Editing Nitrogen cycle

{{Short description|Biogeochemical cycle by which nitrogen is converted into various chemical forms}}
{{pp-semi-indef}}
[[File:Reactive Nitrogen Global Annual Fluxes.jpg|thumb|upright=1.9| Global cycling of reactive nitrogen{{hsp}}<ref name="Fowler 2013" /> including industrial fertilizer production,<ref name="Galloway 2008" /> nitrogen fixed by natural ecosystems,<ref name="Vitousek 2013" /> nitrogen fixed by oceans,<ref name="Voss 2013" /> nitrogen fixed by agricultural crops,<ref name="Mark A 2013" /> NO<sub>x</sub> emitted by biomass burning,<ref name="Vuuren 2011" /> NO<sub>x</sub> emitted from soil,<ref name="Pilegaard 2013" /> nitrogen fixed by lightning,<ref name="Levy 1996" /> NH<sub>3</sub> emitted by terrestrial ecosystems,<ref name="Sutton 2013" /> deposition of nitrogen to terrestrial surfaces and oceans,<ref name="Dentener 2006" /><ref name="Duce 2008" /> NH<sub>3</sub> emitted from oceans,<ref name="Bouwman 2011" /><ref name="Solomon 2007" /><ref name="Duce 2008" /> ocean NO<sub>2</sub> emissions from the atmosphere,<ref name="Sutton 2011" /> denitrification in oceans,<ref name="Voss 2013" /><ref name="Deutsch 2007" /><ref name="Duce 2008" /> and reactive nitrogen burial in oceans.<ref name="Mark A 2013" />|alt=]]

The '''nitrogen cycle''' is the [[biogeochemical cycle]] by which [[nitrogen]] is converted into multiple chemical forms as it circulates among [[atmosphere|atmospheric]], [[terrestrial ecosystem|terrestrial]], and [[marine ecosystem]]s. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include [[nitrogen fixation|fixation]], [[ammonification]], [[nitrification]], and [[denitrification]]. The majority of [[Earth's atmosphere]] (78%) is atmospheric [[nitrogen]],<ref name="CarrollSalt2004p93" /> making it the largest source 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 [[ecologist]]s 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 [[Human impact on the nitrogen cycle|altered the global nitrogen cycle]].<ref name="Kuypers 2011" /><ref name="Galloway 2004" /><ref name="Reis 2016" /> Human modification of the global nitrogen cycle can negatively affect the natural environment system and also human health.<ref name="Gu 2012" /><ref name="Kim 2017" />

== Processes ==
{{biogeochemical cycle sidebar|nutrient}}
Nitrogen is present in the environment in a wide variety of chemical forms including organic nitrogen, [[ammonium]] ({{chem2|NH4+}}), [[nitrite]] ({{chem2|NO2-}}), [[nitrate]] ({{chem2|NO3-}}), [[nitrous oxide]] ({{chem2|N2O}}), [[nitric oxide]] (NO) or inorganic nitrogen gas ({{chem2|N2}}). Organic nitrogen may be in the form of a living organism, [[humus]] or in the intermediate products of organic matter decomposition. The processes in the nitrogen cycle is to transform nitrogen from one form to another. Many of those processes are carried out by [[microorganism|microbes]], either in their effort to harvest energy or to accumulate nitrogen in a form needed for their growth. For example, the [[metabolic waste#Nitrogen wastes|nitrogenous wastes]] in animal [[urine]] are broken down by [[nitrifying bacteria]] in the soil to be used by plants. The diagram alongside shows how these processes fit together to form the nitrogen cycle.

=== Nitrogen fixation ===
{{Main|Nitrogen fixation}}
The conversion of nitrogen gas ({{chem2|N2}}) into nitrates and nitrites through atmospheric, industrial and biological processes is called nitrogen fixation. Atmospheric nitrogen must be processed, or "[[nitrogen fixation|fixed]]", into a usable form to be taken up by plants. Between 5 and 10 billion kg per year are fixed by [[lightning]] strikes, but most fixation is done by free-living or [[symbiosis|symbiotic]] [[bacterium|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 compound]]s. Most biological nitrogen fixation occurs by the activity of [[molybdenum]] (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.<ref name="Moir 2011" /> An example of 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 [[Mutualism (biology)|mutualistic]] relationship with the plant, producing ammonia in exchange for [[carbohydrate]]s. Because of this relationship, legumes will often increase the nitrogen content of nitrogen-poor soils. A few non-legumes can also form such [[symbiosis|symbioses]]. Today, about 30% of the total fixed nitrogen is produced industrially using the [[Haber-Bosch]] process,<ref name="Smith 2004" /> which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia.<ref name="Smil 2000" />

