Paper No. : 12 Principles of Ecology
Module : 31: Ecosystem: Ecosystem Processes-I (Part-3)
Development Team
Paper Coordinator: Prof. D.K. Singh
Department of Zoology, University of Delhi Principal Investigator: Prof. Neeta Sehgal
Head, 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
Description of Module
Subject Name ZOOLOGY
Paper Name Principles of Ecology Module Name/Title Ecosystem
Module Id 31: Ecosystem: Ecosystem Processes-I (Part-3)
Keywords Biogeochemical cycles, Nutrient cycles, Gaseous cycle, sedimentary cycle, Carbon cycle, Nitrogen cycle, Water cycle, Oxygen cycle.
Contents
1. Learning Outcomes 2. Introduction
3. Types of biogeochemical cycle 3.1. Gaseous cycle
3.2. Sedimentary cycle
4. Hydrological cycle or Water cycle
4.1. Impact of human activities on water cycle 5. Gaseous cycle
5.1. Carbon cycle
5.1.1. Impact of human activities on carbon cycle 5.2. Nitrogen cycle
5.2.1. Impact of human activities on nitrogen cycle 5.3. Oxygen cycle
6. Summary
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.
Understand hydrological cycle and its impact on other biogeochemical cycles.
Explain gaseous cycles such as carbon, nitrogen and oxygen cycle.
Understand impact of human activities on these gaseous cycles.
2. Introduction
There are around of 90 chemical elements present in the nature, out of which 30-40 are known to be required by all living organisms. The chemical elements of life such as carbon, hydrogen, oxygen, nitrogen, calcium, phosphorus and other elements are taken up from the environment which is used to form cellular component of an organism and finally through circuitous route of several other organisms, it returned back to the environment to be used again. These more or less circular pathways of chemical elements of the biosphere between the organism and the environment are called as biogeochemical cycles. As the name indicated, the nutrients are circulated through life (bio) and through earth (geo) repeatedly (cycle). The movement of elements and inorganic compounds which are essential to life is designated as nutrient cycling.
The energy flow is profligate as it is being driven by continuous supply of solar energy, nutrient cycle is conservative as the chemical elements are continuously drawn from limited pool and retained within the ecosystem. The Producers uses large number of basic nutrients from abiotic component of ecosystem. These materials then transformed by producers into biomass which in turn utilized by the consumers and are finally returned back to the environment with the help of decomposers. The biogeochemical cycles are driven by energy flow and are crucial for the maintenance of life on earth and biological processes largely determine the main features of the cycles. In each biogeochemical cycle, the two compartments or pools are recognized:
1) Reservoir pool: The large slow moving non-biological component and
2) Labile or cycling pool: A smaller but active portion that is exchanging (moving back and forth) rapidly between organisms and their immediate environment.
The reservoir pool is also called unavailable pool and the active moving pool as available pool. Many elements have multiple reservoir pool and some (such as nitrogen) have multiple labile pools. The figure 1 explains superimposition of a biochemical cycle on an energy flow diagram to explain cycling of matter through one way flow of energy. It is important to emphasize that energy in some form usually must be expanded to recycle materials- a fact of life to remember when it comes to the increasing need for human to recycle water, metal, papers and other materials.
Figure 1: Biogeochemical cycle (black circle) superimposed on a simplified energy flow diagram. PG= Gross primary productivity, PN= net primary productivity, P=
Secondary productivity, R= Respiration (after E.P. Odum).
Elements are neither homogeneously distributed in the nature, nor they are present in the same form throughout the ecosystem. In figure 1, the part of the cycle which is chemically or physically remote from organisms is indicated by the box labeled “nutrient pool” (the reservoir pool), whereas the cycling portion is designated by the circle passing through autotrophs to heterotrophs and return back. The size of reservoir pools is important when one
is assessing the effect of human activities on biogeochemical cycles. Generally, the smallest pool is the first affected by changes in element fluxes. For example, the amount of total carbon in the biosphere, but a small change in this pool has a very large effect on the temperature of the earth.
Human beings are unique in not only requiring 40 essential elements but also using almost all other elements, including the newer synthetic one. Humans activities have also accelerates the movement of many elements which disrupted the self regulating processes that tend to maintain homeostasis are overwhelmed and nutrient cycles tend to become imperfect or acyclic. For example, human mine and process phosphate rock and with such careless abandon that severe local pollution occurs near mines and phosphate mills. The purpose of conservation of natural resources is to make acyclic processes more cyclic. The recycling of water is good initiation because if the hydrologic cycle can be maintained and repaired, there is a better chance of controlling the nutrients that move along with the water.
