Nitrogen Fixation

Nitrogen gas (N₂) makes up 78% of the Earth’s atmosphere, but due to its strong triple bond it is highly unreactive. This means organisms are not able to use it in this state, and it must be converted to more reactive compounds before it can enter the assimilation stage. Fixation is a process which converts nitrogen gas into more reactive nitrogenous compounds, such as ammonia and nitrates. It occurs in two main ways, by bacteria and by lightning.

Bacteria
Particular bacteria in soil and water are able to transform N₂ into ammonia (NH₃) via specialist enzymes.

N₂ → NH₃

Some nitrogen-fixing bacteria have a symbiotic relationship with plants, particularly legumes. They live within the root nodules of the plant and provide the plant with nitrogen in a useable form.

Lightning
The energy from lightning is enough to break the strong N₂ bonds which allows nitrogen to react with oxygen, forming nitrogen oxides (NO_x). The nitrogen oxides precipitate to the soil through rain or snow, where they can form nitrates (NO₃^-) in the soil.

N₂ → NO_x → NO₃^-

Figure 1: An overview of nitrogen fixation