The Carbon and Nitrogen cycle

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Carbon moves through the lower atmosphere and all food webs on its way to and from the ocean's water, sediments, and rocks. Its global movement is called the carbon cycle. Carbon is an element found in all living substances as well as in many inorganic materials. Both diamond and coal are nearly pure carbon, but with different structures. Carbon is a key element for life, composing almost half of the dry mass of the earth’s plants that is the mass when all water is removed. The carbon cycle is the exchange of carbon among three reservoirs or storage places such as the atmosphere, the oceans, and the lands. Besides, human activities also contribute carbon to the atmosphere. The burning of fossil fuels such as coal, oil and gas adds about eight percent as much carbon to the atmosphere as does cellular respiration. The human activity such as open burning or deforestation also contributes large amount of stored carbon to the air, croplands and pastures contain 5-10% as much carbon in plants as is contained in forest plant. In the atmosphere, carbon is attached to some oxygen in a gas called carbon dioxide.

Most of the world's carbon is locked in ocean sediments and rocks. It moves into and out of ecosystem in gaseous form, so its movement is said to be an atmospheric cycle. The carbon cycle also included the geological carbon cycle and the biological carbon cycle. The geologic component of the carbon cycle operates slowly in comparison to the other parts of the global carbon cycle. It is one of the most crucial determinants of the amount of carbon in the atmosphere, and thus of global temperatures. Since the formation of the Earth, geological forces have slowly produced carbonic acid which is a weak acid formed by the reactions between atmospheric carbon dioxide and water. Then is the biological carbon cycle. Biology plays an important role in the movement of carbon between the land, ocean, and atmosphere through the processes of photosynthesis and respiration. Through photosynthesis, green plants use solar energy and water to turn atmospheric carbon dioxide into carbohydrates . For example, sugars, cellulose and starch. The equation for the process is : energy (sunlight) + H2O + 6CO2 ---> 6O2 + C6 H12O6

Carbon dioxide and the other atmospheric gases dissolve in surface waters. Dissolved gases are in equilibrium with the gas in the atmosphere. A weak acid is formed when the carbon dioxide reacts with water in solution that is also known as carbonic acid. Carbonic acid undergoes dissociation to form bicarbonate ions and hydrogen ions. The hydrogen ions and water react with most common minerals such assilicates and carbonates and turns into the minerals. The products of weathering are predominantly clays (a group of silicate minerals) and soluble ions such as calcium, iron, sodium, and potassium. Bicarbonate ions also remain in solution; a remnant of the carbonic acid that was used to weather the rocks.

Sources of Carbon:

Carbon is found as graphite and diamond in nature. It also occurs as carbon dioxide in the atmosphere. Besides, carbon also remains stored as carbonates in the form of marble rocks and limestone in the ocean. Carbonates of earth's crust derived from rocks by chemical reactions to produce carbon dioxide.

Carbon Dioxide Production:

Carbon dioxide is produced when any substance containing carbon is burned. Burning of fossil fuels will release carbon dioxide into the atmosphere. Besides, Carbon dioxide is also a product of breathing and of fermentation. Living organisms use oxygen in respiration and release carbon dioxide, and anaerobic respiration of yeast also give out carbon dioxide. However, man-made carbon dioxide emissions have created an imbalance. There are now more carbon dioxide emissions going into the atmosphere than are being removed. This is primarily due to the burning of fossil fuel from the factories. Burning of wood and fossil fuels increases considerable amount of carbon dioxide into the atmosphere. Besides, carbon dioxide also released by the decomposition of organic wastes and dead bodies by the denitrifying bacteria and fungi. Moreover, volcanic eruptions and weathering of carbonate rocks by the action of acids also contributes to the production of carbon dioxide.


Carbon dioxide fixed by the producers such as plants and trees and entered the food chain and is passed to primary consumers, secondary consumers and decomposers. The main process that brings carbon form the environment into the living world is by the process of photosynthesis, where producers take in carbon dioxide from the atmosphere and convert it into organic compounds. Oxygen and water is released as a by product in the reaction.


