Biogeochemical cycles and Ecological succession
- Energy flow and nutrient circulation are the major functions of the ecosystem.
- Energy is lost as heat forever in terms of the usefulness of the system. On the other
hand, nutrients of food matter never get used up and recycle again and again
- Carbon, hydrogen, oxygen, nitrogen and phosphorus as elements and compounds
makeup 97% of the mass of our bodies and are more than 95% of the mass of all living organisms.
- In addition to these, about 15 to 25 other elements are needed in some form for the
survival and good health of plants and animals.
- These elements or mineral nutrients are always in circulation moving from non-living to living and then back to the non-living components of the ecosystem in a more or less circular fashion. This circular fashion is known as biogeochemical cycling (bio for a living; geo for atmosphere).
- Among the most important nutrient cycles are the carbon nutrient cycle and the
nitrogen nutrient cycle.
- There are many other nutrient cycles that are important in ecology, including a large
number of trace mineral nutrient cycles.
1. Another name of nutrient cycling is biogeochemical cycles (bio: living organism, geo:
rocks, air, water).
2. Nutrient cycles are of two types: (a) gaseous and (b) sedimentary.
3. Environmental factors, e.g., soil, moisture, pH, temperature etc., regulate the rate of
release of nutrients into the atmosphere.
4. Movement of nutrients from the environment into plants and animals and again back to
the environment is essential for life and it is the vital function of the ecology of any
5. Among the most important nutrient cycles are the carbon nutrient cycle and the
nitrogen nutrient cycle. Both of these cycles make up an essential part of the overall soil
The reservoir for a gaseous type of nutrient cycle (e.g., nitrogen, carbon cycle) exists in
the atmosphere. the most important gaseous cycles are – water, carbon and nitrogen.
1. Water cycle (Hydrologic)
- All of we know the importance of water. But water also plays important role in
transportation of nutrients in nutrient cycle and acts as a solvent medium for uptake of nutrients by organisms.
2. Carbon cycle
- Carbon is a minor constituent (0.038%) of the atmosphere as compared to oxygen (21%) and nitrogen (78%) and argon (0.9%).
- Without carbon dioxide life is not possible because it is vital for the production of
carbohydrates through photosynthesis by plants.
- It is the element that anchors all organic substances from coal and oil to DNA
(deoxyribonucleic acid: the compound that carries genetic information).
- Carbon is present in the atmosphere, mainly in the form of carbon dioxide (CO2).
- In the composition of living organisms, carbon constitutes 49 per cent of dry weight of organisms and is next only to water.
- Globally 71% of carbon is found dissolved in oceans. This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere (see image below). In deep oceans such carbon can remained buried for millions of years till geological movement may lift these rocks above sea level. These rocks may be exposed to erosion, releasing their carbon dioxide, carbonates and bicarbonates into streams and rivers.
- The atmosphere only contains about 1% of total global carbon.
- Fossil fuel – it is also a reservoir of carbon.
- Carbon cycling occurs through atmosphere, ocean and through living and dead
- Photosynthesis – photosynthesis fix 4 × 1013 kg of carbon annually.
- A considerable amount of carbon returns to the atmosphere as CO2 through respiratory activities of theproducers and consumers.
- Decomposers also contribute substantially to CO2 pool by their processing of waste
materials and dead organic matter of land or oceans.
- In form of Sediments – Some carbon also enters a long term cycle. It accumulates as undecomposed organic matter inthe peaty layers of marshy soil or as insoluble
carbonatesin bottom sediments of aquatic systems which take a long time to be
- Additional sources which releases CO2 in the atmosphere – Burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity.
- Human activities have significantly influenced the carbon cycle.
- Rapid deforestation and massive burning of fossil fuel for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere. (more in Greenhouse effect).
3. Nitrogen cycle
- Apart from carbon, hydrogen and oxygen, nitrogen is the most prevalent element in living organisms.
- Nitrogen is a constituent of amino acids, proteins, hormones, chlorophylls and many of the vitamins.
- Plants compete with microbes for the limited nitrogen that is available in the soil. Thus, nitrogen is a limiting nutrient for both natural and agricultural ecosystems.
- Nitrogen exists as two nitrogen atoms (N2) joined by a very strong triple covalent bond (N ≡ N).
- In nature, lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (NO, NO2, N2O).
- Industrial combustions, forest fires, automobile exhausts and power generating stations are also sources of atmospheric nitrogen oxides (NO). (Diesel engine emits NO 20 times more than a petrol engine)
Nitrogen Fixing – Nitrogen to Ammonia (N2 to NH3)
- There is an inexhaustible supply of nitrogen in the atmosphere but the elemental form cannot be used directly by most of the living organisms.
- Nitrogen needs to be ‘fixed’, that is, converted to ammonia, nitrites or nitrates, before it can be taken up by plants.
- Nitrogen fixation on earth is accomplished in three different ways:
o By microorganisms (bacteria and blue-green algae),
o By man using industrial processes (fertilizer factories) and
o To a limited extent by atmospheric phenomena such as thunder and lighting.
