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Nitrogen cycle

2007 Schools Wikipedia Selection. Related subjects: General Biology; General
Chemistry

   The nitrogen cycle is the biogeochemical cycle that describes the
   transformations of nitrogen and nitrogen-containing compounds in
   nature.

The basics

   Earth's atmosphere is about 78% nitrogen, making it the largest pool of
   nitrogen. Nitrogen is essential for many biological processes; it is in
   all amino acids, is 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(Smil, 2000).

   Processing, or fixation, is necessary to convert gaseous nitrogen into
   forms usable by living organisms. 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 its own organic compounds.
   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.

   Other plants get nitrogen from the soil by absorption at 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.

Ammonia

   The source of ammonia is the decomposition of dead organic matter by
   bacteria called decomposers, which produce ammonium ions (NH[4]^+). In
   well-oxygenated soil, these ions are then oxygenated first by
   nitrifying bacteria into nitrite (NO[2]^-) and then into nitrate
   (NO[3]^-). This two-step conversion of ammonium into nitrate is called
   nitrification (Smil, 2000).

   Ammonium ions readily bind to soils, especially to humic substances and
   clays. Nitrate and nitrite ions, due to their negative electric charge,
   bind less readily since there are less positively charged ion-exchange
   sites (mostly humic substances) in soil than negative. After rain or
   irrigation, leaching (the removal of soluble ions, such as nitrate and
   nitrite) into groundwater can occur. 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 (Vitousek et al, 1997). Where groundwater recharges stream
   flow, nitrate-enriched groundwater can contribute to eutrophication, a
   process leading to high algal and blue-green bacterial populations and
   the death of aquatic life due to 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. As
   of 2006, the application of nitrogen fertilizer is being 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.

   Ammonia is highly toxic to fish life and the water discharge level of
   ammonia from wastewater treatment plants must often be closely
   monitored. To prevent loss of fish, nitrification prior to discharge is
   often desirable. Land application can be an attractive alternative to
   the mechanical aeration needed for nitrification.

   During anaerobic (low oxygen) conditions, denitrification by bacteria
   occurs. This results in nitrates being converted to nitrogen gas and
   returned to the atmosphere. Nitrate can also be reduced to nitrite and
   subsequently combine with ammonium in the anammox process, which also
   results in the production of dinitrogen gas.

Processes of the Nitrogen Cycle

Nitrogen Fixation

Conversion of N[2]

   There are four ways to convert N[2] (atmospheric nitrogen gas) into
   more chemically reactive forms (Smil, 2000):

   The conversion of dinitrogen (N[2]) from the atmosphere into a form
   available to plants and hence to animals and humans. This is an
   important step in the terrestrial nitrogen cycle:
    1. Biological fixation : some symbiotic bacteria (most often
       associated with leguminous plants) and some free-living bacteria
       are able to fix nitrogen and assimilate it as organic nitrogen. An
       example of mutualistic nitrogen fixing bacteria are the Rhizobium
       bacteria, which live in plant root nodes. These species are
       diazotrophs.
    2. Industrial N-fixation ; in the Haber-Bosch process, N[2] is
       converted together with hydrogen gas (H[2]) into ammonia (NH[3])
       fertilizer.
    3. Combustion of fossil fuels : automobile engines and thermal power
       plants, which release NOx.
    4. Other processes : Additionally, the formation of NO from N[2] and
       O[2] due to photons and lightning, are important for atmospheric
       chemistry, but not for terrestrial or aquatic nitrogen turnover.

   As a result of extensive cultivation of legumes (particularly soy,
   alfalfa, and clover), use of the Haber-Bosch process in the creation of
   chemical fertilizers and pollution emitted by vehicles and industrial
   plants, human beings have more than doubled the annual transfer of
   nitrogen into a biologically available form (Vitousek et al, 1997).
   This has occurred to the detriment of aquatic and wetland habitats
   through eutrophication.

Assimilation

   In plants which have a mutualistic relationship with Rhizobium, some
   nitrogen is assimilated in the form of ammonium ions from the nodules.
   All plants however, can absorb nitrate from the soil via their root
   hairs. These are then reduced to nitrite ions and then ammonium ions
   for incorporation into amino acids, and hence protein, which forms part
   of the plants or animals that they eat (Smil, 2000).

