphorous cycle.

Contents

1. Learning Outcomes

2. Introduction

3. Types of biogeochemical cycle

4. Sedimentary cycle

4.1. Phosphorous cycle

4.1.1. Impact of human activities on phosphorous cycle

4.2. Sulfur cycle

4.2.1. Impact of human activities on sulfur cycle

5. Summary



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1. Learning Outcomes

After studying this module, you shall be able to

Explain nutrient cycle and biogeochemical cycles.

List the major types of biogeochemical cycles.

Explain sedimentary cycles such as Phosphorus and sulfur cycle.

Understand impact of human activities on these sedimentary cycles.

2. Introduction

All living organisms are made up of various chemical elements present in the nature. Out of

all elements occurring in the nature, between 30 and 40 are known to be required by all living

organisms (essential elements). There are certain elements like carbon, hydrogen, oxygen,

and nitrogen are required by organisms in large amount while other elements are required by

organisms in small or in minute quantities. Irrespective of the quantitative need, all essential

elements show definite cycles. The non essential elements which are less closely coupled

with the organism are also flow along with essential elements either in water cycle or because

of their chemical affinity with them.

The biological community operates as a complex processor through which organisms move

nutrients from one place to another within the ecosystem. These biological exchanges of

nutrients interact with physical exchanges and for this reason nutrient cycles can also be

considered as biogeochemical cycle. Bio refers to living organisms and geo refers to earth.

Geochemistry is related with chemical composition of earth and the exchange of various

elements between different parts of earth crust, its atmosphere, and water bodies like sea,

lake, river etc. The concept of the geochemistry is given by Russian Polynov (1937) which

can be explained as the role of chemical elements in the production and decomposition of all

materials with special emphasis on weathering. Biogeochemistry was founded by Russian

V.I. Vernadskij (1926) which involves the study of exchange of materials between biotic and

abiotic components of the ecosphere.



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Figure.1: General pattern of nutrient cycles.

The figures 1 demonstrate the general pattern of nutrients cycles on a global scale. All

nutrient cycles are closed on a global scale but they are open at the local scale. The individual

elements that form the cycle are imperishable and can be recycled by plants and animals. It is

very important to understand and measure the global nutrients cycles because these cycles are

continuously shifting due to anthropogenic activities with possible effects on global climate.

Thus, analysis of nutrient cycling ends with an assessment of human impact on nutrient

cycles and their consequences for animals and plants. Global nutrient cycles correspond to

the summation of local events occurring in different biotic communities. Thus, to understand

the global nutrient cycle, the study must begin at the level of local community.

All nutrients are located in the compartments which represent a distinct space in nature.

These compartments can be defined broadly or very specifically. A compartment is made up

of certain quantity of nutrients present in the standing crop. For example, in lake ecosystem,

Volatiles

bioelements only Evaporation

Marine

food web

Dead organic

matter

H2O and volatile biochemicals

Sinking

OCEAN

Precipitation

Dead organic

matter

Terrestrial

food web

Bioelements

in solution

Loss by water runoff

Decomposition

Weathering Terrestrial biosphere

Volatiles

Bioelements only

Death Uptake



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the phosphorous dissolved in water forms one pool and the phosphorous which is present in

the bodies of consumers is another pool.

The compartments exchange nutrients and thus it is important to measure the uptake and

outflow of nutrients for each compartment. The rate of movement of nutrients between two

compartments is known as flux rate and it is measured as the quantity of nutrients moving

from one pool to another per unit time. The flux rate and pool size together constitutes

nutrient cycle for any particular ecosystem. The ecosystems are not isolated from each other

and nutrients move from one ecosystem to another through meteorological, geological or

biological transport mechanism. Meteorological input includes dissolved matter in the rain

and snow, atmospheric gases and dust particles carried by the wind, geological input includes

weathering and subsurface drainage and biological inputs consists of movements of animals

between different ecosystems.

3. Types of Biogeochemical Cycles

From the view point of the ecosphere as a whole, biogeochemical cycles fall into two basic

groups:

1) The gaseous cycle in which the reservoir is in the atmosphere or hydrosphere (ocean) and

2) Sedimentary or mineral cycle in which the reservoir is in the lithosphere i.e. earth crust.

