Nitrogen (N) is an essential component of DNA,



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Nitrogen (N) is an essential component of DNA, RNA and proteins, the building blocks of life. All organisms require nitrogen to live and grow.

Although the majority of the air we breathe is N2, most of the nitrogen in the atmosphere is unavailable for use by organisms. This is because the strong triple bond between the N atoms in N2 molecules makes it relatively inert. In fact, in order for plants and animals to be able to use nitrogen, N2 gas must first be converted to more a chemically available form such as ammonium (NH4+), nitrate (NO3–), or organic nitrogen [e.g., urea—(NH3)2CO]. Five main processes cycle nitrogen through the biosphere, atmosphere, and geosphere: nitrogen fixation, nitrogen uptake (organismal growth), nitrogen mineralization (decay), nitrification, and denitrification.

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In fact the elemental nitrogen is converted into usable forms by the process of nitrogen fixation and added to the soil.

Nitrogen Fixation:

N2 – NH4+ Nitrogen fixation is the process wherein N2 is converted to ammonium, essential because it is the only way that organisms can attain nitrogen directly from the atmosphere. Certain bacteria, for example those among the genus Rhizobium, are the only organisms that fix nitrogen through metabolic processes.

Nitrogen fixing bacteria often form symbiotic relationships with host plants. This symbiosis is well-known to occur in the legume family of plants (e.g., beans, peas, and clover). In this relationship, nitrogen fixing bacteria inhabit legume root nodules and receive carbohydrates and a favouable environment from their host plant in exchange for some of the nitrogen they fix.

There is also nitrogen fixing bacteria that exist without plant hosts, known as free-living nitrogen fixers. In aquatic environments, blue- green algae (really a bacteria called cyanobacteria) is an important free-living nitrogen fixer.

Nitrogen Uptake:

NH4+ – Organic N The ammonia produced by nitrogen fixing bacteria is usually quickly incorporated into protein and other organic nitrogen compounds, either by a host plant, the bacteria itself, or another soil organism.

Nitrogen Mineralization:

Organic N – NH4+ After nitrogen is incorporated into organic matter, it is often converted back into inorganic nitrogen by a process called nitrogen mineralization, otherwise known as decay. When organisms die, decomposers (such as bacteria and fungi) consume the organic matter and lead to the process of decomposition. During this process, a significant amount of the nitrogen contained within the dead organism is converted to ammonium.

Once in the form of ammonium, nitrogen is available for use by plants or for further transformation into nitrate (NO3–) through the process called nitrification.

Nitrification:

NH3+ – NO3– Some of the ammonium produced by decomposition is converted to nitrate via., a process called nitrification. The bacteria that carry out this reaction gain energy from it. Nitrification requires the presence of oxygen, so nitrification can happen only in oxygen-rich environments like circulating or flowing waters and the very surface layers of soils and sediments. The process of nitrification has some important consequences.

Ammonium ions are positively charged and therefore stick (are sorbed) to negatively charged clay particles and soil organic matter. The positive charge prevents ammonium nitrogen from being washed out of the soil (or leached) by rainfall. In contrast, the negatively charged nitrate ion is not held by soil particles and so can be washed down the soil profile, leading to decreased soil fertility and nitrate enrichment of downstream surface and groundwaters.

Denitrification:

NO3– – N2 + N2O Through denitrification, oxidised forms of nitrogen such as nitrate and nitrite (NO2–) are converted to dinitrogen (N2) and, to a lesser extent, nitrous oxide gas. Denitrification is an anaerobic process that is carried out by denitrifying bacteria, which convert nitrate to dinitrogen in the following sequence: NO3– – NO2– – NO – N2O – N2 Thus a biogeochemical cycle exists and is called as nitrogen cycle.

Carbon Cycle:

Carbon is the basic building element of all the living organisms. By volume, carbon dioxide in the atmosphere is very low, i.

e., about 0.03% v/v. It is absorbed by the plant in the form of CO2 from the atmosphere. It is absolutely essential for the synthesis of carbohydrates and fat through photosynthesis in the presence of sunlight in green plants (producers). Late on the formed in green plants are utilised by consumers (herbivores and carnivores) and thus compound again digested and converted into other forms.

From both the producer and consumer carbon is returned back to the atmosphere during respiration. After the death of producer and consumer certain decomposition agents (like bacteria and fungi) decompose and degrade the complex organic compounds (carbohydrate, fat etc.) in their simplest forms (carbon, hydrogen, oxygen, nitrogen etc.), which are then available for other cycles. Some of the organic carbon becomes incorporated into the earth’s crust in the form of coal, natural gas, petroleum etc.

these fossils (coal, natural gas, petroleum etc.) form the most important sources of energy. CO2 is also released due to the combustion of these fossil fuels. Thus carbon cycle is mainly maintained by the process of photosynthesis, respiration, decomposition and combustion of fossil fuels. Sea is the second major reservoir of carbon.

Sea water contains 50 times mare CO2 than air. This is in the form of carbonates and bicarbonates. The CO2 dissolves in sea water to form carbonic acid which converts carbonate into bicarbonates. Thus the sea regulates the CO content in the atmosphere.

CO2 has the unique property of absorbing infrared rays of the sum, thus keeping the earth warm. But a huge amount of CO2 has been released into atmosphere due to extensive industrialisation and cutting of green plants. This has resulted in an excessive absorption of infrared radiations leading to an increase in atmospheric temperature. A green house effect has been experienced which had effected the functioning of organism adversely.

Sulphur Cycle:

Sulphur is considered a secondary element, along with calcium and magnesium.

Sulphur is required in moderate amounts by plants, but is less likely to limit crop growth than nitrogen, phosphorus or potassium. Sulphur is one of the components that make up proteins and vitamins. Proteins consist of amino acids that contain sulphur atoms.

Sulphur is important for the functioning of proteins and enzymes in plants, and in animals that depend upon plants for sulphur. Plants absorb sulphur when it is dissolved in water. Animals consume these plants, so that they take up enough sulphur to maintain their health.

Most of the earth’s sulphur is tied up in rocks and salts or buried deep in the ocean in oceanic sediments. Sulphur can also be found in the atmosphere. It enters the atmosphere through both natural and human sources. Natural resourses can be for instance volcanic eruptions, bacterial processes, evaporation from water, or decaying organisms. When sulphur enters the atmosphere through human activity, this is mainly a consequence of industrial processes where sulphur dioxide (SO2) and hydrogen sulphide (H2S) gases are emitted on a wide scale.

When sulphur dioxide enters the atmosphere it will react with oxygen to produce sulphur trioxide gas (SO3), or with other chemicals in the atmosphere, to produce sulphur salts. Sulphur dioxide may also react with water to produce sulphuric acid (H2SO4). Sulphuric acid may also be produced from demethylsulphide, which is emitted to the atmosphere by plankton species. All these particles will settle back onto earth, or react with rain and fall back onto earth as acid deposition. The particles will then be absorbed by plants again and are released back into the atmosphere, so that the sulphur cycle will start over again.

A schematic representation of the sulphur cycle:

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