The nitrogen cycle is the process by which nitrogen is
converted between its various chemical forms. This transformation can be
carried out through both biological and physical processes. Important processes
in the nitrogen cycle include fixation,
ammonification,
nitrification,
and denitrification. The majority of Earth's atmosphere (78%) is nitrogen,
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.
Ecological function
Nitrogen is necessary for all known forms of 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 RNA
and DNA. In plants, much of the
nitrogen is used in chlorophyll molecules, which are essential for photosynthesis
and further growth.Nitrogen gas (N2) is the largest constituent of
the Earth's atmosphere, but this form is relatively
nonreactive and unusable by plants. Chemical processing or natural fixation
(through processes such as bacterial conversion—see rhizobium)
are necessary to convert gaseous nitrogen into compounds such as nitrate or
ammonia which can be used by plants. The abundance or scarcity of this
"fixed" nitrogen (also known as reactive nitrogen) frequently limits
plant growth in both managed and wild environments. The nitrogen cycle, like
the carbon cycle, is an important part of every ecosystem.
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
Atmospheric nitrogen must be processed, or "fixed",
to be used by plants. Some fixation occurs in lightning
strikes, but most fixation is done by free-living or symbiotic bacteria
known as diazotrophs.
These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to
produce ammonia,
which is converted by the bacteria into other 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. An example of the free-living bacteria
is Azotobacter.
Symbiotic nitrogen-fixing bacteria such as Rhizobium
usually live in the root nodules of legumes (such as
peas, alfalfa, and locust trees). Here they form a mutualistic relationship with the plant,
producing ammonia in exchange for carbohydrates.
Because of this relationship, legumes will often increase the nitrogen content
of nitrogen-poor soils. A few non-legumes can also form such symbioses.
Today, about 30% of the total fixed nitrogen is produced industrially using the
Haber-Bosch
process, which uses high temperatures and pressures to convert nitrogen gas and
a hydrogen source ( natural gas or petroleum) into ammonia.
Assimilation
Plants take nitrogen from the soil by absorption through
their roots in the form of either nitrate ions
or ammonium
ions. Most nitrogen obtained by terrestrial 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. In plants that have a symbiotic relationship with rhizobia, some nitrogen
is assimilated in the form of ammonium ions directly from the nodules. It is
now known that there is a more complex cycling of amino acids between Rhizobia
bacteroids and plants. The plant provides amino acids to the bacteroids so
ammonia assimilation is not required and the bacteroids pass amino acids (with
the newly fixed nitrogen) back to the plant, thus forming an interdependent
relationship. While many animals, fungi, and other heterotrophic
organisms obtain nitrogen by ingestion of amino acids,
nucleotides
and other small organic molecules, other heterotrophs (including many bacteria) are
able to utilize inorganic compounds, such as ammonium as sole N sources.
Utilization of various N sources is carefully regulated in all organisms.
Ammonification
When a plant or animal dies or an animal expels waste, the
initial form of nitrogen is organic.
Bacteria or fungi convert the organic nitrogen within the remains back into ammonium (NH4+),
a process called ammonification or mineralization. Enzymes involved are:
- 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
The conversion of ammonia 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 Nitrobacter, are responsible for the oxidation of the
nitrites into nitrates (NO3-). It is important for the
ammonia to be converted to nitrates because accumulated nitrites are toxic to
plant life.
Due to their very high solubility
and because soils are largely unable to retain anions, 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. Where groundwater recharges stream flow,
nitrate-enriched groundwater can contribute to eutrophication,
a process that leads to high algal population and growth, especially blue-green
algal populations. 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
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. 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. Denitrification happens in anaerobic
conditions e.g. waterlogged soils. The denitrifying bacteria use nitrates in
the soil to carry out respiration and consequently produce nitrogen gas, which
is inert and unavailable to plants.
Anaerobic ammonium oxidation
Main article:
In this biological process, nitrite and ammonium are
converted directly into molecular nitrogen (N2)
gas. This process makes up a major proportion of nitrogen conversion in the
oceans.
Other processes
Though nitrogen fixation is the primary source of
plant-available nitrogen in most ecosystems, in areas with nitrogen-rich bedrock, the
breakdown of this rock also serves as a nitrogen source.