Ammonification

   Nitrates are the form of nitrogen most commonly assimilated by plant
   species, which, in turn are consumed by heterotrophs for use in
   compounds such as amino and nucleic acids. The remains of heterotrophs
   will then be decomposed into nutrient-rich organic material. Bacteria
   or in some cases, fungi, will convert the nitrates within the remains
   back into ammonia.

Nitrification

   The conversion of ammonia to nitrates is performed primarily by
   soil-living bacteria. The primary stage of nitrification, the oxidation
   of ammonia (NH[3]) is performed by bacteria such as the Nitrosomonas
   species, which converts ammonia to nitrites (NO[2]^-). Other bacterial
   species, such as the Nitrobacter, are responsible for the oxidation of
   the nitrites into nitrates (NO[3]^-) (Smil, 2000)

Anaerobic Ammonium Oxidation

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

Denitrification

   Denitrification is the reduction of nitrates back into the largely
   inert nitrogen gas (N[2]), completing the nitrogen cycle. This process
   is performed by bacterial species such as the Pseudomonas (Smil, 2000)
   .

Nitrogen Cycle in Aquariums

   Schematic representation of the flow of Nitrogen through a common
   aquarium.
   Enlarge
   Schematic representation of the flow of Nitrogen through a common
   aquarium.

   One of the primary goals of the aquarist is to reproduce parts the
   nitrogen cycle on a small scale. While similar to the nitrogen cycle in
   natural environments, the aquarist must supplement some of the
   components necessary for the cycle to complete. When an aquarium is
   initially setup, there is insufficient beneficial bacteria to break
   down fish waste and uneaten food, which allows for unhealthy levels of
   ammonia and nitrite to build up. Hobbyists refer to this situation as
   "New Tank Syndrome"; it is a leading cause of fish deaths with
   newcomers to the hobby. Over time the addition of fish waste, carbon
   dioxide, light, and plant fertilizers will begin to build large
   colonies of beneficial bacteria that will ensure the aquarium remains
   healthy and active (The New Tank Syndrome 2006).

   The primary source of ammonia (NH[3]) is created when fish consume food
   and oxygen (O[2]) and create waste and carbon dioxide (CO[2]). The fish
   waste then decays into Ammonia. Other sources include excess food that
   is not eaten as well as decaying plants and dead fish. The rise in
   ammonia triggers the growth of Nitrosomonas which produce nitrites
   (NO[2]). The nitrites trigger the growth of Nitrobacter to produce
   nitrates (NO[3]). The nitrates and carbon dioxide are consumed by plant
   life which produce oxygen through the process of photosynthesis. Excess
   nitrates are removed by water changes.

Human Influences on the Nitrogen Cycle

   Humans have contributed significantly to the nitrogen cycle by
   artificial nitrogen fertilization (primarily through the Haber Process,
   using energy from fossil fuels to convert N[2] to ammonia gas (NH[3])
   and planting of nitrogen fixing crops (Vitousek et al., 1997). In
   addition, humans have significantly contributed to the transfer of
   nitrogen trace gases from Earth to the atmosphere. N[2]O has risen in
   the atmosphere as a result of agricultural fertilization, biomass
   burning, cattle and feedlots, and other industrial sources (Chapin et
   al. 2002). N[2]O has deleterious effects in the stratosphere, where it
   breaks down and acts as a catalyst in the destruction of atmospheric
   ozone. NH[3] in the atmosphere has tripled as the result of human
   activities. It is a reactant in the atmosphere, where it acts as an
   aerosol, decreasing air quality and clinging on to water droplets,
   eventually resulting in acid rain. Fossil fuel combustion has
   contributed to a 6 or 7 fold increase in NOx flux to the atmosphere. NO
   actively alters atmospheric chemistry, and is a precursor of
   tropospheric (lower atmosphere) ozone production, which contributes to
   smog, acid rain, and increases nitrogen inputs to ecosystems (Smil,
   2000). Ecosystem processes can increase with nitrogen fertilization,
   but anthropogenic input can also result in nitrogen saturation, which
   weakens productivity and can kill plants(Vitousek et al., 1997).
   Decreases in biodiversity can also result if higher nitrogen
   availability increases nitrogen-demanding grasses, causing a
   degredation of nitrogen-poor, species diverse heathlands (Aerts and
   Berendse 1988).
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