CHAPTER – 11
Ecology
Definition
Ecology is the scientific study of different relationships with each
other and with their non-living environment which supports them.
Levels of Organisation of Ecology
Starting with individual organism,
there are several levels at which interactions between the organism and their
environment can be studied. These levels of study are
1. Population
2. Community
3. Ecosystem
4. Biomes
5. Biosphere
1. Population
A group of organisms of same species
living and interacting in the same area is known as population. In other words
members of the same species in an ecosystem constitutes a population.
Population ecology mainly deals with factors that affect population size and
composition.
2. Community
The next level of ecological
organization is a community. It consists of a group of organisms (plants and
animals) belonging to different species which interact in a particular area. In
other words members of different species in an ecosystem form a community.
Community ecology studies various interaction such as predation and competition
among different species.
3. Ecosystem
It is higher level of ecological
study and includes both abiotic factors and community of species of a
particular area. Ecosystem is the basic structural and functional unit of
ecology. It consists of a community of living organisms which interact with
each other and also with their non-living environment to maintain equilibrium
of life. Ecosystem ecology deals with study of the flow of energy and cycling
of material between biotic and abiotic components of a particular area. The
ultimate source of energy for any ecosystem is the sun. Examples of a ecosystem
are a pond, a lake, ocean, forests, desert etc.
4. Biomes
Biomes are assemblage of large and
similar ecosystem which cover vast geographical areas of the earth. For example
coniferous forest biome includes all the comferous forest ecosystems which
extend across the Europe, Asia and North American continents. The desert biome
includes an extensive desert area scattered over many regions of the earth.
Biome can also be described as a large easily recognizable community unit which
is spread over vast geographical areas. Although a biome includes many species
of plants and animals, it is primarily identified by its plant life and is
named after a predominant vegetation, known as the population. For example
forest biome or grass land biome. A biome is also named after some
environmental factor. For example ram forest or temperate grass land.
5. Biosphere
All the ecosystem and biomes
collectively form the biosphere. In other words, the entire portion of planet
earth where life is present is called biosphere. Biosphere consists of the
earth from its surface to about few meter depth, all the water and the gaseous
capsule surrounding the earth. The surrounding air envelop extends to about two
mile above the surface of earth. Beyond the biosphere, no living organism can survive.
Ecosystem
Ecosystem is the basic structural and
functional unit of ecology. It consists of a community of living organisms
which interact with each other and also with their non-living environment, to
maintain equilibrium of life. An ecosystem regulates the flow of energy and
cycling of material between its living (biotic) and non-living (abiotic)
components. The ultimate source of energy for any ecosystem is the sun. There
is no limits or boundaries on the size of an ecosystem. It may be as small as a
bottle containing some Protozoa and Algae or as large as an ocean. Examples of
an ecosystem are a pond, a lake, ocean, forest, desert etc. The term ecosystem
was first coined by Tanslley in 1935.
Components of Ecosystem
An ecosystem is made up of two types
of components. Non-living or abiotic and living or biotic.
Abiotic Components of Ecosystem
Abiotic components consists of
non-living material and energy. These components include every thing that
surrounds and individual organism except other living organism.
Abiotic components are basically of
two types; Material and Energy. The material may be both organic and inorganic
and include carbon, nitrogen, water mineral, carbohydrates, proteins etc. The
energy is in the form of light, heat or chemical energy obtained by breakdown
of chemical bonds.
The abiotic components are controlled by abiotic factors which can be divided into three categories
The abiotic components are controlled by abiotic factors which can be divided into three categories
1. Climate Factors
2. Topographic Factors
3. Edaphic Factors
1. Climate Factors
Light
Light is the ultimate source of energy for an ecosystem and plays an important role in the lives of both plants and animals. Plants use light for photosynthesis. In terrestrial ecosystems, light is not a limiting factor because only about 3% of available light is used for photosynthesis. However, in deep and turbid waters, light can be a limiting factor for photosynthesis. Light affects organisms in three ways; Intensity of light, its duration or photoperiod and its quality (wave length).