3. Types of Biogeochemical cycles
From the point of the biosphere as a whole, biogeochemical cycles can be categorized into two basic groups:
3.1. Gaseous cycle
The gaseous cycle in which the reservoir is in the atmosphere or hydrosphere (ocean).
3.2. Sedimentary cycle
Sedimentary or mineral cycle in which the reservoir is present in the lithosphere i.e. earth crust.
The above classification is based on the input of primary source of nutrient to the ecosystem.
The gaseous cycles like carbon and nitrogen have capacity to quite easily self adjust because of their larger oceanic or atmospheric reservoirs or both. Sedimentary cycles of various elements like phosphorous and sulfur are more liable to disturbed because of the presence of their reservoir in earth’s crust as inactive or immobile form. Although the biogeochemical cycles of various nutrients required by producers and consumers differ in detail, from the perspective of the ecosystem all biogeochemical cycles have a common structure. They share
three basic components: inputs, internal cycling and outputs. Both gaseous and sedimentary cycles involve biotic and abiotic processes which are driven by energy flow and both are tied with the hydrological cycle. Water acts as a medium for the movement of elements and other materials through ecosystem, without which the biogeochemical cycles would cease.
4. Water cycle/ Hydrological cycle
Water is essential for living organisms as it constitutes 70% weight of organisms and it provides medium for biological activities. All organisms, atmosphere and the earth maintain a circulation of water and moisture between them. This cycling of water is called as water cycle or hydrological cycle (figure 2). In other words, the water cycle is the flow of water through various components of the environment, results in the global circulation of water.
Water plays a significant role in the environment for the existence of biogeochemical cycles and functioning of ecosystems. Water is essential for any ecosystem because it is the major constituent of living organisms and it also helps in osmoregulation and thermal regulation of both plants and animals.
The water is not evenly distributed throughout the earth. About 97.3 percent is present in the ocean, about 2.1 percent as ice in the polar caps and permanent glaciers, and the rest is fresh water which is present in the form of atmospheric water vapour, ground water or soil water.
The water cycles rapidly but most of it around 98.7% is blocked in the lithosphere. More than 80 per cent of the total insolation that is not lost immediately as electromagnetic radiation goes to evaporate water. This atmospheric water vapour condenses around dust particles known as nucleation particles. The atmosphere can hold limited amount of water vapour, so the droplets which formed by this means are heavy and fall as precipitation due to force of gravity.
Figure 2: Hydrological cycle
In the hydrologic cycle, the water is evaporated from the surface of ocean which leads to the formation of clouds. When these clouds cool down, they precipitate the water in the form of rain. There are several routes of precipitation of water that falls on land viz. direct evaporation, transpiration, movement of water into ground water system and runoff.
Consequently, the routes of hydrologic cycles can be divided into following main categories.
Evaporation: The surface water of ocean, lakes and other aquatic systems are heated by solar radiation. As a result, water evaporates and converted into water vapour which floats in the air.
Condensation: When the water vapour rises into the air, it slowly cools and condenses to form small droplet of water. These small water droplets accumulate to forms clouds.
Precipitation: The small droplets of water when further condenses and falls on the surface of earth in any form of water like dew, drizzle or rain is called as precipitation.
Runoff: The precipitated rain water accumulates and flows on the surface and sub- surfaces of earth towards rivers, streams or underground stores and finally reaches to oceans. Percolation & infiltration: The process of stored water flowing under earth which merges to the ground water is called percolation and infiltration
Transpiration: The water which directly evaporates from the surface of leaves of plants.
Transpiration acts to move the biogeochemical cycles for all mineral nutrients that enter the food chain via the roots of plants.
Completion of Cycle: All water bodies continue its journey and finally reaches to ocean where they release their water and begins the new water cycle.
Two aspects of the H2O cycle need special emphasis:
1) The amount of water that evaporates from the sea is more than it returns by rainfall, and vice versa for the land. In other words, a significant part of the rain fall that supports land ecosystems comes from water evaporated over the oceans.
2) The human activities like deforestation, compacting agricultural soil etc, increase the rate of runoff, which decrease the recharge of ground water compartment (3rd largest global water reservoir).
4.1. Impact of human activities on water cycle
1) The water cycle is altered by humans by withdrawing large amount of fresh water from lakes, streams, rivers and from underground sources.