Figure 7.x : This equation shows how the carbon dioxide is utilised and converted into glucose and oxygen in the presence of chlorophyll and sunlight.

Carbon dioxide utilization (CDU) uses waste carbon dioxideas a chemical feedstock in the production of a wide range of useful products. In using carbon dioxidewe can remove the need for manufacturing the product from petrochemicals, thus providing a sustainable manufacturing route. Carbon dioxidecan be used to synthesize, polymers, fuels, commodity chemicals such as methanol, pharmaceuticals, building materials and many other products. There are challenges involved which are being solved by new catalysts, integration with renewable energy and optimising production conditions.

Here is how the carbon cycle works:

Step 1:


The producers and consumers breathe and release carbon dioxide into the atmosphere and burning of fossil fuels also produced carbon dioxide.

Step 2:


Producers absorbed carbon dioxide during photosynthesis to make carbohydrates.

Step 3:


Primary consumers consumed on the plants. Consequently passing the carbon compounds along the food chain to secondary consumers, tertiary consumers and decomposers.

Step 4:


When is animals and plants die, they are decomposed bydecomposers. The carbon that was in their bodies is then returned to the atmosphere as carbon dioxide. The decomposed plants and animals may then be available asfossil fuelin the future for combustion.

Chart 7.: It shows the carbon cycle by concentrating more on the role of organisms and observing how biological functions lead to changes in the carbon cycle.

7.4 Nitrogen Cycle

Nitrogen is a crucial element in nucleic acids and amino acids. Even though it is the most common gas in the atmosphere , it is about 80 percent of air, animal cannot use nitrogen in its gaseous form except nitrogen fixing bacteria. Nitrogen fixing bacteria are microorganisms present in the soil or in plant roots that change nitrogen gas from the atmosphere into solid nitrogen compounds that plants can use in the soil. There are two kinds of nitrogen fixer, first is the free living bacteria including the cyanobacteria or also known as blue-green algae, AnabaenaandNostoc which is a non-symbiotic bacteria. Then the second one is mutualistic bacteria which is a symbiotic bacteria such as Rhizobium that associated with leguminous plants and Spirillum lipoferus that is related with cereal grasses. Nitrogen cycle includes the process of nitrogen fixation, nitrification, ammonification and denitrification.

7.4.1 Nitrogen fixation

Nitrogen fixationis an anaerobic process as it occurs without the presence of oxygen. During nitrogen fixation, the atmospheric nitrogen is reduced to ammonia. Bacteria in terrestrial and aquatic environments are involved in this process. These organisms must have a unique enzyme known as dinitrogenase to be able to to this. Nitrogen fixation can be divided into three: Atmospheric nitrogen fixation, biological nitrogen fixation and industrial nitrogen fixation. Atmospheric nitrogen fixation occurs during lightning and thunderstorms which converts atmospheric gaseous nitrogen to oxides of nitrogen. The nitrogen is convert to nitrogen oxide then it dissolved in water forming nitrous acid and nitric acid and then combines with other salts to produce nitrates. Moreover, the industrial nitrogen fixation is utilized in the Haber-Bosch Process, also called the Haber Process, is a complex chemical procedure that takes nitrogen from the air and under high pressure and temperature it combines nitrogen with hydrogen to produce ammonia. This ammonia is the base of the synthetic nitrogen fertilizers widely utilized around the world today. Furthermore, the Biological nitrogen fixation occurs when living organism transform the gaseous of nitrogen into nitrates. It is a process in which nitrogen gas (N2) from the atmosphere is incorporated into the tissue of certain plants.For example, nitrogen fixing bacteria such as Nostac which live in the soil and Rhizobium which live in the root nodules of leguminous plants can convert nitrogen in the surrounding air into ammonium compound by the process of nitrogen fixation. This process may be summarized into the equation : N2 + 16ATP + 8H2------> 2 NH3+ 16ADP + 16 Pi + 2H2. This equation indicated that one molecule of nitrogen gas combines with eight hydrogen ions which is also known as protons to form two molecules of ammonia and two molecules of hydrogen gas. This reaction is conducted by an enzyme known as nitrogenase. The sixteen molecules of Adenosine Triphosphate represent the energy required for the Biological Nitrogen Fixation reaction to take place. ATP is the Adenosine Triphosphate which is also an energy storing compound. This reaction is conducted by enzyme known as nitrogenase. Nitrogenase is an enzyme complex present in nitrogen-fixing microorganism. It catalyses the conversion of atmospheric nitrogen into ammonia. This can then be converted to nitrites, nitrates and for synthesis of amino acids.