Light is the ultimate source of energy for an ecosystem and plays an important role in the lives of both plants and animals. Plants use light for photosynthesis. In terrestrial ecosystems, light is not a limiting factor because only about 3% of available light is used for photosynthesis. However, in deep and turbid waters, light can be a limiting factor for photosynthesis. Light affects organisms in three ways; Intensity of light, its duration or photoperiod and its quality (wave length).
Intensity of
Light
The intensity of light depends upon season,
latitude, angle of incidence and time of day. Plants are classified according
to their requirement of light and shade into heliophytes and sciophytes.
Intensity of light plays important role in plants. In plants intense light may
destroy the chlorophyll. It also affects opening and closing of the stomata,
permeability of the cell membrane, height and surface area of plant etc. In
animals light affects development of pigments.
Photoperiod
Duration of light (photoperiod) affects flowering, leaf fall and other aspects of plant growth. In animals biological rhythms such as reproductive cycles sleep cycles, hibernation, migration, are affected by light duration.
Duration of light (photoperiod) affects flowering, leaf fall and other aspects of plant growth. In animals biological rhythms such as reproductive cycles sleep cycles, hibernation, migration, are affected by light duration.
Quality of
Light
Out of seven colours of visible light
(electromagnetic spectrum), chlorophyll absorbs only red and blue light for
photosynthesis. Light of short wave length such as ultraviolet rays is harmful
for life. Most of the light reaching the earth is converted to heat and thus
light also affects the temperature of the ecosystem.
Temperature
Environmental temperature affects the structure and distribution of organisms because of its effects on biological processes. The living organism can function only between temperatures of 0⁰C and 50⁰C and beyond that range they die. Temperature of water usually does not show vast variations therefore aquatic life possess very few adaptions related to temperature. However, most land animals have adaptions to face sudden changes in temperature.
Environmental temperature affects the structure and distribution of organisms because of its effects on biological processes. The living organism can function only between temperatures of 0⁰C and 50⁰C and beyond that range they die. Temperature of water usually does not show vast variations therefore aquatic life possess very few adaptions related to temperature. However, most land animals have adaptions to face sudden changes in temperature.
Water
Water is most important ecological factor because it is essential for life and directly effects the physiology distribution and structural adaptations of both animal and plants. It acts as a limiting factor in terrestrial ecosystems. The availability of water in an ecosystem depends on factors such as rain, snow, type of the soil, kind of vegetation etc. Water is the raw material for photosynthesis and also plays part in many vital biochemical reactions, It determines the quality and quantity of flora and fauna of an ecosystem. In areas where there is more rain forest are formed and where less water is available deserts are found. Therefore, both plants and animals show many adaptations with regard to water. The water is continuously recycled in nature in the form of hydrological cycle.
Water is most important ecological factor because it is essential for life and directly effects the physiology distribution and structural adaptations of both animal and plants. It acts as a limiting factor in terrestrial ecosystems. The availability of water in an ecosystem depends on factors such as rain, snow, type of the soil, kind of vegetation etc. Water is the raw material for photosynthesis and also plays part in many vital biochemical reactions, It determines the quality and quantity of flora and fauna of an ecosystem. In areas where there is more rain forest are formed and where less water is available deserts are found. Therefore, both plants and animals show many adaptations with regard to water. The water is continuously recycled in nature in the form of hydrological cycle.
Atmosphere
and Wind
Atmosphere is a gaseous cover around
the earth. The main gaseous component of the atmosphere are CO₂,
oxygen, nitrogen and water vapour. The CO₂ is essential for photosynthesis;
oxygen is essential for respiration; while nitrogen is necessary for protein
synthesis. Therefore, most of the components of atmosphere are linked with
bio-geo-chemical cycles.
Wind is the circulation an the moving atmosphere. The velocity of the wind can affect the growth and distribution of vegetation, particularly trees. Plants in exposed areas with high winds show adaptations like presence of strong stem and roots. The dispersal of seeds and spores in plants and migration of birds is affected by the winds. Strong winds increase the rate of transpiration and can create xerophyic conditions.