2) The lands are cleared by deforestation for practicing agriculture, mining, construction etc. This leads to increase runoff and reduction in the recharge of ground water supply. It further increases the risk of flood and accelerates soil erosion.
3) Various nutrients like phosphate, nitrate and other pollutants are added to water due to human activities. These excessive nutrients and pollutants leads to impaired natural ecological balance which purify water. It is estimated that by the year 2100 humans will used or polluted all freshwater reserves and by the year 2230 humans may have to depend only on precipitation.
5.
Gaseous CyclesThe gaseous geochemical cycles are of following types:
5.1. The carbon cycle
The biosphere carbon cycle is primarily concerned with the atmospheric CO2. The atmospheric carbon pool is relatively smaller than carbon present in oceans, fossil fuels and other storage in the lithosphere (figure 3). The burning of fossil fuels and agricultural wastes in addition to deforestation is the major factor for increasing of atmospheric CO2. In addition to CO2, there are two other forms of carbon namely, CO (carbon monoxide) and CH4
(methane), present in the atmosphere in small amount. Both arise from the incomplete or anaerobic decomposition of organic matter. The carbon is a basic constituent of all organic compounds in the living organisms and a major element in the fixation of solar energy by the process of photosynthesis, is closely coupled to energy flow that are inseparable. The source of fixed carbon present in living organisms and fossil deposits is CO2, found in the atmosphere and dissolved in the waters of the earth.
Figure 3: The global Carbon cycle.
The organic compounds formed by the process of photosynthesis consists of carbon which are stored in plant-tissues and is transfer to next trophic level (herbivores) or retained by the plant until it serves as food for detritivores. Small amount of carbon is returned back to the atmosphere in the form of CO2 through plant respiration during which a considerable part of glucose is oxidized to yield CO2, H2O and energy.
The micro-organisms which decompose dead organic material releases carbon back into the atmosphere. Similarly, the carbon taken up by consumers travels through various routes. It may either assimilated into protoplasm till the organism dies, after that it is utilized by
decomposers or it is released through animal respiration (CO2). In addition to respiration, some amounts of carbon undergo into fermentation process and some amount is stored in the body.
As for the storage of carbon in sediments, just as deposition works to store materials, erosion may uncover them, and inorganic chemical weathering of rock can oxidize the carbon contained there. Some carbon is also stored in sediment which is uncovered by erosion or chemical weathering oxidizes the carbon in the rocks (figure 4). Not all the carbon is uncovered from them, some remains in the rock and it may be replaced by carbon dioxide released from volcanic or other geological activities. Human activities has also accelerates the rate of release of carbon from sedimentary rocks. Small amount of carbon, in the ocean, is also present in the form of carbonates. Calcium carbonate (CaCO3) is commonly used by certain animals such as oysters, snails, some protozoans etc. for construction of their shell.
Figure 4: Sources of carbon (sedimentary, marine and anthropogenic) in carbon cycle.
When these animals die, calcium carbonate of shell may either dissolve or will remain in sedimentary form. There are certain control mechanisms present in the carbon cycle. The rate of carbon utilization is dependent on its availability. If a large amount of carbon is used up in
any one phase of cycle, it results in the inhibition or slower down of the other phases of activity. For example, if the pH of water is high (alkaline) then large amount of carbon is incorporated to form carbonate and less amount is present in the solution. This removal of carbon in solution would disturb the equilibrium which is established between the atmospheric and the dissolved carbon dioxide and it leads to the movement of CO2 into solution until equilibrium established. Consequently, the routes of carbon cycles can be divided into following main steps:
CO2 is released from various natural and anthropogenic sources like volcanic eruptions, burning of forests and fuels, decomposition of carbonates, exhausts released by factories & automobile, respiration by animals and plants. The plants also utilize CO2 during photosynthesis.
The major reservoir for carbon dioxide is present in the oceans which is readily dissolves in water. Small amount of carbon is also present in atmosphere and sedimentary rocks.
Plants take up carbon dioxide and convert it into carbohydrates by the process of photosynthesis.
The animals directly or indirectly depend on the plants for food and when these plants and animals die, they form the reservoir of dead organic matter. Decomposers release it to the atmosphere through respiration.