=== Assimilation ===
{{Main|Assimilation (biology)|Nitrogen assimilation}}
Plants can absorb nitrate or ammonium from the soil by 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.<ref name="Willey 2011" /> While many animals, fungi, and other [[heterotrophic]] organisms obtain nitrogen by ingestion of [[amino acid]]s, [[nucleotide]]s, 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 matter|organic]], present in forms such as amino acids and DNA.<ref>{{Cite web |title=The Nitrogen Cycle: Processes, Players, and Human Impact {{!}} Learn Science at Scitable |url=https://www.nature.com/scitable/knowledge/library/the-nitrogen-cycle-processes-players-and-human-15644632/ |access-date=2025-02-13 |website=www.nature.com |language=en}}</ref> Bacteria and fungi convert this organic nitrogen into [[ammonia]] and sometimes ammonium through a series of processes called ammonification or [[Mineralization (soil)|mineralization]]. This is the last step in the nitrogen cycle step involving organic compounds.<ref>{{Citation |last=Strock |first=J.S. |title=Ammonification |date=2008 |pages=162–165 |url=https://www.sciencedirect.com/topics/earth-and-planetary-sciences/ammonification |access-date=2025-02-13 |publisher=Elsevier |language=en |doi=10.1016/B978-008045405-4.00256-1 |isbn=978-0-08-045405-4 |encyclopedia=Encyclopedia of Ecology|url-access=subscription }}</ref> Myriad enzymes are involved including [[Dehydrogenase|dehydrogenases]], [[Protease|proteases]], and [[Deamination|deaminases]] such as [[glutamate dehydrogenase]] and [[glutamine synthetase]].<ref name=":2">{{Citation |last=Cabello |first=P. |title=Nitrogen Cycle |date=2009-01-01 |work=Encyclopedia of Microbiology (Third Edition) |pages=299–321 |editor-last=Schaechter |editor-first=Moselio |url=https://www.sciencedirect.com/science/article/abs/pii/B9780123739445000559 |access-date=2025-03-18 |place=Oxford |publisher=Academic Press |isbn=978-0-12-373944-5 |last2=Roldán |first2=M. D. |last3=Castillo |first3=F. |last4=Moreno-Vivián |first4=C.}}</ref> Nitrogen mineralization and ammonification have a positive correlation with organic nitrogen in the soil,<ref>{{Cite journal |last=Marion |first=G. M. |last2=Black |first2=C. H. |date=1988 |title=Potentially Available Nitrogen and Phosphorus Along a Chaparral Fire Cycle Chronosequence |url=https://acsess.onlinelibrary.wiley.com/doi/abs/10.2136/sssaj1988.03615995005200040048x?getft_integrator=sciencedirect_contenthosting&src=getftr&utm_source=sciencedirect_contenthosting |journal=Soil Science Society of America Journal |language=en |volume=52 |issue=4 |pages=1155–1162 |doi=10.2136/sssaj1988.03615995005200040048x |issn=1435-0661|url-access=subscription }}</ref> soil microbial biomass, and average annual precipitation.<ref name=":3">{{Cite journal |last=Li |first=Zhaolei |last2=Tian |first2=Dashuan |last3=Wang |first3=Bingxue |last4=Wang |first4=Jinsong |last5=Wang |first5=Song |last6=Chen |first6=Han Y. H. |last7=Xu |first7=Xiaofeng |last8=Wang |first8=Changhui |last9=He |first9=Nianpeng |last10=Niu |first10=Shuli |date=2019 |title=Microbes drive global soil nitrogen mineralization and availability |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.14557?getft_integrator=sciencedirect_contenthosting&src=getftr&utm_source=sciencedirect_contenthosting |journal=Global Change Biology |language=en |volume=25 |issue=3 |pages=1078–1088 |doi=10.1111/gcb.14557 |issn=1365-2486|url-access=subscription }}</ref> They also respond closely to changes in temperature.<ref>{{Cite journal |last=Pierre |first=S. |last2=Hewson |first2=I. |last3=Sparks |first3=J. P. |last4=Litton |first4=C. M. |last5=Giardina |first5=C. |last6=Groffman |first6=P. M. |last7=Fahey |first7=T. J. |date=2017 |title=Ammonia oxidizer populations vary with nitrogen cycling across a tropical montane mean annual temperature gradient |url=https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1002/ecy.1863?getft_integrator=sciencedirect_contenthosting&src=getftr&utm_source=sciencedirect_contenthosting |journal=Ecology |language=en |volume=98 |issue=7 |pages=1896–1907 |doi=10.1002/ecy.1863 |issn=1939-9170|url-access=subscription }}</ref> However, these processes slow in the presence of vegetation with high carbon to nitrogen ratios<ref>{{Cite journal |last=Gosz |first=J. R. |date=1981 |title=Nitrogen Cycling in Coniferous Ecosystems |url=https://www.jstor.org/stable/45128679 |journal=Ecological Bulletins |issue=33 |pages=405–426 |issn=0346-6868}}</ref><ref>{{Cite journal |last=Vitousek |first=Peter M. |last2=Gosz |first2=James R. |last3=Grier |first3=Charles C. |last4=Melillo |first4=Jerry M. |last5=Reiners |first5=William A. |date=1982 |title=A Comparative Analysis of Potential Nitrification and Nitrate Mobility in Forest Ecosystems |url=https://esajournals.onlinelibrary.wiley.com/doi/abs/10.2307/1942609?getft_integrator=sciencedirect_contenthosting&src=getftr&utm_source=sciencedirect_contenthosting |journal=Ecological Monographs |language=en |volume=52 |issue=2 |pages=155–177 |doi=10.2307/1942609 |issn=1557-7015|url-access=subscription }}</ref> and fertilization with sugar.<ref>{{Cite journal |last=Zagal |first=Erick |last2=Persson |first2=Jan |date=1994-10-01 |title=Immobilization and remineralization of nitrate during glucose decomposition at four rates of nitrogen addition |url=https://www.sciencedirect.com/science/article/abs/pii/0038071794902127 |journal=Soil Biology and Biochemistry |volume=26 |issue=10 |pages=1313–1321 |doi=10.1016/0038-0717(94)90212-7 |issn=0038-0717|url-access=subscription }}</ref><ref>{{Cite journal |last=DeLuca |first=T. H. |last2=Keeney |first2=D. R. |date=1993-01-01 |title=Glucose-induced nitrate assimilation in prairie and cultivated soils |url=https://link.springer.com/article/10.1007/BF00001116 |journal=Biogeochemistry |language=en |volume=21 |issue=3 |pages=167–176 |doi=10.1007/BF00001116 |issn=1573-515X|url-access=subscription }}</ref>