Wind is the circulation an the moving atmosphere. The velocity of the wind can affect the growth and distribution of vegetation, particularly trees. Plants in exposed areas with high winds show adaptations like presence of strong stem and roots. The dispersal of seeds and spores in plants and migration of birds is affected by the winds. Strong winds increase the rate of transpiration and can create xerophyic conditions.
Fire
Natural fires are mainly caused by lightning, volcanic activity, mutual friction between tree branches etc. It brings about sudden changes in an ecosystem by destroying many species of plants and animals. Fire also have some positive effects. For example they release and recycle many nutrients. In many areas of the world forests have been changed to grass lands due to effect frequent fires. Fire also favours growth of some fungi. These fungi are called pyrophilus fungi.
Natural fires are mainly caused by lightning, volcanic activity, mutual friction between tree branches etc. It brings about sudden changes in an ecosystem by destroying many species of plants and animals. Fire also have some positive effects. For example they release and recycle many nutrients. In many areas of the world forests have been changed to grass lands due to effect frequent fires. Fire also favours growth of some fungi. These fungi are called pyrophilus fungi.
2. Edaphic Factors
The factors relating to condition and
composition of soil are known as edaphic factors. The scientific study of soil
is called pedology. The soil is a layer of materials lying above the rocks of
earth’s crust. It forms a link between the non-living and living components.
The type of soil determines the kind of plants which can grow well in a
particular ecosystem. The structural components of the soil are inorganic
material, organic material and soil water. Formation of soil depends upon two
processes
a. Weathering: It is the breakdown of rocks into fine particles of minerals.
a. Weathering: It is the breakdown of rocks into fine particles of minerals.
3. Topographic Factors
Topography refers to the physical
features of land of an area and includes such components as shape of the surface,
altitude, slope exposure to wind etc. Topographic can influence many climatic
factors and thus plays an important role in ecosystem.
i. Altitude
Higher altitude generally have lower
temperature higher rainfall, increased wind speed, more light or exposure to
sun etc. Therefore vegetation at different altitude is different and show
distinct zonation.
ii. Slope
Steep slope have fast drainage and
therefore the soil becomes thinner and cannot retain water. Consequently,
mountains usually show xerophylic vegetation. In northern hemisphere, south
facing slopes get more sunlight than north facing slopes and therefore, there
is marked difference in the vegetation.
iii.
Direction of Mountains
Mountains and their direction affect
the climate by changing the direction of the rain laden wind. The windward side
of mountain have more rainfall than the other side.
Biotic Components of Ecosystem
The living organisms in an ecosystem
constitute its biotic component and are collectively known as the community. On
trophic or nutritional basis, biotic components are divided into two types
i. Autotrophs
Which can synthesis their own complex
organic foed from simple inorganic molecules. Green plants and some bacteria
fall in this category.
ii.
Heterotrophs
Which cannot synthesis complex
organic food and dependent on autotrophs or other organisms for their
nourishment. Animals, fungi and many bacteria are heterotrophs.
Ecological
Succession
Succession is an orderly and
progressive change in a community structure of an ecosystem over a long period
of time. Such changes mostly take place due to change in environment or due to
some disturbance such as fire, volcanic eruption, flood, movement of glaciers
etc. These disturbances remove the existing community and the disturbed area
may be colonized by a variety of new species. The new species, in turn may
gradually be replaced by still other species over a period of time. The
replacement process goes on till a relatively stable community is developed.
Thus during succession the community passes through many predictable
transitional stages called seres. The first established community is called
pioneers or primary community and the final relatively stable and mature
community is known as the climax community. The word succession was used by
Hult for the first time for orderly changes in community. Two main types of
successions are
1. Primary Succession
2. Secondary Succession
1. Primary Succession
If the succession begins in a
lifeless area where no living organism is present and where soil has not yet
been formed, it is called primary succession. In other words the community is
formed from a scratch. For example succession on a new volcanic island or on
newly formed sand done, is a primary succession. Primary succession require
thousands of venus to form a climax community.
2. Secondary Succession
It occurs in places where an existing
community has been renowned by some disturbance such as fire or other climatic
factors. In such a place there may not be any living organisms but the soil
remains intact. As a result the secondary succession is more rapid than primary
succession. The new communities developed are different from the pressure ones.
