Editing Nitrogen cycle (section)

== 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" />