[[File:Nitrogen Cycle - Reactions and Enzymes.svg|thumb|upright=1.5| {{center|'''Microbial nitrogen cycle'''{{hsp}}<ref name="Sparacino-Watkins 2013" /><ref name="Simon 2013" />}} [[Anammox|ANAMMOX]] is anaerobic ammonium oxidation, [[Dissimilatory nitrate reduction to ammonium|DNRA]] is dissimilatory nitrate reduction to ammonium, and [[Comammox|COMAMMOX]] is complete ammonium oxidation.]]

=== Nitrification ===
{{Main|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 ({{chem2|NH4+}}) is performed by bacteria such as the ''[[Nitrosomonas]]'' species, which converts ammonia to [[nitrites]] ({{chem2|NO2-}}). Other bacterial species such as ''[[Nitrobacter]]'', are responsible for the oxidation of the nitrites ({{chem2|NO2-}}) into [[nitrates]] ({{chem2|NO3-}}). It is important for the [[ammonia]] ({{chem2|NH3}}) to be converted to nitrates or nitrites because ammonia gas is toxic to plants.

Due to their very high [[solubility]] and because soils are highly 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.<ref name="Vitousek 1997" /> 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 ===
{{Main|Denitrification}}
Denitrification is the reduction of nitrates back into nitrogen gas ({{chem|N|2}}), completing the nitrogen cycle. This process is performed by bacterial species such as ''[[Pseudomonas]]'' and [[Paracoccus denitrificans|''Paracoccus'']], under anaerobic conditions. They use the nitrate as an electron acceptor in the place of oxygen during respiration. These facultatively (meaning optionally) 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. Denitrification occurs in free-living microorganisms as well as obligate symbionts of anaerobic ciliates.<ref>{{cite journal |last1=Graf |first1=Jon S. |last2=Schorn |first2=Sina |last3=Kitzinger |first3=Katharina |last4=Ahmerkamp |first4=Soeren |last5=Woehle |first5=Christian |last6=Huettel |first6=Bruno |last7=Schubert |first7=Carsten J. |last8=Kuypers |first8=Marcel M. M. |last9=Milucka |first9=Jana |title=Anaerobic endosymbiont generates energy for ciliate host by denitrification |journal=Nature |date=3 March 2021 |volume=591 |issue=7850 |pages=445–450 |doi=10.1038/s41586-021-03297-6| issn=0028-0836|pmid=33658719 |pmc=7969357 |bibcode=2021Natur.591..445G |doi-access=free }}</ref>