Depending on the habitat, where the succession begins, the succession is
divided into
i. Hydrosere: Succession occurring in
water
ii. Xerosere: Succession occurring on
dry soil or dry terrestrial habitats such as rocks, sand dunes, etc.
iii. Mesosere: Succession on land but
where adequate moisture is available.
i. Hydrosere
OR Hydrach
Primary succession starting in open
water such as ponds, lakes etc. which are ultimately converted to land
community is called hydrosere. There are several seral stages based on dominant
plant types. The seral stages of hydrosere in a pond are
a.
Phytoplankton Stage
The first invaders are phytoplanktons
such as green algae (spirogyra), blue green algae (eynobacteria), diatoms etc.
Since the pioneer community is phytoplankton, this seral stage is called the
phytoplankton stage. Later on Protozoans such as Amoeba, Euglena etc also
invade the aren.
b. Submerged
Stage
The death and decay of planktons make
the soil suitable for growth of rooted hydrophytes such as Hydrilla, Vallisnaria
etc. Animal life at this stage is blue fish, sun fish.
c. Floating
Stage
The death and decay of submerged
hydrophytes and accumulation of sediments from surrounding area decrease the
water level. Therefore pond becomes suitable for growth of rooted plants with
floating leaves. These plants include Nymphea, Nelumbium etc. Some free
floating plants are also seen. These plants lose water through transpiration
and consequently the water level is very much reduced. Animal life is
represented by Hydra, snails, frogs.
d. Reed Swamp
Stage
This stage is also called amphibious
stage. due to death and decay of plants, pond becomes more and more shallow.
Plants are rooted but part of their bodies is above water level. Examples of
such plants are Typha, Sagittaria. Animals are insects, water spiders, giant
bugs.
e. Sedge
Meadow Stage
The water level continuously
decreases and the substratum becomes more suitable for growth. The stem is
usually in the form of rhizome. Transpiration decreases the water level more
and more and finally the marshy soil is exposed. The oxygen of the air oxidises
the nutrients of the wet soil to form nitrates and sulphates. Thus the soil
fertility is increased plants of this stage are Juneus.
f. Woodland
Stage
The water of the soil evaporates
rapidly and the soil becomes more and more drier. The marshy vegetation is
replaced by land plants such as Salix, a shrub and populs, a tree. Animals are
terrestrial like Zebra, Deer etc.
g. Climax
Stage
The climax community is represented
by trees and thus forests are formed. If there is heavy rainfall in the area,
as in tropical regions, rain forests are developed. In areas with moderate rain
fall, mixed forests are developed.
ii. Xerosere
This type of succession starts on a
bare rock on land where there is deficiency of water and organic matter. The
serial stages xerosere are
a. Crustose
Lichen Stage
A bare rock does not possess moisture
and organic matter, but some special types of lichens the crustose lichens, can
survive on such bare surfaces and form the pioneer species. These lichens can
produces acids which help in breaking the rocks and therefore start formation
of the soil on other species. The lichens of this stage are Rhizo-carpon etc.
b. Foliage
Lichen Stage
These lichens are large and leaf like
in structure. They have ability to absorb moisture from the air and retain it.
They can bind the loose soil particles together and thus the soil becomes more
rich. Lichen of this stage are Pasmelia etc.
c. Mass Stage
Due to death and decay of the
lichens, the organic matter is deposited in the soil which becomes richer in
nutrients and can hold water also. As result plants like mosses develop. They
can absorb water from the soil through rhizoids. Plants of this stage are
polytrichum, selaginella etc.
d. Herb Stage
Due to more availability of moisture,
humus and better anchorage, small herbaccous plant develop. In the beginning
annual herbs are formed but later on perennial herbs develop. Some larger
animals such as nematodes, ants and other insects also invade the area.
e. Shrub
Stage
The conditions become suitable for
growth of larger plants (shrubs). They shadow the herbs which die off adding
more humus to the soil. Large number of animals such as snails, millipedes,
some amphibians, reptiles and mammals like shrews, squirrels , foxes some birds
are also found.
f. Climax
Stage
More and more humus is deposited in
the soil due to death and decay of plants and animals. This allows formation of
woody trees. Thus final climax community is formed. The type of forest will
depend on the climate conditions.
