Nitrogen cycle

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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​ and ​RNA​.
In ​plants​, much of the nitrogen is used in
chlorophyll​ molecules which are essential for
photosynthesis​ and further growth.​[1]

What is the nitrogen cycle?

Nitrogen is the major component of earth's atmosphere. It enters the food chain
by means of nitrogen-fixing bacteria and algae in the soil. This nitrogen which has
been 'fixed' is now available for plants to absorb. These types of bacteria form a
symbiotic relationship with legumes--these types of plants are very useful because
the nitrogen fixation enriches the soil and acts as a 'natural' fertilizer. The
nitrogen-fixing bacteria form nitrates out of the atmospheric nitrogen which can be
taken up and dissolved in soil water by the roots of plants. Then, the nitrates are
incorporated by the plants to form proteins, which can then be spread through the
food chain. When organisms excrete wastes, nitrogen is released into the
environment. Also, whenever an organism dies, decomposers break down the
corpse into nitrogen in the form of ammonia. This nitrogen can then be used again
by nitrifying bacteria to fix nitrogen for the plants.

The nitrogen cycle (Higher Tier)

79% of the air around us is ​nitrogen . Living things need nitrogen to make proteins,
but they cannot get it directly from the air because nitrogen gas is too stable to react
inside an organism to make new compounds.

So nitrogen must be changed into a more reactive form to allow plants and animals
to use it. Plants can take up and use nitrogen when it is in the form of ​nitrates or
ammonium salts​. Changing nitrogen into a more reactive substance is called
nitrogen fixation​.

All life requires nitrogen-compounds, e.g., proteins and nucleic acids.

Air, which is 79% nitrogen gas (N​2​), is the major reservoir of nitrogen.
But most organisms cannot use nitrogen in this
form.
Plants must secure their nitrogen in "fixed"
form, i.e., incorporated in compounds such as:
nitrate ions (NO​3​−​)
ammonia (NH​3​)
urea (NH​2​)​2​CO
Animals secure their nitrogen (and all other)
compounds from plants (or animals that have
fed on plants).

Processes of the nitrogen cycle

Four processes participate in the cycling of nitrogen through the biosphere:

nitrogen fixation
decay
nitrification
denitrification

Microorganisms play major roles in all four of these.

Nitrogen fixation

Conversion of N​2
The conversion of nitrogen (N​2​) 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 N​2
(atmospheric nitrogen gas) into more chemically reactive forms:​[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 ​legume root nodules. These species are
diazotrophs​. An example of the free-living bacteria is ​Azotobacter.​
Industrial N-fixation : in the ​Haber-Bosch process, N​2 is converted together with
hydrogen gas (H​2​) into ammonia (NH​3​) which is used to make fertilizer and
explosives.
Combustion of fossil fuels : automobile engines and thermal power plants, which
release various nitrogen oxides (NO​x​).
Other processes : Additionally, the formation of NO from N​2 and O​2 due to photons
and especially lightning, are important for atmospheric chemistry, but not for
terrestrial or aquatic nitrogen turnover.

Decay

The proteins made by plants enter and pass through food webs just as
carbohydrates do. At each ​trophic level​, their metabolism produces organic nitrogen
compounds that return to the environment, chiefly in excretions. The final
beneficiaries of these materials are microorganisms of decay. They break down the
molecules in excretions and dead organisms into ​ammonia​.

Nitrification - ​main article: ​Nitrification

The conversion of ammonia to nitrates is performed primarily by soil-living bacteria

and other nitrifying 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​-​).

​ enitrification
Denitrification - m​ain article: D
Denitrification is the reduction of nitrites back into the largely inert nitrogen gas (N​2​),
completing the nitrogen cycle. This process is performed by bacterial species such as
Pseudomonas and ​Clostridium in anaerobic conditions.​[1] 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.

There are also assimilation, ammonification and annamox.

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.​[1] In plants which have a mutualistic
relationship with rhizobia, amy stone 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.

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.
 ​ nammox
Anaerobic ammonium oxidation - m​ain article: A

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.

Nitrogen fixation
Nitrogen fixation is the process by which ​nitrogen is taken from its natural, relatively
inert molecular form (N​2​) in the ​atmosphere and converted into nitrogen compounds
(such as ​ammonia​, ​nitrate​ and ​nitrogen dioxide​)[1]​
​ .
Nitrogen fixation is performed naturally by a number of different ​prokaryotes​, including
bacteria​, ​actinobacteria​, and certain types of ​anaerobic bacteria. Microorganisms that fix
nitrogen are called ​diazotrophs​. Some higher plants, and some animals (​termites​), have
formed associations with diazotrophs.
Nitrogen fixation also occurs as a result of non-biological processes. These include
lightning​, industrially through the ​Haber-Bosch Process​, and combustion.​[2]
Biological nitrogen fixation was discovered by the Dutch microbiologist ​Martinus
Beijerinck​.