<gallery mode=packed style=float:left heights=270px>
File:Nitrogen cycle.jpg| Classical representation of nitrogen cycle
File:Nitrogen Cycle 2.svg|alt=Diagram of nitrogen cycle above and below ground. Atmospheric nitrogen goes to nitrogen-fixing bacteria in legumes and the soil, then ammonium, then nitrifying bacteria into nitrites then nitrates (which is also produced by lightning), then back to the atmosphere or assimilated by plants, then animals. Nitrogen in animals and plants become ammonium through decomposers (bacteria and fungi).|Flow of nitrogen through the ecosystem. Bacteria are a key element in the cycle, providing different forms of nitrogen compounds able to be assimilated by higher organisms
</gallery>

<gallery mode=packed style=float:right heights=400px>
File:The Nitrogen Cycle.png| Simple representation of the nitrogen cycle. Blue represent nitrogen storage, green is for processes moving nitrogen from one place to another, and red is for the bacteria involved
</gallery>

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=== Dissimilatory nitrate reduction to ammonium ===
{{Main|Dissimilatory nitrate reduction to ammonium}}
Dissimilatory nitrate reduction to ammonium&nbsp;(DNRA), or nitrate/nitrite ammonification, is an [[anaerobic respiration]] process. Microbes which undertake DNRA oxidise organic matter and use nitrate as an electron acceptor, reducing it to [[nitrite]], then [[ammonium]] ({{chem2|NO3- -> NO2- -> NH4+}}).<ref name="Lam 2011" /> Both denitrifying and nitrate ammonification bacteria will be competing for nitrate in the environment, although DNRA acts to conserve bioavailable nitrogen as soluble ammonium rather than producing dinitrogen gas.<ref name="Marchant 2014" />

=== Anaerobic ammonia oxidation ===
{{Main|Anammox}}
The <u>AN</u>aerobic <u>AMM</u>onia <u>OX</u>idation process is also known as the [[Anammox|ANAMMOX]] process, an abbreviation coined by joining the first [[syllable]]s of each of these three words. This biological process is a [[redox]] [[comproportionation]] reaction, in which [[ammonia]] (the [[reducing agent]] giving electrons) and [[nitrite]] (the [[oxidizing agent]] accepting electrons) transfer three [[electron]]s and are converted into one molecule of [[diatomic]] [[nitrogen]] ({{chem|N|2}}) gas and two water molecules. This process makes up a major proportion of nitrogen conversion in the [[ocean]]s. The [[stoichiometry|stoichiometrically]] balanced formula for the ANAMMOX chemical reaction can be written as following, where an [[ammonium]] [[ion]] includes the ammonia molecule, its [[Conjugate (acid-base theory)|conjugated]] [[Base (chemistry)|base]]:  

: {{chem2|NH4+ + NO2- -> N2 + 2 H2O}} (Δ''G''° = {{val|-357 |u=kJ.mol-1}}).<ref name="Anammox" />

This an [[exergonic process]] (here also an [[exothermic reaction]]) releasing energy, as indicated by the negative value of Δ''G''°, the difference in [[Gibbs free energy]] between the products of reaction and the reagents.

=== Other processes ===
Though nitrogen fixation is the primary source of plant-available nitrogen in most [[ecosystem]]s, in areas with nitrogen-rich [[bedrock]], the breakdown of this rock also serves as a nitrogen source.<ref name="NPR 2011" /><ref name="Schuur 2011" /><ref name="Morford 2011" /> Nitrate reduction is also part of the [[iron cycle]], under anoxic conditions Fe(II) can donate an electron to {{chem2|NO3-}} and is oxidized to Fe(III) while {{chem2|NO3-}} is reduced to {{chem2|NO2-, N2O, N2}}, and {{chem2|NH4+}} depending on the conditions and microbial species involved.<ref name="Burgin 2011" /> The [[Whale feces|fecal plumes of cetaceans]] also act as a junction in the marine nitrogen cycle, concentrating nitrogen in the epipelagic zones of ocean environments before its dispersion through various marine layers, ultimately enhancing oceanic primary productivity.<ref name="roman2010">{{cite journal | title = The Whale Pump: Marine Mammals Enhance Primary Productivity in a Coastal Basin | last1 = Roman | first1 = J. | last2 = McCarthy | first2 = J.J. | journal = PLOS ONE | volume = 5 | issue = 10 | page = e13255 |  pmc=2952594| doi = 10.1371/journal.pone.0013255 | date = 2010| pmid = 20949007 | bibcode = 2010PLoSO...513255R | doi-access = free }}</ref>