Approaches to
Ecology
Autecology
(Population Approach)
It is the ecological study related to
a single species (population). For example if you study effects of pollution on
a single mango tree or all the mango trees in an area, it will be autecology.
Synecology
(Community Approach)
It is the ecological study related to
different species (community). In synecology you have to consider together the
individual and the community. The type of ecological study has development the
concept of succession and climax that is the progressive replacement of one
community by more stable community in a particular area.
Ecosystem
Approach
This approach takes into
consideration the whole ecosystem as a unit. Ecosystem approach studies the
biotic and the abiotic components their interaction, the flow of energy and the
cycling of materials between the living and the non living and the non living
components. This approach may take into consideration a small ecosystem such as
a pond or a large ecosystem such as a desert.
Habitat
Approach
Habitat is a particular place or area
with a specific set of physical conditions in which a community of living
organisms live. These organisms possess special adaptations which help them to
survive in that habitat. The flora and launa of different habitat are different
because of different physical conditions. Since defining habitat is relatively
easy, some scientists use this approach for ecological studies. Some ecologists
divide the ecology on the basis of type of habitat in which the organisms live.
Habitat is divided in two main types; aquatic and terrestrial. Aquatic habitat
is of three types; fresh water habitat, marine habitat and estuarine habitat.
Estuaries are places where rivers and streams meet the sea. Terrestrial ecology
can be divided into forests, desert, grass land habitat.
Evolutionary
Approach
The distribution of organisms is
result of long term evolutionary changes and their interaction with
environment. Evolutionary approach tells us about changes since life
originated. This knowledge comes from study of fossil record. Various evolutionary
theories are also part of this approach.
Historical
Approach
It deals with study of various stages
and periods of time through which humans have passed during their evolution.
One aspect of this approach is to study the development of tools in human
history. The first period is called stone age or Neolithic period (1500 to 3000
B.C), during which humans used tools made up of stones.
Interdependence
of Organism
(Interactions in an ecosystem)
The organisms in an ecosystem not
only interact with their physical environment but also with each other. These
interactions may be beneficial or harmful.
Positive
Interactions
In this type of interaction organisms
form associations for mutual benefit or for benefit of one species without
harming the other. Mutualism and commensalisms belong to this type.
Negative
Interactions
In this type of interaction one
organism gets the benefit and the other is harmed. Parasitism, predation and
grazing are considered to be harmful interactions.
The interactions between the organisms lead to close associations between them. There are two basic types of associations:
The interactions between the organisms lead to close associations between them. There are two basic types of associations:
Intraspecific
Association
Associations between members of the
same species. These associations are found in social animals for social
organizations or for formation of social groups.
Interspecific
Associations
Associations between different
species. In these associations, members of one species feed on members of
another species and may also use them as habitat. Following are the types of
interspecific associations
1. Parasitism
Parasitism is an association between
two living organisms in which one lives in or on the body of the other and
derives nourishment from it. In other words, one gets the benefit and the other
is harmed. The organism getting the benefit is called the parasite and the one
which is harmed is called the host. There are following types of parasites
Ectoparasites
They live on the surface of the body of the host. Examples, leech, lice, etc.
They live on the surface of the body of the host. Examples, leech, lice, etc.
Endoparasites
They live in the tissues or cavities of the host. Examples Plasmodium, Tnenia, Entamoeba.
They live in the tissues or cavities of the host. Examples Plasmodium, Tnenia, Entamoeba.
2. Symbiosis
Generally the symbiosis is defined as
an association between two living organisms of different species for mutual
benefit or in which one partner gets the benefit and the other is neither
benefited nor harmed. In other words none of the two partners is harmed. There
are two varieties of symbiosis
a. Commensalism
b. Mutualism
a.