The gaseous cycle has already been discussed in the previous module. In this module we will

learn about sedimentary biogeochemical cycles.

4. Sedimentary or mineral cycle

4.1. Phosphorous cycle

Phosphorus is an important constituent of cell membrane, phospholipids, nucleic acid, bone,

and teeth etc. which make it important to both plants and animals. The reservoir of

phosphorus is present in rock and other natural phosphate deposits (figure 2). These

reservoirs slowly erode and release phosphate from the rocks and minerals by the process of

weathering, erosion, leaching and mining. Only a small fraction of the total phosphorus in the



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soil is available to plants. It occurs naturally as phosphate (PO4¯), either as soluble inorganic

phosphate ions, soluble organic phosphate, as particulate phosphate (insoluble organic or

inorganic molecules) or as mineral phosphate (mineral grain present in rock or sediment).

Phosphate, present in the soil, is absorbed by plants through their roots and incorporated into

tissues. From autotrophs, phosphorus is moved along the grazing food chain with excess

phosphate is excreted trough feces. For example, guano deposits of birds on the desert west

coast of South America. The detritivores in the detritus food chain degrade large organic

molecules containing phosphate and released inorganic ionic phosphate. This form is

immediately absorbed by autotrophs or it incorporated into sediment. The sedimentary phase

remains comparatively slow than the organic phase.

Figure.2: The global phosphorus cycle.

Soil erosion and leaching of dissolved forms release phosphate into the rivers or lakes. Some

phosphate precipitated in the lake sediments whereas, majority of phosphate escapes into the

oceans. In marine and freshwater ecosystems, phytoplanktons absorb organic phosphates



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which are being eaten by zooplankton and detritivores. Zooplanktons may excrete as much

phosphorus daily as it stores in its biomass, returning it to the cycle. It is estimated that more

than half of the phosphorus excreted by zooplanktons are taken up by phytoplanktons. The

remaining phosphorus in organic compounds is used by bacteria, which fail to regenerate

large amount of dissolved inorganic phosphate. Some part of the phosphate is deposited in

the shallow sediments and some is lost to the deep sediments. During the process of ocean

upwelling, the movement of deep waters to the surface brings small amount of phosphates

from the depths to shallow water, where light is available to phytoplanktons for doing

photosynthesis. When the organisms die and sink to the bottom, the level of phosphate at

surface of water became depleted. During upwelling of deep water brings some amount of

phosphorus to the surface of water. Through the uplifting of sediments and harvesting of

fishes from sea, the phosphorus is returned back to land from sea. According to one estimate,

through the fish we consume 60,000 tons of elementary phosphorous returns annually. Sea

birds, by depositing their fecal material (guano deposits), on land also significantly

contributes to returning of phosphorus into the cycle. But this is insufficient to compensate

for the loss from the land to the sea. On a geological timescale, uplifting and subsequent

weathering return this phosphorus to the active cycle (figure 3).

Major reservoir of Phosphorus is found in rocks which releases phosphorus by the

process of weathering. This is carried to the soil by water or air as inorganic

phosphate.

Inorganic phosphate is absorbed and assimilated by plants. In most soils, only 0.01%

of phosphorus is available of the total phosphorus in soil. From autotrophs, organic

form of phosphorus moves through the food chain.

The dead organic matter is acted upon by the phosphatising bacteria to release

inorganic phosphorus from bound organic form. Phosphorus is also lost by runoff

water in deep-ocean sediments.

Phosphorus is returned from shallow marine deposits in fish harvest and guano

deposits of fish eating birds and geological uplift.

The precipitation of phosphorus in marine habitats restricts the primary productivity.

The lake which consists of limited amount of phosphorus forms the oligotrophic lake.



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Figure. 3: Diagrammatic representation of phosphorous cycle.

4.1.1. Impact of human activities on phosphorous cycle

1) Large quantities of phosphate are mobilized by extracting it from rocks for the

manufacturing of fertilizers and detergents. The yield of crops can be increased only

by the use of the fertilizers, so the consumption of these fertilizers will further

increase in future.