== Marine nitrogen cycle ==
[[File:Marine Nitrogen Cycle.jpg|thumb|upright=1.8|{{center|'''Marine nitrogen cycle'''}}]]
[[File:Main marine nitrogen cycles.jpg|thumb|upright=1.8| The main studied processes of the N cycle in different marine environments. Every coloured arrow represents a N transformation: {{chem|N|2}} fixation (red), nitrification (light blue), nitrate reduction (violet), DNRA (magenta), denitrification (aquamarine), N-damo (green), and anammox (orange). Black curved arrows represent physical processes such as advection and diffusion.<ref name=Pajares2019>{{cite journal | last1 = Pajares Moreno | first1 = S. | last2 = Ramos | first2 = R. | year = 2019 | title = Processes and Microorganisms Involved in the Marine Nitrogen Cycle: Knowledge and Gaps | url = | journal = Frontiers in Marine Science | volume = 6 | issue = | page = 739 | doi = 10.3389/fmars.2019.00739 | doi-access = free | bibcode = 2019FrMaS...6..739P }} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>]]

[[File:Marine nitrogen cycle under future ocean acidification.jpg|thumb|upright=1.8|{{center|'''Marine nitrogen cycle under future ocean acidification'''{{hsp}}<ref name="O'Brien2016" />}}]]

The nitrogen cycle is an important process in the ocean as well. While the overall cycle is similar, there are different players<ref name="Moulton 2016" /> and modes of transfer for nitrogen in the ocean. Nitrogen enters the water through the precipitation, runoff, or as {{chem|N|2}} from the atmosphere. Nitrogen cannot be utilized by [[phytoplankton]] as {{chem|N|2}} so it must undergo nitrogen fixation which is performed predominately by [[cyanobacteria]].<ref name="Miller 2008 60–62" /> Without supplies of fixed nitrogen entering the marine cycle, the fixed nitrogen would be used up in about 2000 years.<ref name="Gruber 2008 1–35" /> 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 [[Photic zone|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.<ref name="Miller 2008" /> Ammonification or [[Mineralization (geology)|Mineralization]] is performed by bacteria to convert organic nitrogen to ammonia. [[Nitrification]] can then occur to convert the ammonium to nitrite and nitrate.<ref name="Boyes 2011" /> 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. {{chem|N|2}} 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 most ''[[Prochlorococcus]]'' and some ''[[Synechococcus]]'' that can only take up nitrogen as ammonium.<ref name="Gruber 2008 1–35" />

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.<ref name="Parsons 1997" /> 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 storm]]s) 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.<ref name="Gruber 2008 1–35" /> The amount of ammonium in the ocean is about 3 orders of magnitude less than nitrate.<ref name="Gruber 2008 1–35" /> 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.<ref name="Miller 2008 60–62" /> 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.<ref name="Parsons 1997" /> 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.

===Future acidification===
As illustrated by the diagram on the right, additional [[carbon dioxide]] ({{CO2}}) is absorbed by the [[ocean]] and reacts with water, [[carbonic acid]] ({{chem|H|2|CO|3}}) is formed and broken down into both [[bicarbonate]] ({{Chem2|HCO3-}}) and hydrogen ({{chem|link=Hydronium|H|+}}) ions (gray arrow), which reduces bioavailable [[carbonate]] ({{Chem2|CO3(2-)}}) and decreases ocean [[pH]] (black arrow). This is likely to enhance nitrogen fixation by [[diazotroph]]s (gray arrow), which utilize {{chem|H|+}} ions to convert nitrogen into bioavailable forms such as [[ammonia]] ({{chem|NH|3}}) and [[ammonium]] ions ({{chem2|NH4+}}). However, as pH decreases, and more ammonia is converted to ammonium ions (gray arrow), there is less [[Redox|oxidation]] of ammonia to [[nitrite]] (NO{{su|b=2|p=–}}), resulting in an overall decrease in nitrification and denitrification (black arrows). This in turn would lead to a further build-up of fixed nitrogen in the ocean, with the potential consequence of [[eutrophication]]. Gray arrows represent an increase while black arrows represent a decrease in the associated process.<ref name="O'Brien2016">{{cite journal |title = Implications of Ocean Acidification for Marine Microorganisms from the Free-Living to the Host-Associated |year = 2016|doi = 10.3389/fmars.2016.00047|doi-access = free|last1 = O'Brien|first1 = Paul A.|last2 = Morrow|first2 = Kathleen M.|last3 = Willis|first3 = Bette L.|last4 = Bourne|first4 = David G.|journal = Frontiers in Marine Science|volume = 3| page=47 | bibcode=2016FrMaS...3...47O }} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

