Editing Nitrogen cycle (section)

=== 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" />