- Certain microorganisms are capable of fixing atmospheric nitrogen into ammonia (NH3) and ammonium ions (NH4+). Ammonia is a molecule consisting of nitrogen and hydrogen having molecular NH3, while
ammonium (NH4+) is an ion of ammonia that is formed by accepting a hydrogen ion.
- The enzyme, nitrogenase which is capable of nitrogen reduction is present exclusively in prokaryotes. Such microbes are called N2-fixers. These include:
1. free living nitrogen fixing bacteria (non-symbiotic nitrogen-fixing bacteria or nitrogen fixing soil bacteria) (e.g.aerobic Azotobacter and Beijemickia; anaerobic Clostridium and Rhodospirillum),
2. symbiotic nitrogen-fixing bacteria (e.g.Rhizobium) living in association with
leguminous plants and non-leguminous root nodule plants and
3. some cyanobacteria (a major source of nitrogen fixation in oceans)(blue-green algae.
E.g. Nostoc, Anabaena, Spirulina etc.).
Leguminous: denoting plants of the pea family (Leguminosae), typically having seeds in pods, distinctive flowers, and root nodules containing nitrogen-fixing bacteria.
Nitrification – Ammonia to Nitrates
Ammonium ions can be directly taken up as a source of nitrogen by some plants.
Others absorb nitrates which are obtained by oxidizing ammonia and ammonium ions.
Ammonia and ammonium ions are oxidized to nitrites or nitrates by two groups of
- 1. Ammonium ions are first oxidized to nitrite by the bacteria Nitrosomonas
2. The nitrite is further oxidized to nitrate with the help of the bacterium
These steps are called nitrification. These nitrifying bacteria are chemoautotrophs.
The nitrate thus formed is absorbed by plants and is transported to the leaves.
In leaves, it is reduced to form ammonia that finally forms the amine group of amino
acids, which are the building blocks of proteins. These then go through higher trophic
levels of the ecosystem.
Nitrification is important in agricultural systems, where fertilizer is often applied as
ammonia. Conversion of this ammonia to nitrate increases nitrogen leaching because nitrate is more water-soluble than ammonia.
Nitrification also plays an important role in the removal of nitrogen from municipal
wastewater. The conventional removal is nitrification, followed by denitrification.
Ammonification – Urea, Uric Acid to Ammonia
- Living organisms produce nitrogenous waste products such as urea and uric acid
- These waste products, as well as dead remains of organisms, are converted back into inorganic ammonia and ammonium ions by the bacteria. This process is called
- Some of this ammonia volatilizes and re-enters the atmosphere but most of it is
converted into nitrate by soil bacteria.
Denitrification – Nitrate to Nitrogen
Nitrate present in the soil is reduced to nitrogen by the process of denitrification.
In the soil as well as oceans there are special denitrifying bacteria (Pseudomonas and
Thiobacillus), which convert the nitrates/nitrites to elemental nitrogen. This nitrogen
escapes into the atmosphere, thus completing the cycle.
Step 1: N2 Fixing → Nitrogen → Ammonia or Ammonium Ions
Step 2: Nitrification →Ammonia or Ammonium Ions → Nitrite → Nitrate
Step 3: Ammonification→ Dead Matter + Animal Waste (Urea, Uric Acid) → Ammonia or
Dead Matter + Animal Waste (Urea, Uric Acid) → Ammonia or Ammonium Ions [most of it
escapes into the atmosphere. Rest is Nitrified(Step 2) to nitrates]
Nitrate [some of it is available for plants. Rest is Denitrified(Step 4)]
Step 4: Denitrification – Nitrate → Nitrogen.
The amount of Nitrogen fixed by man through the industrial process has far exceeded the
amount fixed by the Natural Cycle.
As a result, Nitrogen has become a pollutant which can disrupt the balance of nitrogen.
It may lead to acid rain, Eutrophication and Harmful Algal Blooms.
- The sedimentary cycle’s (e.g., sulphur and phosphorus cycle) reservoir is located in Earth’s crust.
- It follows a basic pattern of flow through erosion, sedimentation, mountain building, volcanic activity and biological transport through the excreta of marine birds.
1. Phosphorous cycle
- Phosphorus plays a central role in aquatic ecosystems and water quality.
- Unlike carbon and nitrogen, which come primarily from the atmosphere, phosphorus occurs in large amounts as a mineral in phosphate rocks and enters the cycle from erosion and mining activities.
- This is the nutrient considered to be the main cause of excessive growth of rooted and free-floating microscopic plants (phytoplankton) in lakes [Eutrophication].
- The main storage for phosphorus is in the earth’s crust. On land, phosphorus is usually found in the form of phosphates.
- By the process of weathering and erosion, phosphates enter rivers and streams that
transport them to the ocean.
- In the ocean, phosphorus accumulates on continental shelves in the form of insoluble deposits.
- After millions of years, the crustal plates rise from the seafloor and expose the
phosphates on land.