Biological nitrogen fixation

Biological Nitrogen Fixation (​BNF​) occurs when atmospheric nitrogen is converted to
ammonia by a pair of bacterial enzymes called ​nitrogenase​.[1]​
​ The formula for BNF is:
N​2​ + 8H​+​ + 8e​−​ + 16 ​ATP​ → 2NH​3​ + H​2​ + 16​ADP​ + 16 ​P​i
Although ​ammonia (NH​3​) is the direct product of this reaction, it is quickly ​protonated
into ​ammonium (NH​4​+​). In free-living diazotrophs, the nitrogenase-generated ammonium
is assimilated into ​glutamate through the glutamine synthetase/glutamate synthase
pathway.
In most bacteria, the nitrogenase enzymes are very susceptible to destruction by oxygen
(and many bacteria cease production of the enzyme in the presence of oxygen).​[1] Low
oxygen tension is achieved by different bacteria by: living in anaerobic conditions,
respiring to draw down oxygen levels, or binding the oxygen with a protein such as
Leghemoglobin​ - also spelled leghaemoglobin.​[1]​.
The best-known plants which contribute to nitrogen fixation in nature, are in the ​legume
family - ​Fabaceae​, which includes such taxa as ​clover​, beans, alfalfa, lupines and peanuts.
They contain ​symbiotic bacteria called ​rhizobia within ​nodules in their ​root systems​,
producing nitrogen compounds that help the plant to grow and compete with other plants.
When the plant dies, the fixed nitrogen is released, making it available to other plants and
this helps to fertilize the ​soil​[1]​[3] The great majority of legumes have this association, but
a few genera (e.g., ​Styphnolobium)​ do not. In many traditional and organic farming
practices, fields are rotated through various types of crops, which usually includes one
consisting mainly or entirely of clover or buckwheat (family ​Polygonaceae​), which were
often referred to as "green manure", since the other natural way of adding nitrogen to the
soil is via animal waste products. The entire plant is often ploughed back into the field,
thus not only adding more nitrogen, but also improving the soil's organic content and
volume.

Microorganisms that fix nitrogen

Diazotrophs
Cyanobacteria
Azotobacteraceae
Rhizobia
Frankia

Figure 2: Part of a clover root

system bearing naturally
occurring nodules of
Rhizobium, bacteria that can
fix atmospheric nitrogen.
Each nodule is about 2-3 mm
long.

