Sunday, 9 August 2015

Coral Reefs

Coral reefs are made of tiny animals called “polyps” that belong to a group of animals known as Cnidaria. Polyps have a hard carbonate exoskeletons (outer skeleton) which support and protect the coral polyps. They are connected by living tissue to form a community. Stony corals (scleractinians) are the corals primarily responsible for laying the foundations of, and building up, reef structures

The corals depend upon a symbiotic (endosymbiotic*) relationship with algae-like unicellular flagellate protozoa (zooxanthellae) that are photosynthetic and live within their tissues.

*An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis. Zooxanthellae give coral its coloration.
The zooxanthellae require sunlight for photosynthesis. They provide nutrients to the polyps and reduce the level of carbon dioxide. This makes the conditions for the formation of skeletons more suitable. In turn, the zooxanthellae get a suitable habitat.

The coral species which harbour the zooxanthellae are called hermatypic. Cold water corals lack the symbiotic Dinoflagellates. The species without zoooxanthellae is called ahermatypic.

Not all corals are reef building species. There are also hard corals existing as single, solitary polyps. Some temperate species form small colonies only. Corals that lack the hard outer coverings of calcium carbonate are soft corals.


Corals need to grow in shallow water where sunlight can reach them. Corals depend on the and these algae need sunlight to survive. Corals rarely develop in water deeper than 165 feet (50 meters).

Submarine platform
It should be available at suitable depth (45-55m) so that corals can colonise and grow upwardly and outwardly, to form a massive reef

Clear water
Corals need clear water that lets sunlight through; they don’t thrive well when the water is opaque. Sediment and plankton can cloud water, which decreases the amount of sunlight that reaches the zooxanthellae. Light-absorbing adaptations enable some species to live in dim blue light

Water temperature
Reef-building corals require warm water conditions to survive. Different corals living in different regions can withstand various temperature fluctuations. However, corals generally live in water temperatures of 25–32° C.

Corals need saltwater to survive and they thrive in waters with salinity levels of 27-35 parts per 1000. This is why corals don’t live in areas where rivers drain fresh water into the ocean. Precipitation of calcium necessary for the formation of skeletons. Water temperatures and salinity have to be high and carbon dioxide concentrations have to be low.

Clean water
Corals are sensitive to pollution and sediments. Sediment can create cloudy water and be deposited on corals, blocking out the sun and harming the polyps. Wastewater discharged into the ocean near the reef can contain too many nutrients that cause seaweeds to overgrow the reef (Eutrophication)

Strong wave action 
It ensures: supply of food and oxygen, the distribution of larvae and prevents sediment to settle on the reefs


Fringing reefs 
They grow near the coastline around islands and continents. They are separated from the shore by  narrow, shallow lagoons. Fringing reefs are the most common type of reef.

Barrier Reef
Fringing Reef

Barrier reefs
Barrier reefs also parallel the coastline but are separated by deeper, wider lagoons. At their shallowest point they can reach the water’s surface forming a “barrier” to navigation.  

Difference between Fringing and barrier reefs
1. Barrier reefs have at least some deep portions; fringing reefs do not.
2. Barrier reefs tend to be much farther away from shore than fringing reefs


Atolls are rings of coral that create protected lagoons and are usually located in the middle of the sea. Atolls usually form when islands surrounded by fringing reefs sink into the sea or the sea level rises around them (these islands are often the tops of underwater volcanoes). The fringing reefs continue to grow and eventually form circles with lagoons inside.

Patch reefs

Patch reefs are small, isolated reefs that grow up from the open bottom of the island platform or continental shelf. They usually occur between fringing reefs and barrier reefs. They vary in size, and rarely reach the surface of the water.


1. They are called "rainforests of the sea". Shallow coral reefs form some of the most diverse ecosystems on Earth. They are gene pool centres due to high genetic diversity.

2. They are sensitive bio-indicators. The study of coral reefs is important for providing a clear, scientifically-testable record of climatic events over the past million years or so. 

