8) Nitrogen cycle

Nitrogen cycle

NITROGEN CYCLE

The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out to both biological and non-biological processes. Important processes in the nitrogen cycle include fixation, mineralization, nitrification, and denitrification. The majority of Earth's atmosphere (approximately 78%) is nitrogen,[1] making it the largest pool of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems. The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition. Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle.

A 2011 study found that nitrogen from rocks may also be a significant source of nitrogen, that had not previously been included.

Ecological function

Nitrogen is essential for many processes; it is crucial for any life on Earth. It is a component in all amino acids, as incorporated into proteins, and is present in the bases that make up nucleic acids, such as DNA and RNA. In plants, much of the nitrogen is used in chlorophyll molecules, which are essential for photosynthesis and further growth.[5] Although Earth’s atmosphere is an abundant source of nitrogen, most is relatively unusable by plants.[6] Chemical processing, or natural fixation (through processes such as bacterial conversion—see rhizome), are necessary to convert gaseous nitrogen into forms usable by living organisms, which makes nitrogen a crucial component of food production. The abundance or scarcity of this "fixed" form of nitrogen, (also known as reactive nitrogen), dictates how much food can be grown on a piece of land.
 

The processes of the nitrogen cycle

Nitrogen is present in the environment in a wide variety of chemical forms including organic nitrogen, ammonium (NH4+), nitrite (NO2-), nitrate (NO3-), nitrous oxide (N2O), nitric oxide (NO) or inorganic nitrogen gas (N2). Organic nitrogen may be in the form of a living organism, humus or in the intermediate products of organic matter decomposition. The processes of the nitrogen cycle transform nitrogen from one form to another. Many of those processes are carried out by microbes, either in their effort to harvest energy or to accumulate nitrogen in a form needed for their growth. The diagram above shows how these processes fit together to form the nitrogen cycle.
Nitrogen fixation

Main article: Nitrogen fixation

Atmospheric nitrogen must be processed, or "fixed" (see page on nitrogen fixation), to be used by plants. Some fixation occurs in lightning strikes, but most fixation is done by free-living or symbiotic bacteria. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make their own organic compounds. Most biological nitrogen fixation occurs by the activity of Mo-nitrogenase, found in a wide variety of bacteria and some Archaea. Mo-nitrogenase is a complex two component enzyme that has multiple metal-containing prosthetic groups.[7] Some nitrogen fixing bacteria, such as Rhizobium, live in the root nodules of legumes (such as peas or beans). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form such symbioses. Today, about 30% of the total fixed nitrogen is manufactured in ammonia chemical plants.[8]
 

Conversion of N2

The conversion of nitrogen (N2) from the atmosphere into a form readily available to plants and hence to animals is an important step in the nitrogen cycle, which distributes the supply of this essential nutrient. There are four ways to convert N2 (atmospheric nitrogen gas) into more chemically reactive forms:[5]

    Biological fixation: some symbiotic bacteria (most often associated with leguminous plants) and some free-living bacteria are able to fix nitrogen as organic nitrogen. An example of mutualistic nitrogen fixing bacteria are the Rhizobium bacteria, which live in legume root nodules. These species are diazotrophs. An example of the free-living bacteria is Azotobacter.
    Industrial N-fixation: Under great pressure, at a temperature of 600 C, and with the use of an iron catalyst, hydrogen (usually derived from natural gas or petroleum) and atmospheric nitrogen can be combined to form ammonia (NH3) in the Haber-Bosch process which is used to make fertilizer and explosives.
    Combustion of fossil fuels: automobile engines and thermal power plants, which release various nitrogen oxides (NOx).
    Other processes: In addition, the formation of NO from N2 and O2 due to photons and especially lightning, can fix nitrogen.

Assimilation
Main article: Assimilation

Plants take nitrogen from the soil, by absorption through their roots in the form of either nitrate ions or ammonium ions. All nitrogen obtained by animals can be traced back to the eating of plants at some stage of the food chain.

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.[5] In plants that 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 obtain nitrogen by ingestion of amino acids, nucleotides and other small organic molecules.

Ammonification
Main article: Ammonification

When a plant or animal dies, or an animal expels waste, the initial form of nitrogen is organic. Bacteria, or fungi in some cases, convert the organic nitrogen within the remains back into ammonium (NH4+), a process called ammonification or mineralization. Enzymes Involved:

    GS: Gln Synthetase (Cytosolic & PLastid)
    GOGAT: Glu 2-oxoglutarate aminotransferase (Ferredoxin & NADH dependent)
    GDH: Glu Dehydrogenase:
        Minor Role in ammonium assimilation.
        Important in amino acid catabolism.

Nitrification
Main article: Nitrification

The conversion of ammonium to nitrate is performed primarily by soil-living bacteria and other nitrifying bacteria. In the primary stage of nitrification, the oxidation of ammonium (NH4+) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO2-). Other bacterial species, such as the Nitrobacter, are responsible for the oxidation of the nitrites into nitrates (NO3-).[5] It is important for the nitrites to be converted to nitrates because accumulated nitrites are toxic to plant life.

Due to their very high solubility, nitrates can enter groundwater. Elevated nitrate in groundwater is a concern for drinking water use because nitrate can interfere with blood-oxygen levels in infants and cause methemoglobinemia or blue-baby syndrome.[9] Where groundwater recharges stream flow, nitrate-enriched groundwater can contribute to eutrophication, a process that leads to high algal, especially blue-green algal populations and the death of aquatic life due to the algae's excessive demand for oxygen. While not directly toxic to fish life, like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication. Nitrogen has contributed to severe eutrophication problems in some water bodies. Since 2006, the application of nitrogen fertilizer has been increasingly controlled in Britain and the United States. This is occurring along the same lines as control of phosphorus fertilizer, restriction of which is normally considered essential to the recovery of eutrophied waterbodies.

Denitrification
Main article: Denitrification

Denitrification is the 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.[5] 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.
Anaerobic ammonium oxidation
Main article: Anammox

In this biological process, nitrite and ammonium are converted directly into elemental nitrogen (N2) gas. This process makes up a major proportion of elemental nitrogen conversion in the oceans. 

                   

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                                         B.SC(HONS)GEOGRAPHY,M.SC(Con..)

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