2) Deforestation also causes loss of available phosphate from the soil.

3) Release of sewage waste, runoff of fertilizers from soil and animal waste disrupt the

functioning of aquatic ecosystems (causes eutrophication).

4) The use of phosphorous at the present rate will diminish the reserves within 60-160

years.



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4.2. Sulfur cycle

Like nitrogen, sulfur also forms an important constituent of proteins and amino acids and is

characteristic of organic compounds. It exists in various forms like sulfur, sulfide, sulfur

monoxide and sulfates. Sulfate (SO4) is the most common and important biological form

which is utilized by the plants to incorporate into proteins, sulfur being an essential

constituent of some amino acids like cystine. Sulfur is required by ecosystem in small amount

for the growth of plants and animals. Nevertheless, the sulfur cycle is the key one in the

general pattern of production and decomposition. The sulfur cycle has both sedimentary

phase and gaseous phase. The sedimentary phase of sulfur cycle is tied up in the inorganic

and organic deposits. The sulfur is released from these deposits by the process of weathering

and decomposition which is carried to the terrestrial and aquatic ecosystems. The sulfur cycle

is less pronounced in the gaseous phase and it allows circulation of sulfur on a global scale.

The sulfur enters into atmosphere through combustion of fossil fuels, volcanic activities,

decomposition process and exchange from surface of the seas. It is released into the

atmosphere as hydrogen sulfide (H2S) and reacts promptly with oxygen to form sulfur

dioxide (SO2). It is soluble in water and is carried back to the earth surface through rain as

weak sulfuric acid (H2SO4).

Figure.4: The global sulfur cycle.



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Whatever its source, sulfur in a soluble form, frequently as sulfate (SO4) is absorbed by plant

roots where it is incorporated into various organic molecules like proteins and certain amino

acids (cystine). From these autotrophs, the sulfur is transferred to the consumer level with

excess being excreted in the fecal matter. These excretory materials and dead plants and

animals carrying sulfur releases back into the soil and to the bottoms of ponds, lakes and seas,

acted upon by detritivores (bacteria and fungus).

One group of bacteria called sulfur bacteria reduces hydrogen sulfide (H2S) to elementary

sulfur which is then oxidizes into sulfuric acid. In the presence of light, some green and

purple bacteria use hydrogen sulfide (H2S) during photosynthesis. The purple bacteria which

are found in salty marshes and in the mudflats of estuaries, can transform hydrogen sulfide

into sulfate. Green bacteria can transform hydrogen sulfide into elemental sulfur.

In an aerobic condition, the hydrogen sulfide is oxidized to sulfate by certain bacteria which

are used by the autotrophs. In an anaerobic environment such as bottom of lakes, oxidation

process cannot be occurred due to absence of oxygen. However, in the presence of infrared

radiation, some photosynthetic bacteria can oxidize sulfide into elementary sulfur or sulfate.

The elementary sulfur can also be utilized by other bacteria to form sulphate. Under

anaerobic conditions, elementary sulfur can also be converted into sulfate by certain bacteria

in the presence of nitrate. Under certain conditions, sulfate can also be reduced into sulfide or

sulfur by bacteria.



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Figure. 5: The fate of elementary sulfur in sulfur cycle.

These numbers of reaction in the organic phase of the sulfur cycle provides a mechanism for

regulating the availability of elementary sulfur to the autotrophs (figure 5). If iron is present

in the sediments, sulfur will precipitate as ferrous sulfide (FeS2) in an anaerobic condition.

FeS2 is insoluble in neutral and acidic pH and it is firmly held in mud and wet soil.

Some ferrous sulfide is present in the sedimentary rocks called pyritic rocks may overlying

coal deposits. The FeS2, when exposed to the air in deep and surface mining, oxidizes and in

the presence of water produces ferrous sulfate (FeSO4) and sulfuric acid (H2SO4). In this way,

sulfur present in pyrite rocks, abruptly exposed to weathering by humans, discharges heavy

slugs of sulphur, sulphuric acid, ferric sulphate and ferrous hydroxide into aquatic

ecosystems. These compounds devastate aquatic life and decreases pH of water (acidic).