Marine nitrogen cycle
The nitrogen cycle is an important process in the ocean as
well. While the overall cycle is similar, there are different players and modes
of transfer for nitrogen in the ocean. Nitrogen enters the water through
precipitation, runoff, or as N2 from the atmosphere. Nitrogen cannot
be utilized by phytoplankton as N2 so it must undergo
nitrogen fixation which is performed predominately by cyanobacteria.
Without supplies of fixed nitrogen entering the marine cycle the fixed nitrogen
would be used up in about 2000 years. Phytoplankton need nitrogen in
biologically available forms for the initial synthesis of organic matter.
Ammonia and urea are released into the water by excretion from plankton.
Nitrogen sources are removed from the euphotic
zone by the downward movement of the organic matter. This can occur from
sinking of phytoplankton, vertical mixing, or sinking of waste of vertical
migrators. The sinking results in ammonia being introduced at lower depths
below the euphotic zone. Bacteria are able to convert ammonia to nitrite and
nitrate but they are inhibited by light so this must occur below the euphotic
zone. Ammonification or Mineralization is performed by bacteria to
convert the ammonia to ammonium. Nitrification
can then occur to convert the ammonium to nitrite and nitrate. Nitrate can be
returned to the euphotic zone by vertical mixing and upwelling where it can be
taken up by phytoplankton to continue the cycle. N2 can be returned
to the atmosphere through denitrification.
Ammonium is thought to be the preferred source of fixed
nitrogen for phytoplankton because its assimilation does not involve a redox
reaction and therefore requires little energy. Nitrate requires a redox
reaction for assimilation, but is more abundant so most phytoplankton have
adapted to have the enzymes necessary to undertake this reduction (nitrate
reductase). There are a few notable and well-known exceptions that include Prochlorococcus
and some Synechococcus. These species can only take up nitrogen
as ammonium.
The nutrients in the ocean are not uniformly distributed.
Areas of upwelling provide supplies of nitrogen from below the euphotic zone.
Coastal zones provide nitrogen from runoff and upwelling occurs readily along
the coast. However, the rate at which nitrogen can be taken up by phytoplankton
is decreased in oligotrophic waters year-round and temperate water in
the summer resulting in lower primary production. The distribution of the
different forms of nitrogen varies throughout the oceans as well.
Nitrate is depleted in near-surface water except in
upwelling regions. Coastal upwelling regions usually have high nitrate and chlorophyll
levels as a result of the increased production. However, there are regions of
high surface nitrate but low chlorophyll that are referred to as HNLC (high nitrogen,
low chlorophyll) regions. The best explanation for HNLC regions relates to iron
scarcity in the ocean, which may play an important part in ocean dynamics and
nutrient cycles. The input of iron varies by region and is delivered to the
ocean by dust (from dust storms) and leached out of rocks. Iron is under
consideration as the true limiting element to ecosystem productivity in the
ocean.
Ammonium and nitrite show a maximum concentration at 50–80 m
(lower end of the euphotic zone) with decreasing concentration below that
depth. This distribution can be accounted for by the fact that nitrite and
ammonium are intermediate species. They are both rapidly produced and consumed
through the water column. The amount of ammonium in the ocean is about 3 orders
of magnitude less than nitrate. Between ammonium, nitrite, and nitrate, nitrite
has the fastest turnover rate. It can be produced during nitrate assimilation,
nitrification, and denitrification; however, it is immediately consumed again.
New vs. regenerated nitrogen
Nitrogen entering the euphotic zone is referred to as new
nitrogen because it is newly arrived from outside the productive layer. The new nitrogen can come from below the euphotic
zone or from outside sources. Outside sources are upwelling from deep water and
nitrogen fixation. If the organic matter is eaten, respired, delivered to the
water as ammonia, and re-incorporated into organic matter by phytoplankton it
is considered recycled/regenerated production.
New production is an important component of the marine environment. One reason is that only continual input of new nitrogen can determine the total capacity of the ocean to produce a sustainable fish harvest. Harvesting fish from regenerated nitrogen areas will lead to a decrease in nitrogen and therefore a decrease in primary production. This will have a negative effect on the system. However, if fish are harvested from areas of new nitrogen the nitrogen will be replenished.
New production is an important component of the marine environment. One reason is that only continual input of new nitrogen can determine the total capacity of the ocean to produce a sustainable fish harvest. Harvesting fish from regenerated nitrogen areas will lead to a decrease in nitrogen and therefore a decrease in primary production. This will have a negative effect on the system. However, if fish are harvested from areas of new nitrogen the nitrogen will be replenished.