3.Corals are very important in controlling how much carbon dioxide is in the ocean water. Coral polyp turns carbon dioxide in the water into a limestone shell. Without coral, the amount of carbon dioxide in the water would rise dramatically.

4. They protect coasts from strong currents and waves by slowing down the water before it gets to the shore. They provide a barrier between the ocean and the shore.

5. By providing complex and varied marine habitats, they support a wide range of other organisms.

Example : Fringing reefs just below low tide level have a mutually beneficial relationship with mangrove forests at high tide level and sea grass meadows in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or erode the sediments in which they are rooted, while the mangroves and sea grass protect the coral from large influxes of silt, fresh water and pollutants

6. Reefs are home to a large variety of animals, including fish, seabirds, sponges, cnidarians, worms, crustaceans, mollusks, echinoderms, sea turtles and sea snakes etc

7.They can play a role as nursery ground. The fishing industry depends on coral reefs because many fish spawn there and juvenile fish spend time there before making their way to the open sea

8. Revenue from tourism


Gulf of Kutch
Exclusively consists of fringing reefs. The reefs are relatively less developed due to large range of temperature and high salinity. The entire Gulf of Kutch is also a marine national park.

Exclusively coral atolls with 36 islands of which 10 are inhabited.  

Gulf of Mannar
Fringing reefs with a chain of 21 islands from Rameswaram in the north to Tuticorin in the south. This part of the gulf forms part of the Gulf of Mannar biosphere reserve.

Andaman and Nicobar Islands
Exclusively fringing reefs of about 500 islands


1. Great Barrier Reef, Coral Sea near Australia
2. Red Sea Coral Reef, Red Sea near Israel, Egypt and Djibouti
3. New Caledonia Barrier Reef, Pacific Ocean 
4. Mesoamerican Barrier Reef, Atlantic Ocean near Mexico, Belize, Guatemala and Honduras
5. Florida Reef, Atlantic Ocean and Gulf of Mexico 
6. Great Chagos Bank, The Maldives


Coral bleaching is the loss of intracellular endosymbionts ( zooxanthellae) through either expulsion or loss of algal pigmentation. It occurs when the conditions necessary to sustain the coral's zooxanthellae cannot be maintained.

Coral bleaching is a generalized stress response of corals and can be caused by a number of biotic and abiotic factors, including:

1. increased or reduced water temperatures
2. oxygen starvation caused by an increase in zooplankton levels as a result of overfishing
3. increased solar irradiance (photosynthetically active radiation and ultraviolet light)

4. changes in water chemistry (in particular acidification -Acidification affects the corals' ability to create calcareous skeletons, essential to their survival)
5. increased sedimentation due to silt runoff
6. bacterial infections - Black band disease, Skeletal eroding band , White band disease , White pox disease
7. changes in salinity
8. herbicides
9. low tide and exposure
10. Cyanide fishing - It is a method of collecting live fish mainly for use in aquariums, which involves spraying a sodium cyanide mixture into the desired fish's habitat in order to stun the fish. The practice hurts not only the target population, but also many other marine organisms. Cyanide concentration slows photosynthesis in zooxanthellae, which results in coral reefs losing color; it also eliminates one of their major food sources.
11.elevated sea levels due to global warming 
12. mineral dust from African dust storms caused by drought

While most of these triggers may result in localized bleaching events (tens to hundreds of kilometers), mass coral bleaching events occur at a regional or global scale and are triggered by periods of elevated thermal stress resulting from increased sea surface temperatures.
The coral reefs that are more subject to continued bleaching threats are the ones located in warm and shallow water with low water flow. Physical factors that can prevent or reduce the severity of bleaching are available for the reefs located under conditions that include low light, cloud cover, high water flow and higher nutrient availability

  1. Storms and tidal emersions
  2. El Niño: increased sea surface temperatures, decreased sea level and increased salinity from altered rainfall
  3. Predation by fishes, marine worms, barnacles, crabs, snails and sea stars
  4. Dust outbreaks
  5. Chemical pollution
  6. Land development
  7. Ocean acidification

Friday, 3 July 2015

Environment and Ecology: Part 3

Biogeochemical Cycles Contd.