Nitrogen Fixation by
Cyanobacteria
Cyanobacteria​ inhabit nearly
all illuminated environments on Earth and play key roles in the carbon and ​nitrogen
cycle​ of the ​biosphere​. Generally, ​cyanobacteria​ are able to utilize a variety of
inorganic and organic sources of combined nitrogen, like ​nitrate​, ​nitrite​, ​ammonium​,
urea​ or some ​amino acids​. Several cyanobacterial strains are also capable of
diazotrophic​ growth. ​Genome sequencing​ has provided a large amount of
information on the genetic basis of ​nitrogen metabolism​ and its control in different
cyanobacteria. ​Comparative genomics​, together with functional studies, has led to a
significant advance in this field over the past years. 2-oxoglutarate has turned out to
be the central signalling molecule reflecting the carbon/nitrogen balance of
cyanobacteria. Central players of nitrogen control are the global transcriptional
factor NtcA, which controls the expression of many genes involved in nitrogen
metabolism, as well as the PII signalling protein, which fine-tunes cellular activities
in response to changing C/N conditions. These two proteins are sensors of the
cellular 2-oxoglutarate level and have been conserved in all cyanobacteria. In
contrast, the adaptation to nitrogen starvation involves heterogeneous responses in
different strains.
Chemical nitrogen fixation
Nitrogen can also be artificially fixed for use in ​fertilizer​, explosives, or in other products.
The most popular method is by the ​Haber process​. This artificial fertilizer production has
achieved such scale that it is now the largest source of fixed nitrogen in the ​Earth​'s
ecosystem​.
The Haber process requires high pressures and very high temperatures and active
research is committed to the development of catalyst systems that convert nitrogen to
ammonia at ambient temperatures. Many compounds can react with atmospheric nitrogen
under ambient conditions (eg lithium makes lithium nitride if left exposed), but the
products of such reactions are not easily converted into biologically accessible nitrogen
sources. After the first dinitrogen ​complex was discovered in 1965 based on ​ammonia
coordinated to ​ruthenium ([Ru(NH​3​)​5​(N​2​)]​2+​)​[​, research in chemical fixation focused on
transition metal complexes. Since that time a large number of transition metal compounds
that contain dinitrogen as ligand have been discovered. The dinitrogen ligand can be
bound either to a single metal or bridge two (or more) metals. The coordination chemistry
of dinitrogen is rich and under intense study. This research may lead to new ways of
using dinitrogen in synthesis and on an industrial scale.
In contrast to the graphic shown above, the major product of this reaction is ammonia
(NH3) and not an ammonium salt ([NH4][X]). In fact, approximately 75% of the
ammonia produced can be distilled away from the reaction vessel (suggesting the
ammonia is not protonated) into a vessel containing HCl as a trap. This method of
trapping the NH3 was doubtlessly chosen because it makes the product easier to handle.
Also, note that because only 1 equiv of Cl anion is available under catalytic conditions
(via reduction of the precatalyst molbdenum chloride, shown) therefore it is unlikely that
the product ammonium salt would always have this counterion.
Note also that although the dinitrogen complex is shown in brackets this species can be
isolated and characterized. Here the brackets do not indicate that the intermediate is not
observed.
Nitrification
Nitrification is the biological ​oxidation of ​ammonia with oxygen into ​nitrite followed by
the oxidation of these nitrites into ​nitrates​. Degradation of ammonia to nitrite is usually
the rate limiting step of nitrification. Nitrification is an important step in the ​nitrogen
cycle in ​soil​. This process was discovered by the ​Russian ​microbiologist​, ​Sergei
Winogradsky​.
The oxidation of ammonia into nitrite is performed by two groups of organisms,
ammonia oxidizing ​bacteria and ammonia oxidizing ​archaea​[1]​. Ammonia oxidizing
bacteria can be found among the β- and γ-proteobacteria [2]​ ​ . In soils the most studied
ammonia oxidizing bacteria belong to the genera ​Nitrosomonas and ​Nitrosococcus​.
Although in soils ammonia oxidation occurs by both bacteria and archaea in harsher
environments like oceans ammonia oxidation is dominated by archaea​[3]​. The second step
(oxidation of nitrite into nitrate) is (mainly) done by bacteria of the genus ​Nitrobacter.​
Both steps are producing energy to be coupled to ATP synthesis. Nitrifying organisms are
chemoautotrophs​, and use ​carbon dioxide​ as their ​carbon​ source for growth.
Nitrification also plays an important role in the removal of ​nitrogen from municipal
wastewater​. The conventional removal is nitrification, followed by ​denitrification​. The
cost of this process resides mainly in ​aeration (bringing oxygen in the reactor) and the
addition of an external carbon source (e.g. ​methanol​) for the denitrification.
In most environments both organisms are found together, yielding nitrate as the final
product. It is possible however to design systems in which selectively nitrite is formed
(the ​Sharon process)​ .
Together with ​ammonification​, nitrification forms a ​mineralization process which refers
to the complete decomposition of organic material, with the release of available nitrogen
compounds. This replenishes the ​nitrogen cycle​.

Chemistry
Nitrification is a process of nitrogen compound ​oxidation (effectively, loss of electrons
from the nitrogen atom to the oxygen atoms) :

NH​3​ + O​2​ → NO​2​−​ + 3H​+​ + 2e​−

NO​2​−​ + H​2​O → NO​3​−​ + 2H​+​ + 2e​−

Denitrification
Denitrification is a microbially facilitated process of dissimilatory nitrate reduction that
may ultimately produce molecular ​nitrogen (N​2​) through a series of intermediate gaseous
nitrogen oxide products. This respiratory process ​reduces oxidized forms of nitrogen in
response to the oxidation of an ​electron donor such as ​organic matter​. The preferred
nitrogen ​electron acceptors in order of most to least thermodynamically favourable
include: ​nitrate (NO​3​-​), ​nitrite (NO​2​-​), ​nitric oxide (NO), and ​nitrous oxide (N​2​O). In terms
of the general ​nitrogen cycle​, denitrification performs the opposite function of nitrogen
fixation, which converts gaseous nitrogen into a more oxidised and biologically available
form. The process is performed primarily by ​heterotrophic ​bacteria (such as ​Paracoccus
denitrificans and various ​pseudomonads​), although autotrophic denitrifiers have also
been identified (e.g.,​Thiobacillus denitrificans​). Denitrifiers are represented in all main
proteolytic groups. Generally several species of bacteria are involved in the complete
reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway has
been identified in the reduction process.​[1]
Denitrification takes place under special conditions in both terrestrial and marine
ecosystems​. In general, it occurs where ​oxygen​, a more energetically favourable electron
acceptor, is depleted, and ​bacteria ​respire nitrate as a substitute terminal ​electron
acceptor​. Due to the high concentration of oxygen in our atmosphere, denitrification only
takes place in environments where oxygen consumption exceeds the rate of oxygen
supply, such as in some ​soils and ​groundwater​, ​wetlands​, poorly ventilated corners of the
ocean, and in seafloor sediments.
Denitrification generally proceeds through some combination of the following
intermediate forms:
NO​3​-​ → ​NO​2​-​ → ​NO​ → ​N​2​O​ → ​N​2​ gas

