The nitrogen cycle: fixation

The main source of nitrogen on Earth is the atmosphere. But most living things cannot use nitrogen gas directly: it must first be fixed into compounds. Atmospheric nitrogen is fixed in three main ways.

Figure to be added

Figure 15.18 — the nitrogen cycle: nitrogen (N₂) in the air fixed by lightning, by industry (chemical fertiliser) and by bacteria; plants and animals, their waste and dead bodies decomposed to ammonium (NH₄⁺), then nitrite (NO₂⁻) and nitrate (NO₃⁻) by nitrifying bacteria; nitrate returned to the air by denitrifying bacteria.

Biological fixation

Some bacteria living free in the soil (Azotobacter), and bacteria such as Rhizobium living symbiotically in the root nodules of legumes, turn atmospheric nitrogen into ammonia.

Atmospheric fixation

When lightning strikes, atmospheric nitrogen becomes nitric oxide and nitrogen dioxide.

Industrial fixation

Turning atmospheric nitrogen into nitrates as chemical fertilisers is done industrially.

Why nitrogen needs fixing

Air is almost four-fifths nitrogen, yet plants can starve of it. The two atoms of an N₂ molecule are held by a very strong triple bond, and ordinary living things cannot break it. Every living thing needs nitrogen to make proteins and DNA, so something has to split N₂ and join it to hydrogen or oxygen first. Only a few bacteria can do this, using special enzymes; lightning does it with enormous energy; and factories do it with high temperature and pressure. This is why farmers grow legumes such as beans and gram in rotation with other crops: their root-nodule bacteria enrich the soil with nitrogen for free.

Three ways to fix nitrogen

Bacteria (Azotobacter, Rhizobium), lightning and industrial fertiliser-making turn atmospheric nitrogen into compounds plants can use.