Oxygen cycle
It describes the movement of oxygen within and between its three main reservoirs: The atmosphere, the biosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis.


Vast amount of molecular oxygen is contained in rocks and minerals within the Earth (99.5%) Only a small fraction has been released as free oxygen to the biosphere (0.01%) and atmosphere (0.49%). The main source of oxygen within the biosphere and atmosphere is photosynthesis, which breaks down carbon dioxide and water to create sugars and oxygen:

6CO2 + 6H2O+ energy → C6H12O6 + 6O2

Atmospheric oxygen also comes from photolysis, where high energy ultraviolet radiation breaks down atmospheric water and nitrous oxide into component molecules. The free H and N atoms escape into space leaving O2 in the atmosphere:

2H2O + energy → 4H + O2
2N2O + energy → 4N + O2

The main way oxygen is lost from the atmosphere is via respiration and decay mechanisms in which animals consume oxygen and releases carbon dioxide.
The lithosphere also consumes free oxygen by chemical weathering and surface reactions. Eg)  formation of iron-oxides (rust)
4FeO + 3O2 → 2Fe2O3

Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere create carbonate shell material (CaCO3) rich in molecular oxygen. When the organism dies, its shell is deposited on the shallow sea floor and buried over time to create limestone rock.

Weathering processes initiated by organisms can also free oxygen from the land mass. Plants and animals extract nutrient minerals from rocks and release oxygen in the process.

NOTE : Factual Data

Storage Capacity of O: Lithosphere > Atmosphere > Biosphere
Residence time of O2      : Lithosphere > Atmosphere > Biosphere

Gain of atmospheric O2 (in descending order of contribution)
  1. Photosynthesis (land)
  2. Photosynthesis (ocean)
  3. Photolysis of N2O    
  4. Photolysis of H2O

Loss of atmospheric O2 (in descending order of contribution)
  1. Aerobic Respiration
  2. Microbial Oxidation
  3. Combustion of Fossil Fuel  
  4. Photochemical Oxidation
  5. Chemical Weathering
  6. Fixation of N2 by Lightning , Surface Reaction of O3
  7. Fixation of N2 by Industry
  8. Oxidation of Volcanic Gases


Ozone or trioxygen(O3), an inorganic molecule, is an allotrope* of oxygen that is much less stable than the diatomic allotrope O2, breaking down in the lower atmosphere to normal dioxygen.

*Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of these elements. Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner.

The highest levels of ozone in the atmosphere are in the stratosphere - ozone layer between about 10 and 50 km above the surface. It is present in low concentrations, nevertheless, the ozone layer is extremely important to modern life, as it absorbs harmful ultraviolet radiation.

1. Creation: Ozone is formed from dioxygen by the action of ultraviolet light and also atmospheric electrical discharges.

O2 + uv energy → 2O

Each oxygen atom then quickly combines with an oxygen molecule to form an ozone molecule:
O + O2 + uv energy → O3

2. Ozone-Oxygen Cycle / Chapman cycle: Ozone molecules formed, absorb radiation having an appropriate wavelength and triatomic ozone molecule becomes diatomic molecular oxygen plus a free oxygen atom
O3 + uv energy → O2 + O

The atomic oxygen produced quickly reacts with another oxygen molecule to reform ozone:
O + O2 → O3 + Heat energy

Thus, the absorbed solar energy also raises the temperature of the atmosphere within the ozone layer, creating a thermal barrier that helps trap the atmosphere below, as opposed to bleeding out into space.

3. Removal: if an oxygen atom and an ozone molecule meet, they recombine to form two oxygen molecules:
3 + O· → 2O2
And if two oxygen atoms meet, they react to form one oxygen molecule:
2O· → O2

The overall amount of ozone in the stratosphere is determined by a balance between production by solar radiation and removal. The removal rate is slow, since the concentration of O atoms is very low.