Consequently, the routes of sulfur cycle can be divided into following main steps:

Sulfur is present in the nature as elementary sulfur or sulfides and sulfates. The fossil

fuels and volcanic activities release H2S and SO2 gas in atmosphere which eventually



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returns to the soil as sulphuric acid along with rain, forming sulfate compounds by the

activities of various bacteria.

The elementary sulfur present in the rocks is also converted into sulfate by weathering

process and is also released into the soil. Sulfur in the form of sulfate is absorbed by

plants and form various organic molecules.

The sulfate utilized by the plants is transferred to consumers of the food chain. After

the death of plants and animals as well as their waste materials are decomposed by

detritivores into sulfur. The elementary sulfur is converted into sulfate by certain

bacteria which is again available for the plants and cycle continues.

4.2.1. Impact of human activities on sulfur cycle

As in the case of other biogeochemical cycles, sulfur cycle is also being affected by human

activities, such as air pollution, mining etc.

1) The oxides of sulfur are toxic and constituted about one third of the industrial air

pollutants discharged into the air. Burning of sulfur containing coal, automobile

exhausts and oil to produce electric power increases the concentration of these

oxides (SO2 and SO4) into the air which are also damaging to photosynthesis.

2) The oxides of sulfur and nitrogen interact with water vapour to produce sulfuric

and nitric acid (H2SO4 and H2NO3) that falls to earth surface in the form of acid

rain. It has greatest impact on fresh water bodies like lakes or streams and acidic

soils that lacks pH buffers (such as carbonates, calcium salts etc.).

3) The conversion of sulfur containing metallic mineral ores into free metals like

copper, lead, and zinc releases large amount of SO2 into the environment.

4) Refining of sulfur containing petroleum to make gasoline, heating oil etc. also

releases large amount of SO2 into the atmosphere.

5. Summary

All living organisms are made up of various chemical elements such as C, H, N, O, present in

the nature. Out of all elements occurring in the nature, 30 to 40 elements are known to be



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required by living organisms. The biological community operates in a complex manner in

which organisms move nutrients from one place to another within the ecosystem. These

biological exchanges of nutrients interact with physical exchanges and for this reason nutrient

cycles are also considered as biogeochemical cycle. The ecosystems are not isolated from

each other and nutrients move from one ecosystem to another through meteorological,

geological or biological transport mechanism.

From the view point of the ecosphere as a whole, biogeochemical cycles can be divided into

two basic categories, gaseous cycle in which the reservoir is in the atmosphere or

hydrosphere and sedimentary cycle in which the reservoir is in the lithosphere.

The sedimentary cycle includes phosphorus and sulfur cycle. Phosphorus is an important

constituent of cell membrane, phospholipid, nucleic acid, bone and teeth etc. The reservoir of

phosphorus is present in rock and other natural phosphate deposits which releases phosphorus

by the process of weathering. This is carried to the soil by water or air as inorganic phosphate

which is absorbed and assimilated by plants. From autotrophs, organic form of phosphorus

moves through the food chain. Dead organic matter is acted upon by the phosphatising

bacteria to release inorganic phosphorus from bound organic form. Phosphorus is returned

from shallow marine deposits in fish harvest and guano deposits of fish eating birds and

geological uplift.

Like nitrogen, sulfur also forms an important constituent of proteins. Sulfate (SO4) is the

most common and important biological form which is utilized by the plants to incorporate

into proteins. Sulfur is present in the nature as elementary sulfur or sulfides and sulfates. The

fossil fuels and volcanic activities release H2S and SO2 gas in atmosphere which eventually

returns to the soil as sulfuric acid along with rain, forming sulfate compounds by the

activities of various bacteria. The elementary sulfur present in the rocks is also converted into

sulfate by weathering process and is also released into the soil. The sulfate utilized by the

plants is transferred to consumers of the food chain. When plants and animals die, their body

is decomposed by detritivores and free sulfur is released. The elementary sulfur is converted

into sulfate by certain bacteria which is again available for the plants and cycle continues.

paper : 12 module 32 ecosystem: ecosystem processes -i

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