nitrogen fixation

There is nitrogen everywhere — four-fifths of the air — and almost none of it is usable. "Fixing" nitrogen means taking the inert N2 of the atmosphere and turning it into a chemically reactive, life-usable form, above all ammonia (NH3) or nitrate (NO3-). It is one of the most consequential reactions on Earth, because nitrogen built into proteins and DNA must first be wrenched out of the air, and the air guards it jealously behind a triple bond.

Fixation happens by three main routes. Biological fixation is done by certain bacteria — free-living ones and those living in the root nodules of legumes such as peas and beans — using the enzyme nitrogenase, which contains iron and molybdenum and quietly reduces N2 to NH3 at ordinary temperature and pressure, a feat chemists cannot yet match cheaply. Atmospheric fixation happens when lightning supplies enough energy to combine N2 and O2 into nitrogen oxides, which dissolve in rain. Industrial fixation is the Haber-Bosch process, which combines N2 and H2 over an iron catalyst at high temperature and pressure to make ammonia on a colossal scale. All three share the same hard step: breaking or activating the N≡N triple bond.

Industrial fixation reshaped human history. Roughly half the nitrogen atoms in the average person's body today passed through a Haber-Bosch reactor, because synthetic fertiliser feeds a large fraction of the world's population. But the same flood of fixed nitrogen now disrupts ecosystems: excess nitrate runs off into rivers and seas, causing algal blooms and dead zones, and nitrogen oxides from combustion add to smog and acid rain. Fixation is therefore both one of chemistry's greatest gifts and one of its largest environmental headaches.