Peculiarities of carbon cycle: The carbon cycle shows the basic similarity with other biogeochemical cycles but it is unusual in that the organic phase is not essentially a complete cycle within itself. The organic phase and atmospheric phases are so closely entangled that the rapid cycling of the organic phase is present. The various paths along which carbon can flow is typical of biogeochemical cycles which provides a well-buffered system with sufficient feedback mechanisms to assure an adequate supply of carbon. It is important that all phases of the carbon cycle yields carbon dioxide at some time and carbon dioxide acts as raw material for them. Thus, although carbon is present in relative low concentration in the atmosphere (0.03 per cent), carbon in a form in which it can be used by the organisms is almost always present.
5.1.1. Impact of human activities on carbon cycle
1) The burning fossil fuels like coal and wood releases large amount of CO2 into the atmosphere.
2) Cutting of trees and other plants that absorb CO2 faster than they can regenerate themselves cause further increases the CO2 into the atmosphere.
3) The above two activities add more CO2 into the atmosphere which results in the increase of earth’s average temperature. This higher concentration CO2 along with other greenhouse gases influence earth’s natural green house effect which results in the warming the troposphere and surface of earth called global warming. It adversely affects global food production and wildlife habitat. It also changes the temperature and precipitation pattern that raise the average sea level in different parts of the world.
5.2. The nitrogen cycle
The significance of nitrogen lies in the fact that it is the essential part of proteins and nucleic acids which are essential constituents of living organisms and also regulates various biological functions. Nitrogen is an essential component of organic molecules like proteins, pigments, nucleic acids and vitamins. The plants and animals require large amount of nitrogen for their existence and growth. It also forms the major constituent of the atmosphere (79%) in gaseous state which cannot be utilized by the animals unless it is converted into some usable forms. The organisms utilize nitrogen either in the inorganic form such as ammonia, nitrate and nitrites or as organic forms like protein or nucleic acids. Thus, the atmospheric nitrogen can be used only when it has been fixed as one of the inorganic form.
This fixation is done by biological or physiochemical processes. The later process is done by high-energy fixation such as cosmic radiation, meteorite trails, thundershower and lightning in which nitrogen combines with oxygen and hydrogen of water to form ammonia and nitrates. These compounds are carried to the earth surface through rain water. However the fixation of nitrogen by physiochemical processes is very negligible and the major fixing of nitrogen (about 90%) is contributed by biological process (figure 5).
Figure 5: The global Nitrogen cycle.
In biological process, some nitrogen fixing bacteria, fungi and blue-green algae, take molecular nitrogen from the atmosphere and combine it with hydrogen to form ammonia.
This ammonia is directly available to the producers.
Figure 6: The process of ammonification in nitrogen cycle.
Some of the nitrogen-fixing bacteria are free-living present in the soil (Azotobacter and Clostridium), some algae in water (Nostoc, Calothrix and Anabaena) and fixed large quantities of free atmospheric nitrogen. Another group of bacteria called symbiotic bacteria (eg. Rhizobium) lives in the root of leguminous plants (e.g., peas, beans, clover and alfalfa) and fixes atmospheric nitrogen in the agricultural land. Other non-leguminous angiosperms plants like, Alnus, Ceanothus, Shepherdia, Elaeagnus and Myrica also perform the same function.
The bacteria invade the roots and stimulate the formation of root-nodules. The secretion of the plant root helps in multiplying the bacteria. The excretion of bacteria dissolved the cell wall of root hair, through which they enter inside root and multiply to form root nodules. The combination of symbiotic bacteria and cells of root remains able to fix atmospheric nitrogen.
Because of this nitrogen fixing property, leguminous plants are often grown to restore fertility of soil (figure 7).
Figure 7: Root nodules of symbiotic bacteria in leguminous plants.
These bacteria may fix around 50 kg to 100 kg of nitrogen per acre per year, and free soil bacteria can fix up to 12 kg per acre per year. Furthermore, free soil bacteria (Azotobacter and Clostridium) produce ammonia as first stable compound and the process is activated by
molybdenum as an activator whereas, accumulation of nitrates and ammonia in soil inhibit the process.
Certain species of lichens like Collema tunaeforme and Peltigera rufescens are also concerned in nitrogen fixation. These lichens possess nitrogen-fixing blue green algae component. In addition to perform the function of nitrogen fixing, some of the microorganisms like fungus and bacteria (detritivores) degrade nitrogenous wastes and carrion of animals, nitrogen is converted to the amino form (eg. L-Alanine). The amino group (–NH2) is released from organic molecules to form ammonia and this process is known as deamination. Certain bacteria especially from the genus Nitrosomonas, oxidize ammonia to nitrite (NO2—). This reaction can occur in the soil, lake or sediment where ammonia is being released and oxygen is present. As the nitrite is produced in the environment, other bacteria like Nitrobacter, convert nitrite into nitrate (NO3—) in the presence of oxygen. Both of these reactions performed by Nitrosomonas and Nitrobacter are together called as nitrification.