- After more time, weathering will release them from rock and the cycle’s geochemical
phase begins again.
2. Sulphur cycle
- The sulphur reservoir is in the soil and sediments where it is locked in organic (coal, oil and peat) and inorganic deposits (pyrite rock and sulphur rock) in the form of
sulphates, sulphides and organic sulphur.
- It is released by weathering of rocks, erosional runoff and decomposition of organic
matter and is carried to terrestrial and aquatic ecosystems in a salt solution.
- The sulphur cycle is mostly sedimentary except two of its compounds – hydrogen
sulphide (H2S)and sulphur dioxide (SO2) which add a gaseous component.
- Sulphur enters the atmosphere from several sources like volcanic eruptions,
combustion of fossil fuels (coal, diesel etc.), from the surface of the ocean and from gases released by decomposition. Atmospheric hydrogen sulphide also gets oxidized into sulphur dioxide.
- Atmospheric sulphur dioxide is carried back to the earth after being dissolved in
rainwater as weak sulphuric acid.
- Whatever the source, sulphur in the form of sulphates is taken up by plants and incorporated through a series of metabolic processes into sulphur bearing amino acid which is incorporated in the proteins of autotroph tissues. It then passes through the grazing food chain.
- Sulphur bound in a living organism is carried back to the soil, to the bottom of ponds and lakes and seas through excretion and decomposition of dead organic material.
Biotic communities are dynamic in nature and change over a period of time. The process
by which communities of plant and animal species in an area are replaced or changed
into another over a period of time is known as ecological succession.
It is the process of change in the species structure of an ecological community over
time. The time scale can be decades (for example, after a wildfire), or even millions of
years after a mass extinction.
The community begins with relatively few pioneering plants and animals and develops
through increasing complexity until it becomes stable or self-perpetuating as a climax
The ʺengineʺ of succession, the cause of ecosystem change, is the impact of established
species upon their own environments.
The first plant to colonize an area is called the pioneer community.
The final stage of succession is called the climax community.
Successional stages or sere
The stage leading to the climax community is called successional stages or series.
Succession is characterized by the following – increased productivity, the shift of
nutrients from the reservoirs, increased diversity of organisms with increased niche
development, and a gradual increase in the complexity of food webs.
Succession would occur faster in area existing in the middle of the large continent. This
is because, here seeds of plants belonging to the different series would reach much
faster, establish and ultimately result in the climax community.
Succession that occurs on land where moisture content is low for e.g. on the bare rock is known as xerarch. Succession that takes place in a water body, like ponds or lake is
- Primary succession takes place an over a bare or unoccupied areas such as rocks
outcrop, newly formeddeltas and sand dunes, emerging volcano islands and lava flows as well as glacial moraines(muddy area exposed by a retreating glacier) where no community has existed previously.
- The first inhabitants are lichens or plants—those that can survive in such an environment.
- Over hundreds of years these “pioneer species” convert the rock into soil by secreting acids to dissolve rock, helping in weathering and soil formation that can support simple plants like bryophytes, which are able to take hold in the small amount of soil.
- These plants further modify the soil, which is then colonized by other types of plants.
- Each successive stage modifies the habitat by altering the amount of shade and the
composition of the soil.
- The final stage of succession is a climax community, which is a very stable stage that can endure for hundreds of years.
- In primary succession in water, the pioneers are the small phytoplanktons, they are replaced with time by free-floating angiosperms, then by rooted hydrophytes, sedges, grasses and finally the trees.
- The climax again would be a forest. With time the water body is converted into
- Secondary succession follows a major disturbance, such as a fire or a flood.
- The stages of secondary succession are similar to those of primary succession;
however, primary succession always begins on a barren surface, whereas secondary
succession begins in environments that already possess soil.
- In addition, through a process called old-field succession, farmland that has been
abandoned may undergo secondary succession.
- An example of Secondary Succession by stages:
1. A stable deciduous forest community
2. A disturbance, such as a wild fire, destroys the forest
3. The fire burns the forest to the ground
4. The fire leaves behind empty, but not destroyed, soil
5. Grasses and other herbaceous plants grow back first
6. Small bushes and trees begin to colonize the area
7. Fast growing evergreen trees develop to their fullest, while shade-tolerant trees
develop in the understory
8. The short-lived and shade intolerant evergreen trees die as the larger deciduous trees overtop them. The ecosystem is now back to a similar state to where it began.
- When succession is brought about by living inhabitants of that community itself, the process is called autogenic succession.
- It is driven by the biotic components of an ecosystem.
- Change brought about by outside forces is known as allogenic succession.
- Allogenic succession is driven by the abiotic components of the ecosystem.
Auto·trophic and Heterotrophic succession
- Succession in which, initially the green plants are much greater is quantity is known as autotrophic succession.
- Succession in which the heterotrophs are greater in quantity is known as heterotrophic succession.
- Succession would occur faster in area existing in the middle of the large continent. This is because here all propagules or seeds of plants belonging to the different series would reach much faster, establish and ultimately result in the climax community.