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What Converts Atmospheric Nitrogen Into Ammonium?

You owe more than you probably ever realized to soil bacteria -- because without them, neither you nor any of the plants you see on dry land would be alive. That's because these soil bacteria convert atmospheric nitrogen into ammonia; some of them also convert ammonia into nitrates. Plants exploit this fixed nitrogen, and you in turn obtain the nitrogen you need from plants. To fix the atmospheric nitrogen, these bacteria use an enzyme called a nitrogenase.

Enzyme Catalysis
- The reaction between nitrogen and hydrogen has a very high activation energy barrier, meaning a lot of energy is needed to "kickstart" the reaction and make it happen. That's partly because the triple bond in the N2 molecule is exceptionally strong. The nitrogenase enzyme is a catalyst that provides another pathway by which the reaction can happen; this alternate pathway has a lower activation energy barrier, so the reaction will now happen more quickly.
Components
- The nitrogenase enzyme contains two components. One of these components feeds electrons into the second component; the second component actually has the machinery that will make the reaction happen. This machinery is still a focus of intense study by chemists and biochemists. The enzyme uses adenosine triphosphate (usually abbreviated ATP) to drive the reaction, and since cells like bacteria use sugars to get the energy they need to make ATP, this process ultimately costs the bacterium energy. The bacteria need nitrogen; however, and this process offers them the means to obtain a usable form.
Symbiosis
- Nitrogen-fixing bacteria often form interesting relationships with plants. Legume plants like soybeans, for example, grow nodules in their roots that shelter nitrogen-fixing bacteria. The plant provides a few nutrients for these unusual guests, and in exchange the bacteria furnishes the plant with nitrogen. That's why planting legumes can enrich the soil -- it increases nitrogen content so other plants can grow there later on. Other nitrogen bacteria live in the soil, in the ocean or in association with other plants.
Other Sources
- Not all of the nitrogen in you came from bacteria; a small fraction was fixed by lightning, while a larger fraction was fixed by the Haber-Bosch process. This process is a method for synthesizing ammonia from nitrogen and hydrogen, and large quantities of fertilizer are manufactured from ammonia produced in this way every year. The Haber process enables farmers to grow more crops on soil than it would otherwise support, so it's become an essential part of modern agriculture.

Nature