Figure 8: Nitrification process during nitrogen cycle.
In nitrification process, ammonia is oxidized into nitrate and nitrite through which energy is released. This energy is utilized by the bacteria to synthesize organic materials directly from carbon dioxide and water. The action of these nitrifying bacteria is controlled by the
environmental and chemical nature of the soil. Both of these types of bacteria work in alkaline medium. Nitrate is used up by the autotrophs at the beginning of food chain. Under certain conditions nitrate is either not produced in the nitrogen cycle or it is degraded before it can be utilized by the autotrophs. This process of degradation of nitrate is called denitrification and it occurs when oxygen concentration is low.
Figure 9: Degradation of nitrate into free nitrogen by denitrification process.
Denitrifying bacteria like Pseudomonas breakdown the nitrate into nitrite, ammonia, or molecular nitrogen and utilizes the released energy to drive their metabolism. Consequently, the routes of nitrogen cycle can be divided into following main steps (figure 10):
The nitrogen is present in atmosphere in the large amount (78 %) which is fixed either by physical (lightening) or biological process (rhizobium, azotobacter and cyanobacteria).
These microorganisms convert nitrogen into nitrates which is used by plants for biosynthesis of different organic molecules such as amino acids, proteins, vitamins etc. and later passes into the food chain.
When the plant and animals die, the complex nitrogen present in the dead tissues is decomposed by several nitrifying bacteria into ammonia, nitrite and nitrates, which is again taken up by the plants.
Some denitrifying bacteria convert nitrates in to the molecular nitrogen (N2) under anaerobic condition. This free nitrogen is again release into the atmosphere and the cycle continues (figure 11).
Figure 10: Diagrammatic presentation of global nitrogen cycle.
Figure 11: Diagrammatic representation of nitrogen transformation in nitrogen cycle.
5.2.1. Impact of humans activities on Nitrogen cycle
1) The large amount of NO is released into the atmosphere due to burning of any fuel at high temperature. The NO is converted into NO2 gas and HNO3 in the atmosphere and return back as acid rain which is dangerous to humans and other living organisms.
2) The N2O is continuously added into atmosphere due to action of anaerobic bacteria which acts on livestock wastes and commercial inorganic fertilizers added into the soil. This gas is responsible for global warming and also depletes O3 in the stratosphere.
3) The inorganic fertilizers consist of NO3-
that seeps through the soil, reaches into water table and make the water unfit for drinking.
4) Excess nitrates in agricultural land runoff and municipal wastes impair the aquatic ecosystem.
5) The nitrogen is removed from top layer of soil during harvesting nitrogen rich crops, excessive irrigation and burning or clearing of grassland and forest before doing agricultural practices.
6) Enrichment of soil with nitrogen or other nutrients causes the invasion of alien species or the opportunist “weedy type” species that can grow and over compete in high nitrogen conditions.
7) Excess nitrogen when present in water, food or in the air becomes threat to human health. Excess nitrate can occurs in the drinking water due to some exotic legumes.
For example, the introduction of legume tangan-tangan (leucaena leucocephala) from the Philippines after World war-II has poisoned the ground water in much of Guam.
5.3. The oxygen cycle
Oxygen is formed as a by-product of photosynthesis which is involved in the oxidation of carbohydrates to form carbon dioxide, water and energy. The oxygen play important role in biological oxidation as a hydrogen acceptor. Hydrogen is removed from organic molecules by enzymatic activity through a series of reactions which is finally accepted by the oxygen to form water molecule. Although, oxygen is necessary for life but due to its higher reactivity, molecular oxygen may be toxic to living cells. So, to protect the cell from toxic effects of oxygen, the cells bears peroxisomes, a cell organelle which mediate oxidative reactions
resulting in the production of hydrogen peroxide (H2O2). The H2O2 is then used as an acceptor in oxidizing other compounds through the mediation of other enzymes.
The major pool of free oxygen is present in the atmosphere which supports life. There are mainly two sources of atmospheric oxygen.
1) Photodissociation in which most of the hydrogen is released from water vapour and escapes into outer space (figure 12).
Figure 12: Photodissociation of oxygen molecule.
2) Photosynthesis starts only since life began on the earth. The photosynthesis and respiration are cyclic, so it involves both release and utilization of oxygen and seems to balance each other (figure 13).