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== Human influences on the nitrogen cycle ==
[[File:Global - Global Fertilizer and Manure, Version 1 Nitrogen Fertilizer Application (6074011960).jpg|thumb|Nitrogen fertilizer application]]
[[File:Global Global Fertilizer and Manure, Version 1 Nitrogen in Manure Production (6173194512).jpg|thumb|Nitrogen in manure production]]
{{Main|Human impact 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 production of chemical [[fertilizer]]s, and pollution emitted by vehicles and industrial plants, human beings have more than doubled the annual transfer of nitrogen into biologically available forms.<ref name="Vitousek 1997" /> 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.<ref name="Holland 1999" />

Generation of Nr, [[reactive nitrogen]], has increased over 10 fold in the past century due to global [[industrialisation]].<ref name="Galloway 2008" /><ref name="Gu 2012 dup?" /> This form of nitrogen follows a cascade through the [[biosphere]] via a variety of mechanisms, and is accumulating as the rate of its generation is greater than the rate of [[denitrification]].<ref name="Cosby 2003" /> Nr burial in lakes and oceans has been increasing in tandem with anthropogenic input, now double the Nr burial flux pre-[[Industrial Revolution|industrial revolution]]. Reactive nitrogen can be denitrified in water or buried in sediments to accumulate. This buried Nr lies latent until its sediments are disturbed through events like [[Storm|storms]] or [[Flood|floods]], when large amounts of nitrogen are reintroduced to the water where it can be denitrified and impact the environment.<ref>{{Cite journal |last1=Wang |first1=Mei |last2=Houlton |first2=Benjamin Z. |last3=Wang |first3=Sitong |last4=Ren |first4=Chenchen |last5=van Grinsven |first5=Hans J. M. |last6=Chen |first6=Deli |last7=Xu |first7=Jianming |last8=Gu |first8=Baojing |date=2021-11-28 |title=Human-caused increases in reactive nitrogen burial in sediment of global lakes |url=https://www.sciencedirect.com/science/article/pii/S2666675821000837 |journal=The Innovation |volume=2 |issue=4 |pages=100158 |doi=10.1016/j.xinn.2021.100158 |pmid=34704084 |bibcode=2021Innov...200158W |issn=2666-6758|pmc=8527044 }}</ref>

[[Nitrous oxide]] ({{chem|N|2|O}}) has risen in the atmosphere as a result of agricultural fertilization, biomass burning, cattle and feedlots, and industrial sources.<ref name="Chapin 2002" /> {{chem|N|2|O}} 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.<ref name="Howarth 2009" />

[[Ammonia]] ({{chem|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 to water droplets, eventually resulting in [[nitric acid]] ([[Hydrogen|H]][[nitrate|NO<sub>3</sub>]]) that produces [[acid rain]]. Atmospheric ammonia and nitric acid also damage respiratory systems.

The very high temperature of lightning naturally produces small amounts of {{chem|NO|x}}, {{chem|NH|3}}, and {{chem|HNO|3}}, but high-temperature [[combustion]] has contributed to a 6- or 7-fold increase in the flux of {{chem|NO|x}} to the atmosphere. Its production is a function of combustion temperature - the higher the temperature, the more {{chem|NO|x}} is produced. [[Fossil fuel]] combustion is a primary contributor, but so are biofuels and even the burning of hydrogen. However, the rate that hydrogen is directly injected into the combustion chambers of internal combustion engines can be controlled to prevent the higher combustion temperatures that produce {{chem|NO|x}}.

