Sunday, July 27, 2008


The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. It is a gaseous cycle.
Earth's atmosphere is about 78% nitrogen, making it the largest pool of nitrogen. Nitrogen is essential for many biological processes; and is crucial for any life here on Earth. It is in all amino acids, is incorporated into proteins, and is present in the bases that make up nucleic acids, such as DNA(Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses) and RNA(Ribonucleic acid (RNA) is a nucleic acid and consists of a long chain of nucleotide units). In plants, much of the nitrogen is used in chlorophyll molecules which are essential for photosynthesis and further growth.
Nitrogen Fixation
The conversion of nitrogen (N2) from the atmosphere into a form readily available to plants and hence to animals and humans is an important step in the nitrogen cycle, that determines the supply of this essential nutrient. There are four ways to convert N2 (atmospheric nitrogen gas) into more chemically reactive forms. They are biological fixation, industrail n-fixation, combustion of fossil fuels and other processes.
Nitrogen Mineralization
In most ecosystems nitrogen is primarily stored in living and dead organic matter. This organic nitrogen is converted into inorganic forms when it re-enters the biogeochemical cycle via decomposition. Decomposers, found in the upper soil layer, chemically modify the nitrogen found in organic matter from ammonia (NH3 ) to ammonium salts (NH4+ ). This process is known as mineralization and it is carried out by a variety of bacteria, actinomycetes, and fungi.
Nitrification
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.
Denitrification
Through denitrification, oxidized 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. They use the nitrate as an electron acceptor in the place of oxygen during respiration. These facultatively anaerobic bacteria can also live in aerobic conditions.
Ammonification
When a plant or animal dies, or an animal excretes, the initial form of nitrogen is organic. Bacteria, or in some cases, fungi, converts the organic nitrogen within the remains back into ammonia, a process called ammonification or mineralization.
Assimilation
Plants can absorb nitrate or ammonium ions from the soil via their root hairs. If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll. In plants which have a mutualistic relationship with rhizobia, some nitrogen is assimilated in the form of ammonium ions directly from the nodules. Animals, fungi, and other heterotrophic organisms absorb nitrogen as amino acids, nucleotides and other small organic molecules.
Currently, much research is devoted to understanding the effects of nitrogen enrichment in the air, groundwater and surface water. Scientists are also exploring alternative agricultural practices that will sustain high productivity while decreasing the negative impacts caused by fertilizer use. These studies not only help us quantify how humans have altered the natural world, but increase our understanding of the processes involved in the nitrogen cycle as a whole.


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Environmentalist Blogged:1:32 PM