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Dr. Christopher S. Baird

How do plants get their nitrogen from the air?

Category: Chemistry      Published: February 21, 2014

green trees
Plants get their nitrogen from the soil and not directly from the air. Public Domain Image, source: Christopher S. Baird.

Plants do not get their nitrogen directly from the air. Although nitrogen is the most abundant element in the air, every nitrogen atom in the air is triple-bonded to another nitrogen atom to form molecular nitrogen, N2. This triple bond is very strong and very hard to break (it takes energy to break chemical bonds whereas energy is only released when bonds are formed). As a result, even though nitrogen in the air is very common, it is energetically unfavorable for a plant to split the nitrogen molecule in order to get the raw atoms that it can use. The strong triple bond of N2 also makes it hard for molecular nitrogen to react with most other chemicals. This is, in fact, part of the reason there is so much nitrogen in the air to begin with. Also, the stability and symmetry of the nitrogen molecule makes it hard for different nitrogen molecules to bind to each other. This property means that molecular nitrogen can be cooled to very low temperatures before becoming liquid, leading liquid nitrogen to be a very effective cryogenic liquid.

The act of breaking apart the two atoms in a nitrogen molecule is called "nitrogen fixation". Plants get the nitrogen that they need from the soil, where it has already been fixed by bacteria and archaea. Bacteria and archaea in the soil and in the roots of some plants have the ability to convert molecular nitrogen from the air (N2) to ammonia (NH3), thereby breaking the tough triple bond of molecular nitrogen. Such organisms are called "diazotrophs". From here, various microorganisms convert ammonia to other nitrogen compounds that are easier for plants to use. In this way, plants get their nitrogen indirectly from the air via microorganisms in the soil and in certain plant roots. Note that lightning and high-energy solar radiation can also split the nitrogen molecule, and therefore also fixes the nitrogen in the air. However, the amount of nitrogen fixed by lightning and solar radiation is insignificant compared to the amount fixed by diazotrophs in the soil and in roots. In his book Nitrogen Fixation, John Postgate states,

The fixation of nitrogen – the conversion of atmospheric nitrogen into a form that plants can use – is a process fundamental to world agriculture. It comes about as a consequence of spontaneous, anthropogenic and biological activities. The existence and importance of the biological component have been recognized for more than a century, but scientific advances over the past few decades have radically altered our understanding of its nature and mechanisms.

the nitrogen cycle
The biological nitrogen cycle crucially depends on soil-inhabiting and root-inhabiting bacteria and archaea, which split apart the nitrogen molecules from the air. Public Domain Image, source: EPA.

Topics: air, ammonia, atmosphere, diazotroph, nitrogen, nitrogen fixation, plants