Or expressed as a ​redox​ reaction:

2NO​3​-​ + 10e​-​ + 12H​+​ → N​2​ + 6H​2​O
Denitrification is the second step in the nitrification-denitrification process, the
conventional way to remove nitrogen from ​sewage and municipal ​wastewater​. It is also
an instrumental process in ​wetlands and ​riparian zones for the removal of excess nitrate
from groundwater with excess nitrate levels, commonly by extensive agricultural or
residential fertiliser usage.
Direct reduction from nitrate to ​ammonium​, a process known as dissimilatory nitrate
reduction to ammonium or DNRA, is also possible for organisms that have the nrf-​gene​.
This is less common than denitrification in most ecosystems as a means of nitrate
reduction.
Reduction under anoxic conditions can also occur through process called anaerobic
ammonia oxidation (​Anammox​); this reaction is expressed as the following:
 NH​4​+​ + NO​2​-​ → N​2​ + 2H​2​O
In some ​wastewater treatment plants​, small amounts of ​methanol are added to the
wastewater to provide a ​carbon source​ for the denitrification bacteria.

Anammox

Enrichment culture of the anammox bacterium ​Kuenenia stuttgartiensis,​ Radboud

University Nijmegen
Anammox​, an acronym for ​anaerobic ammonium oxidation,​ is a stage in the ​nitrogen
cycle​. In this biological process, ​nitrite and ​ammonium are converted directly into
dinitrogen gas. This process contributes up to 50% of the dinitrogen gas produced in the
oceans. It is thus a major sink for ​fixed nitrogen and so limits oceanic primary
productivity. The overall catabolic reaction is:
NH​4​+​ + NO​2​-​ → N​2​ + 2H​2​O.
The bacteria that perform the anammox process belong to the bacterial phylum
planctomycetes​, of which Planctomyces and Pirellula are the best known genera.
Currently four genera of anammox bacteria have been (provisionally) defined: ​Brocadia,​
Kuenenia,​ ​Anammoxoglobus (all fresh water species), and ​Scalindua (marine species).
The anammox bacteria are characterized by several striking properties: they all possess
one ​anammoxosome​, a membrane bound compartment inside the ​cytoplasm which is the
locus of anammox catabolism. Further, the membranes of these bacteria mainly consist of
ladderane lipids so far unique in biology. Of special interest is the turnover of ​hydrazine
(normally used as a high-energy rocket fuel and poisonous to most living organisms) as
an intermediate. A final striking feature of the organism is the extremely slow growth
rate. The doubling time is nearly two weeks.
The application of the anammox process lies in the removal of nitrogen in wastewater
treatment. Instead of the conventional ​nitrification​-denitrification process, only half of the
nitrogen has to be oxidized partly to ​nitrite​. For the enrichment of the anammox
organisms a biofilm system seems to be especially suited in which the necessary sludge
age of more than 20 days can be ensured. Other possibilities are ​sequencing batch
reactors (SBR) or gas-lift-loop reactors using granular sludge. The cost reduction
compared to conventional nitrogen removal is considerable; the technique, however, is
still young. The first full scale sludge-water treatment plant using the biological process
of anammox was built 2000 in Germany (Hattingen). As of 2006 there are three full scale
processes in The Netherlands. One on a municipal wastewater treatment plant (in
Rotterdam​), one on an industrial treatment plant (tannery) and one full scale application
using SBR at the wastewater treatment plant Strass, Austria.

Nitrogen Fixation

N2 → NH4+
Nitrogen fixation is the process wherein N​2 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 (Figure 2) and
receive carbohydrates and a favorable environment from their host plant in
exchange for some of the nitrogen they fix. There are 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.

In addition to nitrogen fixing bacteria, high-energy natural events such as lightning,

forest fires, and even hot lava flows can cause the fixation of smaller, but significant
amounts of nitrogen (Figure 3). The high energy of these natural phenomena can
break the triple bonds of N2 molecules, thereby making individual N atoms available
for chemical transformation.

Within the last century, humans have become as important a source of fixed
nitrogen as all natural sources combined. Burning fossil fuels, using synthetic
nitrogen fertilizers, and cultivation of legumes all fix nitrogen. Through these
activities, humans have more than doubled the amount of fixed nitrogen is pumped
into the biosphere every year (Figure 3), the consequences of which are discussed
below.

Nitrogen Uptake

NH​4​+​ → 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. When organisms nearer the top of the food
chain (like us!) eat, we are using nitrogen that has been fixed initially by nitrogen
fixing bacteria.