Figure 7.: It is the nitrogen fixing Rhizobium

7.4.2 Ammonification

Next, the ammonification is the process that involved breaks down of proteins, amino acids, and other nitrogen-containing compounds in dead and waste organic matter into the form ammonia. Bacteria can use enzyme such as protease to break down protein into amino acids, which are compounds containing an amine (NH2) group. The amino acids, and other compounds with amine groups, such as nucleic acids andurea, are then decayed by microorganisms known as ammonifying bacteria, releasing ammonia (NH3). This dissolves in water, and usually forms ammonium (NH4+) ions, by combining with hydrogen (H+) ions, which are bounteous in most soils. This ammonium is oxidized to nitrites by nitrosomonas and then to nitrates by nitrobacter. The nitrosomonas and nitrobacter are the example of nitrifying bacteria which carry out the process of nitrification. Ammonification is carried out by a diverse array of microorganisms that perform ecological decomposition services, and its product is ammonia or ammonium ion. Ammonium is a suitable source of nutrition for many species of plants, especially those living in acidic soils. However, most plants cannot utilize ammonium effectively, and they require nitrate as their vital source of nitrogen sustenance.

7.4.3 Nitrification

Furthermore, nitrification is the process of oxidation of ammonia into nitrites and then into nitrates in the presence of nitrifying bacteria. It reduces forms of inorganic and organic nitrogen, primarily ammonia, are oxidized to nitrate.

Plant and animal protein and metabolic wastes are decomposed by bacteria into ammonia. The ammonia may be utilized by plants, or it may be converted by Nitrosomonas and Nitrobacter species to nitrate, which is also used by plants. Ammonia is converted into nitrites by Nitrosomonas and Nitrococcus bacteria. followed by the oxidation of nitrite to nitrate by the nitrite‐oxidizing bacteria that are Nitrobacter and Nitrocystis and now it is available for plant absorption. Plants absorb nitrogen from the soil by their roots in the form of eithernitrateionsornitriteions. Plants cannot assimilate ammonium ions. Nitrate is a crossroads compound, it can be used by plants for their nutritional needs, or it can be liberated as atmospheric nitrogen by certain microorganism. Most nitrogen acquired by terrestrial animals can be traced back to the eating of plants at some phase of thefood chain. Ammonia also known NH3 is a form ofnitrogenthat is found in soils. Nitrogen enters the soil in organic form from decay plants, waste from creatures, and lightning striking the Earth. While in this organic form plants are unable to absorb it. Therefore, nitrogen needs to be converted into a usable inorganic form. Bacteria that are also found in the soil convert the nitrogen through a chemical process as outlined in the following section.

7.4.4 Denitrification

The cycle is balanced by a continuous return of nitrogen to the atmosphere by denitrifying bacteria which break down nitrates and release nitrogen back into the atmosphere through the process of denitrification. Micrococcus denitrificans, Bacillus and Pseudomonas aeruginosa are the examples of the denitrifying bacteria.


Figure 7.: It shows the various steps of nitrogen cycle.