Editing Nitrogen cycle (section)

{{Short description|Biogeochemical cycle by which nitrogen is converted into various chemical forms}}
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[[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" />