In all of these biological nitrogen-containing molecules, nitrogen is in a reduced state with an oxidation number of -3, as in ammonia. Living organisms require a supply of reduced nitrogen to synthesize these molecules, but most of them cannot get it from atmospheric . Fortunately, there are bacteria that can "fix" nitrogen by converting to . Energy is required to accomplish this, but this expenditure of energy is worthwhile to the bacterium because it provides a source of reduced nitrogen for synthetic purposes. When organisms die, most of the reduced nitrogen in proteins and other compounds remains reduced and is reused by other organisms. However, there are inevitable losses in this process, and new reduced nitrogen is required. The rest of the living world cannibalizes the efforts of nitrogen-fixing bacteria to keep the system going. Many such bacteria live in the root nodules of legumes such as soybeans, which is why agricultural land sometimes is replenished by growing a crop of soybeans and plowing the crop under at maturity. If given nitrates (with nitrogen ON = +5) instead of ammonia (with nitrogen ON = -3), plants can reduce them and incorporate them into protein. Thus the critical factor is not the oxidation state of nitrogen, it is the unreactivity of gaseous and the inability of plants or animals to do anything with it.

The triple bond in the molecule is so stable and resistant to attack that reactions involving , either oxidation or reduction, are hopelessly slow. Nitrogen-fixing bacteria, alone among living organisms, have catalytic enzymes that speed up these reactions.

A limited amount of is fixed in the atmosphere each year by electrical discharge in lightning, whereby the relative unreactivity of is overcome by energy from the lightning discharge. The products include acidic oxides of nitrogen, which means that the rain during a thunderstorm actually is a very dilute solution of nitric acid. More important as a rival to bacterial processes is industrial nitrogen fixation by methods involving catalysts and high pressures that were developed by Fritz Haber in Germany during World War I:



Once fixed as ammonia, nitrogen then can be used directly as fertilizers to grow crops, or oxidized to nitrates for explosives to blow them up.