Commensalism
It is an association between two
living organism of different species in which one is benefited and the other is
not affected, that is it is neither benefited nor harmed. Following are some
examples
Epiphytes are plants that grow on trunks and branches of trees for
support only but can synthesis their own food. In htis way the epiphyte is
benefited without harming the tree. Epiphytes are common in tropical rain
forests. Orchids and mosses are common exampels.
Small Fishes live in the cloaca of sea cucumber for protection only. No
harm is done to sea cucumber.
b. Mutualism
It is an association between two
species of living organisms for mutual benefit. The two partners are dependent
on each other for their survival and growth. The organisms in this association
may be two animals, two plants or a plant and an animal. Example of mutualism
are
Nitrogen
Fixing Bacteria
They live in the nodules of
leguminous plants. The bacteria fix atmospheric nitrogen for the plant and in
return the plant provides shelter for the bacteria.
Lichens
It is an association between green algae and fungus. The algae cells are protected by the fungus and derive moisture from it, while the fungus obtains oxygen and carbohydrates from the algae.
It is an association between green algae and fungus. The algae cells are protected by the fungus and derive moisture from it, while the fungus obtains oxygen and carbohydrates from the algae.
Sea Anemone
and Hermit Crab
The sea anemone attaches itself to
shell of hermit crab. The anemone gets free transport while the crab is
camouflaged by the anemone and is protected from its predators.
3. Predation
In this relationship one population
called predators feeds on the members of another population known as prey.
Normally predators are larger than the prey and quickly capture and kill the
prey. Predators are usually animals bet a few like pitcher plant and other
carnivorous plants are plant predators. Mostly the predators are secondary or
tertiary consumers of an ecosystem.
The prey-predators interaction keeps the population of both the prey and the predator in equilibrium. Any species of a prey without its natural predator, disturbs the balance of the ecosystem due to explosive increase of their population. The degree of harm caused by this interaction, depends upon the relative population of the prey and predators. For example Ospreys (a kind of hawk) fed on fishes of the ocean. The supply of fishes is so large that this interaction does not cause much harm. However any disturbance in the relative number of prey and predator may result in extinction of any one or both.
The prey-predators interaction keeps the population of both the prey and the predator in equilibrium. Any species of a prey without its natural predator, disturbs the balance of the ecosystem due to explosive increase of their population. The degree of harm caused by this interaction, depends upon the relative population of the prey and predators. For example Ospreys (a kind of hawk) fed on fishes of the ocean. The supply of fishes is so large that this interaction does not cause much harm. However any disturbance in the relative number of prey and predator may result in extinction of any one or both.
4. Grazing
Grazing is a feeding association
between herbivores and plants. Natural herbivores include rabbits, deer,
zebras, wild horses, elephants etc. Man has also introduced cattle and sheep in
the grass land both direct and indirect effects on the ecosystem. Moderate
grazing is not harmful, but the over grazing greatly surfaces the growth
capacity of grasses and other plants. Consequently all the producers are
eliminated and green pastures are turned into barren lands. The indirect effect
of overgrazing is hardening of the soil and soil erosion. The plant roots and
under ground stem of grasses bind the soil together. Their destruction by
grazing makes the soil loose which erodes easily. Too many animals trampling
the soil makes it hard and poorly aerated. This may lead to conversion grass
lands into deserts. Grasses have many adaptation to save them from effects of
grazing.
Biogeochemical
Cycles
(Cycles in Ecosystem)
There is a constant recycling of
essential nutrients within the biosphere from environment to organism and back
to environment. Since the path of recycling involves living organism (bio),
soil (geo) and chemical reaction these cycles or circular paths are called
biogeochemical cycles. The elements which are recycled include Carbon,
Hydrogen, Oxygen, Nitrogen, Phosphate and many others. Carbon, Hydrogen and
Oxygen are closely associated with each other. Therefore, they form a single
carbo, hydrogen, oxygen cycle, but the nitrogen and phosphorus form independent
cycles.
Nitrogen Cycle
The nitrogen is an essential part of
protoplasm, proteins, nucleic acids, chlorophyll etc and is therefore,
necessary for living organisms. The exchange of nitrogen between soil and
atmosphere and its circulation through bodies of living organisms takes place
through inter-related processes known as nitrogen cycle.