Ammonia and nitrous oxides actively alter [[atmospheric chemistry]]. They are precursors of [[troposphere|tropospheric]] (lower atmosphere) ozone production, which contributes to [[smog]] and [[acid rain]], damages [[plant]]s and increases nitrogen inputs to ecosystems. [[Ecosystem]] processes can increase with [[nitrogen fertilization]], but [[human impact on the environment|anthropogenic]] input can also result in nitrogen saturation, which weakens productivity and can damage the health of plants, animals, fish, and humans.<ref name="Vitousek 1997" />

Decreases in [[biodiversity]] can also result if higher nitrogen availability increases nitrogen-demanding grasses, causing a degradation of nitrogen-poor, species-diverse [[Heath (habitat)|heathlands]].<ref name="Aerts 1988" />

== Consequence of human modification of the nitrogen cycle ==
[[File:Estimated nitrogen surplus across Europe 2005.png|thumb|Estimated nitrogen surplus (the difference between inorganic and organic fertilizer application, atmospheric deposition, fixation, and uptake by crops) for the year 2005 across Europe.]]

=== Impacts on natural systems ===
Increasing levels of [[Deposition (aerosol physics)|nitrogen deposition]] is shown to have several adverse effects on both terrestrial and [[aquatic ecosystem]]s.<ref name="Bobbink 2010" /><ref name="Liu 2011" /> Nitrogen gases and [[aerosol]]s can be directly toxic to certain plant species, affecting the aboveground physiology and growth of plants near large [[Point source pollution|point sources]] of nitrogen pollution. Changes to plant species may also occur as nitrogen compound accumulation increases availability in a given ecosystem, eventually changing the species composition, plant diversity, and nitrogen cycling. Ammonia and ammonium – two reduced forms of nitrogen – can be detrimental over time due to increased toxicity toward sensitive species of plants,<ref name="Britto 2002" /> particularly those that are accustomed to using nitrate as their source of nitrogen, causing poor development of their roots and shoots. Increased nitrogen deposition also leads to soil acidification, which increases base cation leaching in the soil and amounts of [[Aluminium|aluminum]] and other potentially toxic metals, along with decreasing the amount of [[nitrification]] occurring and increasing plant-derived litter. Due to the ongoing changes caused by high nitrogen deposition, an environment's susceptibility to ecological stress and disturbance – such as pests and [[pathogen]]s – may increase, thus making it less resilient to situations that otherwise would have little impact on its long-term vitality.

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.<ref name="Camargoa 2006" /> 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 zone (ecology)|dead zones]] near the mouth of the Mississippi in the [[Gulf of Mexico]] are a well-known example of [[algal bloom]]-induced [[Hypoxia (environmental)|hypoxia]].<ref name="Rabalais 2002" /><ref name="Dybas 2005" /> Even though there have been some efforts at reducing Nitrogen agricultural runoff, there has been no significant reduction in dead zone size.<ref>{{Cite web |date=2024-08-01 |title=Gulf of Mexico 'dead zone' larger than average, scientists find {{!}} National Oceanic and Atmospheric Administration |url=https://www.noaa.gov/news-release/gulf-of-mexico-dead-zone-larger-than-average-scientists-find |access-date=2025-04-08 |website=www.noaa.gov |language=en}}</ref> The New York [[Adirondack Lake]]s, [[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.<ref name="2lTxW" />

[[Fresh water|Freshwater]] has a lower ability to neutralize acidity, so acidification can occur with less nitrogen deposition.<ref>{{Cite web |title=FAQs about Ocean Acidification - Woods Hole Oceanographic Institution |url=https://www.whoi.edu/know-your-ocean/ocean-topics/how-the-ocean-works/ocean-chemistry/ocean-acidification/faqs-about-ocean-acidification/ |access-date=2025-03-19 |website=Woods Hole Oceanographic Institution |language=en-US}}</ref> This acidification can negatively impact [[fish]] and aquatic [[Invertebrate|invertebrates]]<ref>{{Cite journal |last1=Bednaršek |first1=Nina |last2=Newton |first2=Jan A. |last3=Beck |first3=Marcus W. |last4=Alin |first4=Simone R. |last5=Feely |first5=Richard A. |last6=Christman |first6=Natasha R. |last7=Klinger |first7=Terrie |date=2021-04-15 |title=Severe biological effects under present-day estuarine acidification in the seasonally variable Salish Sea |url=https://www.sciencedirect.com/science/article/abs/pii/S0048969720362185 |journal=Science of the Total Environment |volume=765 |pages=142689 |doi=10.1016/j.scitotenv.2020.142689 |pmid=33077233 |bibcode=2021ScTEn.76542689B |issn=0048-9697}}</ref> while favoring phytoplankton that can handle more acidic environments.<ref>{{Cite journal |last1=Erisman |first1=Jan Willem |last2=Galloway |first2=James N. |last3=Seitzinger |first3=Sybil |last4=Bleeker |first4=Albert |last5=Dise |first5=Nancy B. |last6=Petrescu |first6=A. M. Roxana |last7=Leach |first7=Allison M. |last8=de Vries |first8=Wim |date=2013-07-05 |title=Consequences of human modification of the global nitrogen cycle |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=368 |issue=1621 |pages=20130116 |doi=10.1098/rstb.2013.0116 |pmc=3682738 |pmid=23713116}}</ref>