Ozone Depletion

Ozone depletion is largely a result of man-made substances. Certain free radicals like hydroxyl (OH), nitric oxide (NO) and chlorine (Cl) and bromine (Br), catalyze the recombination reaction, leading to an ozone layer that is thinner than it would be if the catalysts were not present. Most of the OH and NO are naturally present in the stratosphere, but human activities like emissions of chlorofluorocarbons (CFCs) and halons, has greatly increased the Cl and Br concentrations, leading to ozone depletion.

Chlorine, fluorine and carbon atoms are extremely stable. This extreme stability allows CFCs to slowly make their way into the stratosphere (most molecules decompose before they can cross into the stratosphere from the troposphere). This prolonged life in the atmosphere allows them to reach great altitudes where photons are more energetic. When the CFC's come into contact with these high energy photons, their individual components are freed from the whole.

Cl + O3 → ClO + O2
ClO + O. → Cl + O2
Overall reaction : O3 + O. → 2O2

Chlorine initiates the breakdown of ozone and combines with a freed oxygen to create two oxygen molecules. After each reaction, chlorine begins the destructive cycle again with another ozone molecule. One chlorine atom can thereby destroy thousands of ozone molecules. Because ozone molecules are being broken down they are unable to absorb any ultraviolet light so we experience more intense UV radiation at the earths surface. 

Ozone depleting substances (ODSs)

Substances which deplete the ozone layer and are widely used in refrigerators, airconditioners, fire extinguishers, in dry cleaning, as solvents for cleaning, electronic equipment and as agricultural fumigants.

The Montreal Protocol on Substances that Deplete the Ozone Layer - is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances that are responsible for ozone depletion. Ozone depleting substances controlled by Montreal Protocol include:

Chlorofluorocarbons (CFCs) - used in refrigeration, air conditioning and heat pump systems
Halon - used historically as fire suppression agents and fire fighting
Carbon tetrachloride/ Tetrachloromethane (CCl4) - limited solvent use in laboratories and chemical and pharmaceutical industry
Methyl chloroform (CH3CCl3) - limited solvent use in laboratories and chemical and pharmaceutical industry
Hydrobromofluorocarbons (HBFCs) - historically used in fire suppression systems and fire fighting
Hydrochlorofluorocarbons (HCFCs) - used in refrigeration, air conditioning and heat pump systems
Methyl bromide (CH3Br) - historically used in fumigation, soil treatment, pest control, quarantine, market gardening
Bromochloromethane (CH2BrCl) - historically used in the manufacture of biocides.

The Ozone Hole

Refers to the reduced concentrations of ozone directly over the continent of Antarctica, that an enormous hole in the ozone layer had developed over the region (it had begun to develop in the mid 1970s)

Reasons :
As Antarctica is surrounded by water, winds over the continent blow in a unique clockwise direction creating a so called "polar vortex" that effectively contains a single static air mass over the continent. So, air over Antarctica does not mix with air in the rest of the earth's atmosphere. 

Antarctica has the coldest winter temperatures on earth. These chilling temperatures result in the formation of polar stratospheric clouds (PSCs) which are a conglomeration of frozen H2O and HNO3. PSCs form an electrostatic attraction with CFC molecules and other halogenated compounds

During spring, the PSCs melt in the stratosphere and release all of the halogenated compounds that were previously absorbed. In the antarctic summer, high energy photons are able to photolyze the halogenated compounds, freeing halogen radicals that then catalytically destroy O3. As Antarctica is constantly surrounded by a polar vortex, radical halogens are not able to be diluted over the entire globe. The ozone hole develops as result of this process.

In years with a strong polar vortex, the ozone hole is seen to expand in diameter, whereas in years with a weaker polar vortex, the ozone hole is noted to shrink.