Human influences on the nitrogen cycle
As a result of extensive cultivation of legumes
(particularly soy, alfalfa, and clover), growing
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 biologically
available forms. In addition, humans have significantly contributed to the
transfer of nitrogen trace gases from Earth to the atmosphere and from the land
to aquatic systems. Human alterations to the global nitrogen cycle are most
intense in developed countries and in Asia, where vehicle emissions and industrial agriculture are highest.
Nitrous oxide (N2O) has risen in the
atmosphere as a result of agricultural fertilization, biomass burning, cattle
and feedlots, and industrial sources. N2O has deleterious effects in
the stratosphere,
where it breaks down and acts as a catalyst in the
destruction of atmospheric ozone. Nitrous oxide is also a greenhouse
gas and is currently the third largest contributor to global
warming, after carbon dioxide and methane. While
not as abundant in the atmosphere as carbon dioxide, it is, for an equivalent
mass, nearly 300 times more potent in its ability to warm the planet.
Ammonia (NH3) 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 to water droplets, eventually resulting in nitric acid
(HNO3)
that produces acid
rain. Atmospheric ammonia and nitric acid also damage respiratory systems.
The very-high temperature of lightning naturally produces
small amounts of NOx, NH3, and HNO3, but
high-temperature combustion has contributed to a 6 or 7 fold increase in
the flux of NOx to the atmosphere. Its production is a function of
combustion temperature - the higher the temperature, the more NOx is
produced. Fossil
fuel combustion is a primary contributor, but so are biofuels and even the
burning of hydrogen. The higher combustion temperature of hydrogen produces
more NOx than natural gas combustion.
Ammonia and nitrous oxides actively alter atmospheric chemistry. They are precursors of
tropospheric
(lower atmosphere) ozone production, which contributes to smog and acid rain,
damages plants and
increases nitrogen inputs to ecosystems. Ecosystem
processes can increase with nitrogen fertilization, but anthropogenic input can also result
in nitrogen saturation, which weakens productivity and can damage the health of
plants, animals, fish, and humans.
Decreases in biodiversity
can also result if higher nitrogen availability increases nitrogen-demanding
grasses, causing a degradation of nitrogen-poor, species diverse heathlands.
Wastewater treatment
Onsite sewage facilities such as septic
tanks and holding tanks release large amounts of nitrogen into the environment
by discharging through a drainfield into the ground. Microbial activity
consumes the nitrogen and other contaminants in the wastewater. However, in
certain areas, microbial activity is unable to process all the contaminants and
some or all of the wastewater, with the contaminants, enters the aquifers. These
contaminants accumulate and eventually end up in drinking water. One of the
contaminants most concerned about is nitrogen in the
form of nitrate.
A nitrate concentration of 10 ppm (parts per million) or 10 milligrams per liter
is the current EPA limit for drinking
water and typical household wastewater can produce a range of 20–85 ppm.
One health risk associated with drinking water (with >10
ppm nitrate) is the development of methemoglobinemia
and has been found to cause blue baby syndrome. Several American states have
now started programs to introduce advanced wastewater treatment systems to the
typical onsite sewage facilities. The result of these systems is an overall
reduction of nitrogen, as well as other contaminants in the wastewater.
Environmental impacts
Additional risks posed by increased availability of
inorganic nitrogen in aquatic ecosystems include water acidification; eutrophication
of fresh and saltwater systems; and toxicity issues for animals, including
humans. Eutrophication often leads to lower dissolved oxygen levels in the
water column, including hypoxic and anoxic conditions, which can cause death of
aquatic fauna. Relatively sessile benthos, or bottom-dwelling creatures, are
particularly vulnerable because of their lack of mobility, though large fish
kills are not uncommon. Oceanic dead zones near the mouth of the Mississippi in
the Gulf of Mexico are a well-known example of algal bloom-induced hypoxia. The New York Adirondack Lakes, Catskills,
Hudson Highlands, Rensselaer Plateau and parts of Long Island display the
impact of nitric acid rain deposition, resulting in the killing of fish and
many other aquatic species.
Ammonia (NH3) is highly toxic to fish and the
level of ammonia discharged from wastewater treatment facilities must be
closely monitored. To prevent fish deaths, nitrification via aeration prior
to discharge is often desirable. Land application can be an attractive
alternative to the aeration.
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