Nitrogen Mineralization

Organic N → NH​4​+

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

NH4​+​ → 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

NO​3​-​ → N​2​ + N​2​O

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 in the following sequence:

NO​3​-​ → NO​2​-​ → NO → N​2​O → N​2​.

Nitric oxide and nitrous oxide are both environmentally important gases. Nitric
oxide (NO) contributes to smog, and nitrous oxide (N2O) is an important
greenhouse gas, thereby contributing to global climate change.

Once converted to dinitrogen, nitrogen is unlikely to be reconverted to a biologically

available form because it is a gas and is rapidly lost to the atmosphere.
Denitrification is the only nitrogen transformation that removes nitrogen from
ecosystems (essentially irreversibly), and it roughly balances the amount of
nitrogen fixed by the nitrogen fixers described above.

Fertilizer
Spreading ​manure​, an organic fertilizer
Fertilisers (​also spelled fertilizer) are chemical compounds given to ​plants to promote
growth; they are usually applied either through the soil, for uptake by plant roots, or by
foliar feeding​, for uptake through leaves. Fertilisers can be ​organic (composed of organic
matter), or ​inorganic (made of simple, inorganic chemicals or minerals). They can be
naturally occurring compounds such as ​peat or ​mineral deposits, or manufactured through
natural processes (such as ​composting​) or chemical processes (such as the ​Haber
process​). These chemical compounds leave lawns, gardens, and soils looking beautiful as
they are given different essential nutrients that encourage plant growth.
They typically provide, in varying ​proportions​, the three ​major plant nutrients (​nitrogen​,
phosphorus​, ​potassium​: N-P-K), the ​secondary plant nutrients (​calcium​, ​sulfur​,
magnesium​) and sometimes ​trace elements (or micronutrients) with a role in plant or
animal nutrition: ​boron​, ​chlorine​, ​manganese​, ​iron​, ​zinc​, ​copper​, ​molybdenum and (in
some countries) ​selenium​.
Both organic and inorganic fertilizers were called "manures" derived from the French
expression for manual ​tillage​, but this term is now mostly restricted to organic ​manure​.
Though ​nitrogen is plentiful in the earth's atmosphere, relatively few plants engage in
nitrogen fixation (conversion of atmospheric nitrogen to a biologically useful form). Most
plants thus require nitrogen compounds to be present in the soil in which they grow.

Inorganic fertilizers (mineral fertilizer)

Naturally occurring inorganic fertilizers include Chilean ​sodium nitrate​, mined ​rock
phosphate​, and ​limestone​ (a calcium source).

Macronutrients and micronutrients

Fertilizers can be divided into macronutrients or micronutrients based on their
concentrations in plant dry matter. There are six macronutrients: nitrogen, phosphorus,
and potassium, often termed "primary macronutrients" because their availability is
usually managed with NPK fertilizers, and the "secondary macronutrients" — calcium,
magnesium, and sulphur — which are required in roughly similar quantities but whose
availability is often managed as part of liming and manuring practices rather than
fertilizers. The macronutrients are consumed in larger quantities and normally present as
a whole number or tenths of percentages in plant tissues (on a dry matter weight basis).
There are many micronutrients, required in concentrations ranging from 5 to 100 parts
per million (ppm) by mass. Plant micronutrients include ​iron (Fe), ​manganese (Mn),
boron​ (B), ​copper​ (Cu), ​molybdenum​ (Mo), ​nickel​ (Ni), ​chlorine​ (Cl), and ​zinc​ (Zn.

Macronutrient fertilizers
Synthesized materials are also called ​artificial​, and may be described as ​straight, where
the product predominantly contains the three primary ingredients of ​nitrogen (N),
phosphorus (P), and ​potassium (K), which are known as ​N-P-K fertilizers or ​compound
fertilizers when elements are mixed intentionally. They are named or labeled according
to the content of these three elements, which are macronutrients. The mass fraction
(percent) nitrogen is reported directly. However, phosphorus is reported as ​phosphorus
pentoxide (P​2​O​5​), the ​anhydride of ​phosphoric acid​, and potassium is reported as
potassium oxide (K​2​O), which is the anhydride of ​potassium hydroxide​. Fertilizer
composition is expressed in this fashion for historical reasons in the way it was analyzed
(conversion to ash for P and K); this practice dates back to ​Justus von Liebig (see more
below). Consequently, an 18-51-20 fertilizer would have 18% nitrogen as N, 51%
phosphorus as P​2​O​5​, and 20% potassium as K​2​O, The other 11% is known as ​ballast and
may or may not be valuable to the plants, depending on what is used as ballast. Although
analyses are no longer carried out by ashing first, the naming convention remains. If
nitrogen is the main element, they are often described as ​nitrogen fertilizers.
In general, the mass fraction (percentage) of elemental phosphorus, [P] = 0.436 x [P​2​O​5​]
and the mass fraction (percentage) of elemental potassium, [K] = 0.83 x [K​2​O]
(These conversion factors are mandatory under the UK fertilizer-labelling regulations if
elemental values are declared in addition to the N-P-K declaration.​[5]​)
An 18−51−20 fertilizer therefore contains, by weight, 18% elemental nitrogen (N), 22%
elemental phosphorus (P) and 16% elemental potassium (K).