Low level ozone / tropospheric ozone

It is an atmospheric pollutant, formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides. Ozone reacts directly with some hydrocarbons and thus begins their removal from the air, but the products are key components of smog.

Ozone photolysis by UV light leads to production of the hydroxyl radical HO•, which plays a part in the removal of hydrocarbons from the air, but is also the first step in the creation of components of smog such asperoxyacyl nitrates, which can be powerful eye irritants.

Ozone acts as a greenhouse gas, absorbing some of the infrared energy emitted by the earth
There is evidence of significant reduction in agricultural yields because of increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species.

The atmospheric lifetime of tropospheric ozone is about 22 days.

Tuesday, 30 June 2015

Evergreen Books for Prelims and Mains examination of UPSC Civil Services

UPSC offers a diverse and distinctive questions from various areas in the prelims and mains examination. However there are certain books are must be relied upon in any case as they are very important for conceptual understanding of the subjects. As UPSC is moving towards more basics and opinion based upon those things, it is necessary to follow these books.

For Modern Indian History
1. India's Struggle for Independence by Bipin Chandra (Click here)
2. Modern Indian history by Bipin Chandra (Click here)
3. Indian Since independence by Bipin Chandra (Click here)

For Indian Culture
1. Indian Culture by Spectrum/ NCERT (Click here)

For Polity
1. Indian Polity by Laxmikant (Click here)
2. Indian Polity by Sriram IAS Academy

For Economics
1. Indian Economy by Ramesh Singh (Click here)
2. Indian Economy by Sriram IAS Academy

For Geography
1. NCERT 11th and 12th (Click here)
2. Certificate Physical and Human Geograhy by Goh Cheng Leong (Click here)
There are various books on ecology and environment and are of much importance looking into the fact that Indian Forest Service exam is same as that of Civil Service. So, there is a need for special emphasis on that.

For Environment and Ecology
1. Shankar IAS notes- As Recommended by many
2. Majid Hussain (Click Here)
3. PD Sharma (CLick Here)

For Science
1. NCERT/ S. Chand of Class X

P.S. : The list of in-comprehensive and is not exhaustive. It is based upon my experience with books. Readers can take guidance from other sources as well.

Friday, 26 June 2015

Environment and Ecology: Part 2


In ecosystems flow of nutrients is cyclical. The nutrients cycle from dead remains of organisms released back into the soil by detrivores which are absorbed again i.e. nutrient absorbed from soil by the root of green plants are passed on to herbivores and then carnivores.

This recycling of the nutrients is called biogeochemical or nutrient cycle (Bio = living, geo = rock, chemical = element). The transfer of matter involves biological, geological and chemical processes; hence the name. These cycles facilitate the transfer of matter from one form to another and from one location to another on planet earth. 

It is a circuit or pathway by which a chemical element (or molecule) moves through both biotic and abiotic compartments of an ecosystem. Abiotic factors - water (hydrosphere), land (lithosphere), and air (atmosphere); the living factors of the planet can be referred to collectively as the biosphere. All chemical elements occurring in organisms are part of biogeochemical cycles.

It thus provides a clear demonstration of the harmonious interactions between organisms and their environment, both biotically and abiotically.
The entire earth or biosphere is a closed system i.e. nutrients are neither imported nor exported from the biosphere. There are two important components of a biogeochemical cycle

(1) Reservoir pool

Though components of the biogeochemical cycle are not completely lost, they can be held for long periods of time in one place. This place, called a reservoir is a place or region or location where a biogeochemical element is in its highest concentration.  Eg) Coal deposits that are storing carbon for a long period of time, atmosphere/rocks which stores large amounts of nutrients.

Influx : difference between the amount of elements entering a reservoir and the amount leaving the reservoir.

(2) Cycling pool /exchange pool/ compartments of cycle

They are relatively short storages of carbon in the form of plants and animals. Eg) plants and animals, which temporarily use carbon in their systems and release it back into a particular reservoir. Carbon is held for a relatively short time in plants and animals when compared to coal deposits. The amount of time that a chemical is held in one place is called its residence time.