Figure 13: The global Oxygen cycle.
The main non-living oxygen pool consists of molecular oxygen, water and carbon dioxide.
All these are closely linked to each other in photosynthesis and several other oxidation- reduction reactions. The cycling of oxygen is very complex process (figure 13). As a
component of CO2, it circulates throughout the biosphere. Some of CO2 present in the aquatic system combines with calcium to form calcium carbonates. Oxygen when combines with nitrogen compounds through physical or biological process to form nitrates, with iron to form ferric oxides and with several minerals to form various oxides. In these forms, oxygen remains temporarily out of the cyclic pathway. In photosynthesis, the oxygen atom of water molecule split which again reconstituted into water during respiration of plants and animals.
Some part of the atmospheric oxygen when reaches to troposphere layer, it is reduced to ozone (O3) by high energy ultraviolet radiation. Consequently, the routes of oxygen cycle can be divided into following main steps:
Atmosphere consists of about 21% oxygen. Almost all living organisms required oxygen for deriving energy from oxidation of organic molecules.
When water moves from atmosphere to earth surface and back into hydrological cycle, oxygen is also cycled through the environment. Plants begin the oxygen cycle by producing oxygen through the process of photosynthesis. The animal form the other half of the oxygen cycle, during the respiration.
There is continuous exchange of O2 between the atmosphere and water bodies on the earth. The amount of O2 present in the biosphere is relatively constant, so that the oxygen cycle may get stable (figure 14).
Atmospheric oxygen
Organic molecule C6H12O6
Photosynthesis
Respiration
Carbon dioxide (CO2)
Oxygen cycle
Water (H2O)
Figure 14: Diagrammatic representation of global oxygen cycle.
6. Summary
The chemical elements of life such as carbon, hydrogen, oxygen, nitrogen etc. are taken up by organisms from the environment to form cellular component and finally returned back to the environment to be used again, which forms biogeochemical cycles.It can be categorized into gaseous cycle in which the reservoir is in the atmosphere or hydrosphere and Sedimentary cycle in which the reservoir is present in the lithosphere.
Both gaseous and sedimentary cycles involve biotic and abiotic processes which are driven by energy flow and both are tied with the hydrological cycle.
All organisms, atmosphere and the earth maintain a circulation of water and moisture between them. This cycling of water is called as water cycle or hydrological cycle. In the hydrologic cycle, the water is evaporated from the surface of ocean which leads to the formation of clouds. When these clouds cool down, they precipitate the water in the form of rain which enters into water bodies. These water bodies continue its journey and finally reach to ocean where they release their water and begin the new water cycle.
The carbon cycle is gaseous cycle which primarily concerned with the atmospheric CO2. The atmospheric carbon pool is relatively smaller than carbon present in oceans, fossil fuels and other storage in the lithosphere. CO2 is released from various natural and anthropogenic sources. Plants utilize this carbon dioxide and convert it into carbohydrates by the process of photosynthesis. The animals directly or indirectly depend on the plants for food and when these plants and animals die, they form the reservoir of dead organic matter. Decomposers release it to the atmosphere through respiration.
The nitrogen is the essential part of proteins and nucleic acids which are constituents of living organisms and also regulates various biological functions. It is present in atmosphere in the large amount (78 %) and is fixed either by physical process such as lightening or biological process by several microorganisms like rhizobium, azotobacter and cyanobacteria etc. These microorganisms convert nitrogen into nitrates which is used by plants for biosynthesis of different organic molecules which later passes into the food chain. After the death of plant and animals, the complex nitrogen present in the dead tissues is decomposed by several
nitrifying bacteria into ammonia, nitrite and nitrates, which are again taken up by the plants.
Some denitrifying bacteria convert nitrates in to the molecular nitrogen (N2) under anaerobic condition. This free nitrogen is again release into the atmosphere and the cycle continues.
Oxygen is a gaseous cycle which formed as a by-product of photosynthesis. Almost all living organisms required oxygen for deriving energy from oxidation of organic molecules. When water moves from atmosphere to earth surface and back into hydrological cycle, oxygen is also cycled through the environment. Plants begin the oxygen cycle by producing oxygen through the process of photosynthesis. The animal forms the other half of the oxygen cycle, during the respiration. There is continuous exchange of O2 between the atmosphere and water bodies on the earth. The amount of O2 present in the biosphere is relatively constant, so that the oxygen cycle may get stable.
