What is Nitrogen cycle?
[[File:Nitrogen_Cycle.jpg|right|thumb|Nitrogen cycle|300px]] The '''nitrogen cycle''' is the way that [[nitrogen]] in [[nature]] is changed into many different forms that are used by living organism[s]. [[Air]] is about 78% nitrogen. Nitrogen chemicals are needed for life. Nitrogen is a necessary part of [[proteins]], [[DNA]], and [[RNA]]. In plants, nitrogen is needed for [[photosynthesis]] and growth. <ref name=Smil>{{cite book |author=Smil V. |year=2000 |title=Cycles of life |publisher=[[Scientific American]] Library, New York}}</ref> However, living things cannot use the elemental nitrogen in the air for these things. [[Nitrogen fixation]] is needed to change the nitrogen in air (N<sub>2</sub>) into forms that can be used by life. Most nitrogen fixation is done by [[microorganism]]s called [[bacteria]]. These bacteria have an [[enzyme]] that combines N<sub>2</sub> with [[hydrogen]] gas (H<sub>2</sub>) to make [[ammonia]] (NH<sub>3</sub>). Some of these bacteria live in the roots of [[plant]]s (mostly [[legumes]]). In these roots, they make ammonia for the plant and the plant gives them [[carbohydrates]]. Other plants take nitrogen [[Chemical compound|compounds]] out of the [[soil]] through their roots. All nitrogen in [[animal]]s comes from eating plants. [[Ammonium]] (NH<sub>4</sub>) in soil is made by nitrogen-fixing bacteria and [[decomposer]]s, [[bacteria]] and [[fungi]] that break down dead life into its parts. This process is called ''ammonification''. Ammonium has a [[positive charge]]. It easily joins to [[clay]] and [[humus]] in the soil. Ammonia and ammonium are [[poison]]ous to [[fish]] and other animals. [[Sewage]] and other [[waste]]-[[water]] is regularly measured because of this. If ammonia levels are too high, ''nitrification'' must happen. Nitrification is the [[oxidation]] of ammonia and ammonium to [[nitrite]] (NO<sub>2</sub><sup>−</sup>) and then to [[nitrate]] (NO<sub>3<