Note: Generally, reservoirs are abiotic factors while exchange pools are biotic factors

Elements transported in the biogeochemical cycles are categorized as:

Micro elements - elements required by living organisms in smaller amounts. Eg) boron used mainly by green plants, copper used by some enzymes and molybdenum used by nitrogen-fixing bacteria.
Macro elements - elements required by living organisms in larger amounts. Eg) carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur

Importance of biogeochemical cycles

1. They enable the transformation of matter from one form to another. This transformation enables the utilization of matter in a form specific to particular organisms.

Eg) Nitrogen (N2), despite its abundance in the atmosphere, often is the most limiting nutrient for plant growth, as most plants can only take up nitrogen in two solid forms: ammonium ion (NH4+) and the ion nitrate (NO3-). So, biogeochemical cycles enable the provision of elements to organisms in utilizable forms.

2. They enable the transfer of molecules from one locality to another.
Eg) Some elements like N2 are highly concentrated in the atmosphere, but some of the atmospheric N2 is transferred to soil through the N2 cycle

3. These cycles facilitate the storage of elements. Elements carried through the biogeochemical cycles are stored in their natural reservoirs, and are released to organisms in small consumable amounts.
Eg) Through N2 cycle and with the help of N2 fixing bacteria, green plants utilize N2 in bits though it is abundant in the atmosphere.

4. They assist in functioning of ecosystems, ie proper functioning in a state of equilibrium. Whenever any imbalances occur, the ecosystem through the biogeochemical cycles restores to the equilibrium state. The adjustment is such that the disturbing factor is eliminated.

5. Biogeochemical cycles link living organisms with living organisms, living organisms with the non-living entities and non-living entities with non-living entities. This is because all organisms depend on one another and, the biotic and abiotic component of the ecosystem are linked by flow on nutrients engineered by the biogeochemical cycles.

6. They regulate the flow of substances. As the biogeochemical cycles pass through different spheres, the flow of elements is regulated since each sphere has a particular medium and the rate at which elements flow is determined by the viscosity and density of the medium. Therefore elements in the biogeochemical cycles flow at differing rates within the cycle and this regulates the flow of the elements in those cycles.

Lifespan And Rate Of Biogeochemical Cycles

It is the time a particular element or molecule of a substance being carried in the biogeochemical cycle takes to make one complete cycle. It may range from several days to millions of years.
Eg) water droplet of average size may stay in the atmosphere for about ten days before precipitation, carbon atoms may reside in the earth crust for the age of the Earth.

The speed of the cycles depends on the medium in which the molecule being cycled is and the surrounding conditions. So, climatic conditions have a significant impact on the biogeochemical cycles.

Cycles that involve molecules or ions in a gaseous state are generally shorter than cycles that involve solid or liquid state transfer because of the slow rate at which molecules move through the lithosphere.

Some of the important Biogeochemical cycles :


Nitrogen is a very important element in that it is part of both proteins (present in the composition of the amino acids that make those proteins) & nucleic acids, such as DNA and RNA (present in nitrogenous bases). Our atmosphere contains nearly 79% of nitrogen but it can’t be used directly by the majority of living organisms.

Nitrogen cycles from gaseous phase to solid phase then back to gaseous phase through the activity of a wide variety of organisms. Cycling of nitrogen is vitally important for all living organisms. There are five main processes :

NITROGEN FIXATION : Nitrates can then be used by plants or animals Involves conversion of gaseous nitrogen into Ammonia, a form in which it can be used by plants. Atmospheric nitrogen can be fixed by the following three methods

Atmospheric fixation: Lightening, combustion and volcanic activity help in the fixation of nitrogen.