The atmosphere contains about 78 percent nitrogen gas. It is an inert gas and the organisms cannot use it directly. Therefore, nitrogen is first changed to soluble nitrogen compounds such as nitrates (NO₃) which the plants can absorb from the soil. Conversion of atmospheric nitrogen to soluble nitrogen compounds is called nitrogen fixation.
The atmosphere contains about 78 percent nitrogen gas. It is an inert gas and the organisms cannot use it directly. Therefore, nitrogen is first changed to soluble nitrogen compounds such as nitrates (NO₃) which the plants can absorb from the soil. Conversion of atmospheric nitrogen to soluble nitrogen compounds is called nitrogen fixation.
Source of Nitrates
There are three sources of nitrates
for living organisms
i. Nitrogen Fixation by Thunderstorm
ii. Nitrogen Fixation by Living
Organisms
iii. Decomposition of Dead Organic
Material
i. Nitrogen Fixation By Thunderstorm
The first source of nitrates is
thunderstorm and lightening. Nitrogen can combine with atmospheric oxygen at
very high energy level. The lightening during rains provides this energy which
is used to combine atmospheric nitrogen and oxygen to form nitrogen oxides.
These oxides dissolve in water to form nitric acid which falls down into the
soil and react with mineral ions there to forms nitrates.
ii. Nitrogen Fixation by Living Organisms
The second source of nitrates are
bacteria and cynobacteria (blue, green, algae) which can covert free
atmospheric nitrogen into soluble nitrates and nitrates. There are two types
a. Symbiotic
Nitrogen Fixation
Symbiotic bacteria, such as
Rhizobium, live in the nodulated roots of legumes such as bean, pea etc. These
bacteria have the ability to absorb free atmospheric nitrogen and convert it to
organic compounds such as nitrates which enter the soil.
b.
Non-Symbiotic Nitrogen Fixation
It takes place by the activity of
free living soil bacteria like clostridium and Azobaeter and blue green algae
like Nostoc etc. They absorb free nitrogen from air and convert it to nitrates
of the soil.
Nitrates from
Decomposition
The third source of nitrogen for
organisms is soil nitrates from decomposition of dead organisms and excretory
wastes. This process occurs in two steps
i. Ammonification
ii. Nitrification
i. Ammonification
The dead animals, plants and
excretory waste is decomposed by the saprophytic bacteria and fungi to form
simple compounds like amino acids, water and carbondioxide. The amino acids are
then changed to ammonia and ammonium ions. This process is called
ammonification.
ii. Nitrification
Ammonia and ammonium, formed as a
result of ammonification is converted to nitrites and nitrates by a process
known as nitrification. It takes place by the activity of two groups of
bacteria together called the nitrifying bacteria. Nitrite bacteria, such as
Nitrosomonas, first convert ammonia to nitrites. The second group the nitrate
bacteria such as Nitrobacter, then convert nitrites into nitrates.
The nitrates from all the above
sources are taken up from the soil by the plants and are used for formation of
plant proteins. These proteins are then taken up by animals as food and
converted to animals protein. The dead plants, animals, the excretory waste is
decomposed and the nitrates are recycled as described a above.
Denitrification
The loss of soil nitrogen is called denitrification and mostly takes place through the activity of denitrifying bacteria such as Pseudomonas. They convert nitrites and nitrates back to molecular nitrogen and return it to atmosphere. These bacteria are anaerobic and use nitrates as oxidizing agent instead of oxygen. Therefore, in well acrated soils their activity is restricted and soil nitrogen is not lost. The lost nitrogen of the soil is made up by fixation of nitrogen from atmosphere.
The loss of soil nitrogen is called denitrification and mostly takes place through the activity of denitrifying bacteria such as Pseudomonas. They convert nitrites and nitrates back to molecular nitrogen and return it to atmosphere. These bacteria are anaerobic and use nitrates as oxidizing agent instead of oxygen. Therefore, in well acrated soils their activity is restricted and soil nitrogen is not lost. The lost nitrogen of the soil is made up by fixation of nitrogen from atmosphere.
0 comments :
Post a Comment