Agricultural versus horticultural

In general, agricultural fertilizers contain only one or two macronutrients. Agricultural
fertilizers are intended to be applied infrequently and normally prior to or alongside
seeding. Examples of agricultural fertilizers are granular ​triple super-phosphate​,
potassium chloride​, ​urea​, and ​anhydrous ammonia​. The commodity nature of fertilizer,
combined with the high cost of shipping, leads to use of locally available materials or
those from the closest/cheapest source, which may vary with factors affecting
transportation by rail, ship, or truck. In other words, a particular ​nitrogen source may be
very popular in one part of the country while another is very popular in another
geographic region only due to factors unrelated to agronomic concerns.
Horticultural or specialty fertilizers, on the other hand, are formulated from many of the
same compounds and some others to produce well-balanced fertilizers that also contain
micronutrients. Some materials, such as ​ammonium nitrate​, are used minimally in large
scale production farming. The 18-51-20 example above is a horticultural fertilizer
formulated with high phosphorus to promote bloom development in ornamental flowers.
Horticultural fertilizers may be water-soluble (instant release) or relatively insoluble
(controlled release). Controlled release fertilizers are also referred to as sustained release
or timed release. Many controlled release fertilizers are intended to be applied
approximately every 3-6 months, depending on watering, growth rates, and other
conditions, whereas water-soluble fertilizers must be applied at least every 1-2 weeks and
can be applied as often as every watering if sufficiently dilute. Unlike agricultural
fertilizers, horticultural fertilizers are marketed directly to consumers and become part of
retail product distribution lines.
Nitrogen fertilizer
Nitrogen fertilizer is often synthesized using the ​Haber-Bosch process​, which produces
ammonia​. This ammonia is applied directly to the soil or used to produce other
compounds, notably ​ammonium nitrate and ​urea​, both dry, concentrated products that
may be used as fertilizer materials or mixed with water to form a concentrated liquid
nitrogen fertilizer, ​UAN​. Ammonia can also be used in the ​Odda Process in combination
with rock phosphate and potassium fertilizer to produce compound fertilizers such as
10-10-10 or 15-15-15.
The production of ammonia currently consumes about 5% of global natural gas
consumption, which is somewhat under 2% of world energy production.​[7] Natural gas is
overwhelmingly used for the production of ammonia, but other energy sources, together
with a hydrogen source, can be used for the production of nitrogen compounds suitable
for fertilizers. The cost of natural gas makes up about 90% of the cost of producing
ammonia.​[8] The price increases in natural gas in the past decade, among other factors
such as increasing demand, have contributed to an increase in fertilizer price.
Nitrogen-based fertilizers are most commonly used to treat fields used for growing ​maize​,
followed by ​barley​, ​sorghum​, ​rapeseed​, ​soyabean​ and ​sunflower​.

Health and sustainability issues

Inorganic fertilizers sometimes do not replace trace mineral elements in the soil which
become gradually depleted by crops grown there. This has been linked to studies which
have shown a marked fall (up to 75%) in the quantities of such minerals present in fruit
and vegetables.​[9] One exception to this is in ​Western Australia where deficiencies of
zinc​, ​copper​, ​manganese​, ​iron and ​molybdenum were identified as limiting the growth of
crops and pastures in the 1940s and 1950s. Soils in Western Australia are very old, highly
weathered and deficient in many of the major nutrients and trace elements. Since this
time these trace elements are routinely added to inorganic fertilizers used in agriculture in
this state.
In many countries there is the public perception that inorganic fertilizers "poison the soil"
and result in "low quality" produce. However, there is very little (if any) scientific
evidence to support these views. When used appropriately, inorganic fertilizers enhance
plant growth, the accumulation of organic matter and the biological activity of the soil,
preventing overgrazing and soil erosion. The nutritional value of plants for human and
animal consumption is typically improved when inorganic fertilizers are used
appropriately.
There are concerns though about ​arsenic​, ​cadmium and ​uranium accumulating in fields
treated with ​phosphate fertilizers​. The phosphate minerals contain trace amounts of these
elements and if no cleaning step is applied after mining the continuous use of phosphate
fertilizers leads towards an accumulation of these elements in the soil. Eventually these
can build up to unacceptable levels and get into the produce. (See ​cadmium poisoning​.)
Another problem with inorganic fertilizers is that they are presently produced in ways
which cannot be continued indefinitely. Potassium and phosphorus come from mines (or
from saline lakes such as the ​Dead Sea in the case of ​potassium fertilizers​) and resources
are limited. Nitrogen is unlimited, but nitrogen fertilizers are presently made using ​fossil
fuels such as ​natural gas​. Theoretically fertilizers could be made from ​sea water or
atmospheric nitrogen using ​renewable energy​, but doing so would require huge
investment and is not competitive with today's unsustainable methods. Innovative ​thermal
depolymerization biofuel schemes are trialling the production of byproducts with 9%
nitrogen fertilizer sourced from organic waste​[10]​[11]