Industrial fixation: At high temperature (400°C) and high pressure (200 atm.), molecular nitrogen is broken into atomic nitrogen which then combines with hydrogen to form ammonia (Haber-Bosch process)

Bacterial fixation: There are two types of bacteria-
  • Symbiotic bacteria e.g. Rhizobium in the root nodules of leguminous plants.
  • Free-living or symbiotic e.g. Nostoc, Azobacter, Cyanobacteria 

can combine atmospheric or dissolved nitrogen with hydrogen to form ammonia.

NITRIFICATION: The conversion of ammonia to nitrate is performed primarily by soil-living bacteria and other nitrifying bacteria.
In the primary stage of nitrification, the oxidation of ammonium (NH4+) to nitrites (NO2-), is performed by ammonium oxidizing bacteria (AOB) represented by the "Nitrosomonas" species.
The second reaction is oxidation of nitrite (NO2-) to nitrate (NO3-) by nitrite-oxidizing bacteria (NOB), represented by the “Nitrobacter” species. It is important for ammonia to be converted to nitrates or nitrites because ammonia gas is toxic to plants.

ASSIMILATION: In this process nitrogen fixed by plants is converted into organic molecules such as proteins, DNA, RNA etc. These molecules make the plant and animal tissue.

AMMONIFICATION: 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 by the bacteria. This process is called ammonification. Ammonifying bacteria help in this process.

DENITRIFICATION: Reduction of nitrates back into the largely inert nitrogen gas (N2), completing the nitrogen cycle. This process is performed by bacterial species such as Pseudomonas and Clostridium in anaerobic conditions (in oxygen free medium), eg) waterlogged soils. 
The denitrifying bacteria use nitrates in the soil to carry out respiration and consequently produce nitrogen gas- inert and unavailable to plants. Denitrification is reverse of nitrogen fixation.

Human influences on the nitrogen cycle

Huge increase in transfer of nitrogen into biologically available forms – Due to extensive cultivation of legumes, growing use of the Haber–Bosch process in the creation of chemical fertilizers, and pollution emitted by vehicles and industrial plants

Transfer of nitrogen trace gases from Earth to the atmosphere and from the land to aquatic systems.

Nitrous oxide (N2O) has risen in the atmosphere due to agricultural fertilization, biomass burning, cattle and feedlots, and industrial sources. N2O in the stratosphere breaks down and acts as a catalyst in the destruction of atmospheric ozone.

Nitrous oxide is also a greenhouse gas (GHG) and is currently the third largest contributor to global warming, after carbon dioxide and methane. It is 300 times more potent in its ability to warm the planet, than carbon dioxide.

Ammonia (NH3) in the atmosphere (increasing due to human activities) acts as an aerosol, decreasing air quality and clinging to water droplets, eventually resulting in nitric acid (HNO3) that causes acid rain. Atmospheric ammonia and nitric acid also damage respiratory systems.

The very-high temperature of lightning naturally produces small amounts of NOx, NH3, and HNO3, but high-temperature combustion has contributed to a 6 or 7 fold increase in the flux of NOx to the atmosphere. The higher the temperature, the more NOx is produced. 

Ammonia and nitrous oxides are precursors of tropospheric (lower atmosphere) ozone production, which contributes to smog and acid rain, damages plants and increases nitrogen inputs to ecosystems.

Decrease in biodiversity can also result if higher nitrogen availability increases nitrogen-demanding grasses, causing a degradation of nitrogen-poor species.

Onsite sewage facilities release large amounts of nitrogen into the environment. Microbial activity consumes the nitrogen and other contaminants in the wastewater. But in certain areas, microbial activity is unable to process all the contaminants and wastewater with the contaminants, enters the aquifers.

One health risk associated with drinking water (with >10 ppm nitrate) is the development of methemoglobinemia or blue baby syndrome.  

Additional risks of increased availability of inorganic nitrogen in aquatic ecosystems are water acidification, eutrophication of fresh and saltwater systems and toxicity issues for animals. Eutrophication often leads to lower dissolved oxygen levels in the water column, including hypoxic and anoxic conditions, which can cause death of aquatic fauna.