How does nitrogen travel from air into life and back again?

Most air is nitrogen gas, yet a plant cannot usually use that gas directly to build its proteins. The two nitrogen atoms in each nitrogen-gas molecule are tightly joined. Nitrogen must pass through particular transformations before most organisms can incorporate it into living material.

Certain microbes can break the nitrogen-gas bond and make ammonia, much of which becomes ammonium in water. This biological nitrogen fixation requires energy. Some fixing microbes live freely, while others exchange resources with plants such as legumes. Lightning and industrial fertilizer production provide additional routes into forms that can participate readily in biological processes.

Plants can absorb ammonium or nitrate through their roots. Other microbes transform ammonium into nitrite and then nitrate in a process called nitrification, commonly where oxygen is available. Plants incorporate nitrogen into amino acids and other essential molecules. Animals obtain nitrogen by eating plants or other animals; they do not gain their protein-building nitrogen simply by breathing nitrogen gas.

When organisms release waste or die, decomposers break down nitrogen-containing material. Nitrogen can return to ammonium and become available again. But another route leads out of the immediate food web. In oxygen-poor environments, some microbes use nitrate in their metabolism and convert it through several steps into nitrogen gases. Complete denitrification returns nitrogen gas to the atmosphere.

The routes depend on local conditions. A waterlogged patch and an airy soil pore can support different transformations. Water can also carry nitrate away before nearby roots take it up. Consequently, adding usable nitrogen to one field can affect a river, lake or coastal ecosystem farther away. The same element is involved, but its chemical form and location change what can happen next.

For example, a nitrogen atom in a fallen leaf can return to a plant through decomposition without visiting the atmosphere first.

The pictured cycle is therefore a network of transformations, not a rule that every atom follows one prescribed loop. Nitrogen can spend time in living matter, soil, water or air. Some pathways also release nitrous oxide, a greenhouse gas. Understanding the network explains both why nitrogen is indispensable to life and why an excess of readily usable nitrogen can become an environmental problem.