Organic fertilizers

A compost bin
Naturally occurring organic fertilizers include ​manure​, ​slurry​, ​worm castings​, ​peat​,
seaweed​, ​sewage , and ​guano​. ​Green manure crops are also grown to add nutrients to the
soil​. Naturally occurring minerals such as mine rock phosphate, sulphate of potash and
limestone are also considered Organic Fertilizers.
Manufactured organic fertilizers include ​compost​, ​bloodmeal​, ​bone meal and ​seaweed
extracts. Other examples are natural enzyme digested proteins, ​fish meal​, and feather
meal.
The decomposing ​crop residue from prior years is another source of fertility. Though not
strictly considered "fertilizer", the distinction seems more a matter of words than reality.
Some ambiguity in the usage of the term 'organic' exists because some of synthetic
fertilizers, such as ​urea and ​urea formaldehyde​, are fully organic in the sense of ​organic
chemistry​. In fact, it would be difficult to chemically distinguish between urea of
biological origin and that produced synthetically. On the other hand, some fertilizer
materials commonly approved for organic agriculture, such as powdered ​limestone​,
mined ​rock phosphate and ​Chilean saltpeter​, are inorganic in the use of the term by
chemistry.
Although the density of nutrients in organic material is comparatively modest, they have
some advantages. Some or all organic fertilizer can be produced on-site, lowering
transport costs. The majority of nitrogen supplying organic fertilizers contain insoluble
nitrogen and act as a slow-release fertilizer.
Modern theories of organic agriculture admit the obvious success of ​Leibig's theory, but
stress that there are serious limitations to the current methods of implementing it via
chemical fertilization. They re-emphasize the role of humus and other organic
components of soil, which are believed to play several important roles:

Mobilizing existing soil nutrients, so that good growth is achieved with lower
nutrient densities while wasting less
Releasing nutrients at a slower, more consistent rate, helping to avoid a
boom-and-bust pattern
Helping to retain soil moisture, reducing the stress due to temporary moisture
stress
Improving the ​soil structure

Organics also have the advantage of avoiding certain problems associated with the
regular heavy use of artificial fertilizers:

the possibility of "burning" plants with the concentrated chemicals (i.e. an over
supply of some nutrients)
the progressive decrease of real or perceived "soil health", apparent in loss of
structure, reduced ability to absorb precipitation, lightening of soil color, etc.
the necessity of reapplying artificial fertilizers regularly (and perhaps in increasing
quantities) to maintain fertility
extensive runoff of soluble nitrogen and phosphorus, leading to eutrophication
the cost (substantial and rising in recent years) and resulting lack of independence

Organic fertilizers can have disadvantages:

As, typically, a dilute source of nutrients when compared to inorganic fertilizers,

applying significant amounts of nutrients in a distant location from the source
would incur increased costs for transportation
The composition of organic fertilizers tends to be more complex and variable than a
standardized inorganic product.
Improperly-processed organic fertilizers may contain ​pathogens from plant or
animal matter that are harmful to humans or plants. However, proper ​composting
should remove them.​[12]

In non-​organic farming a compromise between the use of artificial and organic fertilizers
is common, often using inorganic fertilizers supplemented with the application of
organics that are readily available such as the return of crop residues or the application of
manure.