Ammonia ({{chem|NH|3}}) is highly toxic to fish, and the level of ammonia discharged from [[Sewage treatment|wastewater treatment facilities]] must be closely monitored. Nitrification via [[aeration]] before discharge is often desirable to prevent fish deaths. Land application can be an attractive alternative to aeration.

=== Impacts on human health: nitrate accumulation in drinking water ===
Leakage of [[Reactive nitrogen|Nr (reactive nitrogen)]] from human activities can cause nitrate accumulation in the natural water environment, which can create harmful impacts on human health. Excessive use of N-fertilizer in agriculture has been a significant source of nitrate pollution in groundwater and surface water.<ref name="Power 1989" /><ref name="Strebel 1989" /> Due to its high solubility and low retention by soil, nitrate can easily escape from the subsoil layer to the groundwater, causing nitrate pollution.  Some other [[Nonpoint source pollution|non-point sources]] for nitrate pollution in groundwater originate from livestock feeding, animal and human contamination, and municipal and industrial waste. Since groundwater often serves as the primary domestic water supply, nitrate pollution can be extended from groundwater to surface and drinking water during [[Drinking water|potable water]] production, especially for small community water supplies, where poorly regulated and unsanitary waters are used.<ref name="Fewtrell 2004" />

The [[World Health Organization|WHO]] standard for [[drinking water]] is 50&nbsp;mg {{chem2|NO3-}} L<sup>−1</sup> for short-term exposure, and for 3&nbsp;mg {{chem2|NO3-}} L<sup>−1</sup> chronic effects.<ref name="c3aLP" /> Once it enters the human body, nitrate can react with organic compounds through [[nitrosation]] reactions in the [[stomach]] to form [[nitrosamine]]s and [[N-Nitrosamides|nitrosamides]], which are involved in some types of cancers (e.g., [[oral cancer]] and [[gastric cancer]]).<ref name="Canter 2019" />

=== Impacts on human health: air quality ===
Human activities have also dramatically altered the global nitrogen cycle by producing nitrogenous gases associated with global atmospheric nitrogen pollution. There are multiple sources of atmospheric [[reactive nitrogen]] (Nr) fluxes. Agricultural sources of reactive nitrogen can produce atmospheric emission of [[ammonia]] ({{chem2|NH3}}), [[nitrogen oxide]]s ({{chem|NO|x}}) and [[nitrous oxide]] ({{chem|N|2|O}}). Combustion processes in energy production, transportation, and industry can also form new reactive nitrogen via the emission of {{chem|NO|x}}, an unintentional waste product. When those reactive nitrogens are released into the lower atmosphere, they can induce the formation of smog, [[particulate matter]], and aerosols, all of which are major contributors to adverse health effects on human health from air pollution.<ref name="Kampa 2008" /> In the atmosphere, {{chem|NO|2}} can be [[Redox|oxidized]] to [[nitric acid]] ({{chem|HNO|3}}), and it can further react with {{chem|NH|3}} to form [[ammonium nitrate]] ({{Chem2|NH4NO3}}), which facilitates the formation of particulate nitrate. Moreover, {{chem|NH|3}} can react with other acid gases ([[Sulfuric acid|sulfuric]] and [[hydrochloric acid]]s) to form ammonium-containing particles, which are the precursors for the secondary [[Organic compound|organic]] [[aerosol]] particles in [[Smog|photochemical smog]].<ref name="Erisman 2013" />

== See also ==
* {{annotated link|Planetary boundaries}}
* {{annotated link|Phosphorus cycle}}

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}}

{{Biogeochemical cycle}}

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[[Category:Nitrogen cycle| ]]
[[Category:Nitrogen|Cycle]]
[[Category:Biogeochemical cycle]]
[[Category:Soil biology]]
[[Category:Metabolism]]
[[Category:Biogeography]]