Risks of fertilizer use

The problem of over-fertilization is primarily associated with the use of artificial
fertilizers, because of the massive quantities applied and the destructive nature of
chemical fertilizers on soil nutrient holding structures. The high solubilities of chemical
fertilizers also exacerbate their tendency to degrade ​ecosystems​, particularly through
eutrophication​.
Storage and application of some nitrogen fertilizers in some weather or soil conditions
can cause emissions of the ​greenhouse gas ​nitrous oxide (N​2​O). ​Ammonia gas (NH​3​) may
be emitted following application of inorganic fertilizers, or manure or slurry. Besides
supplying nitrogen, ammonia can also increase soil ​acidity (lower ​pH​, or "souring").
Excessive nitrogen fertilizer applications can also lead to pest problems by increasing the
​ [15]​
birth rate, longevity and overall fitness of certain pests.​[13]​ [14]​ ​ [16]​
​ [17]​
​ [18]
​
The concentration of up to 100 mg/kg of ​cadmium in ​phosphate minerals (for example,
minerals from ​Nauru​[19] and the ​Christmas islands​[20]​) increases the contamination of soil
with cadmium, for example in New Zealand.​[21] ​Uranium is another example of a
​
contaminant often found in phosphate fertilizers.​[22]​[23]​ [24]
For these reasons, it is recommended that knowledge of the nutrient content of the soil
and nutrient requirements of the crop are carefully balanced with application of nutrients
in inorganic fertilizer especially. This process is called ​nutrient budgeting​. By careful
monitoring of soil conditions, farmers can avoid wasting expensive fertilizers, and also
avoid the potential costs of cleaning up any pollution created as a by-product of their
farming.
It is also possible to over-apply organic fertilizers; however, their nutrient content, their
solubility, and their release rates are typically much lower than chemical fertilizers. By
their nature, most organic fertilizers also provide increased physical and biological
storage mechanisms to soils, which tend to mitigate their risks.

Global issues
The growth of the ​world's population to its current figure has only been possible through
intensification of ​agriculture associated with the use of fertilizers.​[25] There is an ​impact
on the sustainable consumption​ of other ​global resources​ as a consequence.
The use of fertilizers on a global scale ​emits significant quantities of ​greenhouse gas into
the atmosphere. Emissions come about through the use of: [26] ​

animal ​manures and ​urea​, which release ​methane​, ​nitrous oxide​, ​ammonia​, and
carbon dioxide in varying quantities depending on their form (solid or liquid) and
management (collection, storage, spreading)
fertilizers that use ​nitric acid or ​ammonium bicarbonate​, the production and
application of which results in emissions of ​nitrogen oxides​, ​nitrous oxide​, ​ammonia
and ​carbon dioxide​ into the atmosphere.

By changing processes and procedures, it is possible to mitigate some, but not all, of
these effects on ​anthropogenic climate change​.
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.​[2] 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.
N​2​O has risen in the atmosphere as a result of agricultural fertilization, biomass burning,
cattle and feedlots, and other industrial sources.​[3] N​2​O has deleterious effects in the
stratosphere​, where it breaks down and acts as a ​catalyst in the destruction of atmospheric
ozone​. ​Ammonia (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. NOx 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.​[1] ​Ecosystem processes can increase with nitrogen ​fertilization​, but
anthropogenic input can also result in nitrogen saturation, which weakens productivity
and can kill plants.​[2] Decreases in ​biodiversity can also result if higher nitrogen
availability increases nitrogen-demanding grasses, causing a degradation of
nitrogen-poor, species diverse heathlands.​[4]

Wastewater
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 the soil is unsuitable to handle some or all of the wastewater,
and as a result, the wastewater with the contaminants enters the ​aquifers​. These
contaminants accumulate and eventually end up in drinking water. One of the
contaminants concerned about the most is ​nitrogen in the form of ​nitrates​. A nitrate
concentration of 10 ppm 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 (milligrams
per liter).
The health risk associated with drinking >10 ppm nitrogen water is the development of
methemoglobinemia and has been found to cause ​blue baby syndrome​. Several 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.

Inorganic Nitrogen Sources

Calcium cynamide; urea; Chilean nitrate of soda or saltpeters, ammonium nitrate;
ammonium phosphate; ammonium sulfate; ammonium sulfate nitrate; calcium nitrate;
potassium nitrate. Ammonia, anhydrous contain 82% N; urea contains 45%N; ammonium
nitrate contains 33.5% N. Nitrogen also fixed by legumes like: alfalfa; red clover; sweet
clover; peas; soybean. Rhizobium and Azotobacter Clostridium are capable of fixing
nitrogen and supplying atmospheric N to the plants.
Nitrogen deficiency effects on photosynthesis, so leaves turn yellow, it also effects on
root growth, carbohydrate utilization, fruit firmness, hardness of plants, maturity time,
diseases incidence. In order to remove N deficiency the consideration of soil, pH and
time of application is need to be considered.
Conclusion
Nitrogen is an important constituent of protein and protoplasm. Essential for growth of
plants. Nitrogen in plant can be classified in to three groups:

Inorganic fraction, in form of NO​3​-​ and NH​4​+

Low molecular weight organic fraction-contains amino acids, amides and amines
High molecular weight organic bipolymers. Protein and nucleic acids
Conversion of nitrate to ammonia is as under: NO​3​- ----- NO​2​- ----- N​2​O​2​-2 ----- NH​4​OH
----- NH​3