How do plants get their nitrogen from the air?

Plants do not directly obtain their nitrogen from the air. While nitrogen is the most abundant element in the atmosphere, it exists primarily as molecular nitrogen, denoted as $N_2$. Each nitrogen atom in this molecule is triple-bonded to another nitrogen atom, creating a very strong bond that is difficult to break.

The Challenge of Nitrogen Fixation

The triple bond in molecular nitrogen is exceptionally strong, making it energetically unfavorable for plants to split the nitrogen molecule to access the individual nitrogen atoms. This process of breaking apart the nitrogen atoms is known as “nitrogen fixation.”

The energy required to break these chemical bonds is significant, and plants lack the necessary mechanisms to perform this task. Instead, plants acquire the nitrogen they need from the soil, where it has already been converted into usable forms by certain microorganisms.

Role of Diazotrophs

In the soil, specific bacteria and archaea, collectively referred to as “diazotrophs,” possess the unique ability to convert atmospheric nitrogen ($N_2$) into ammonia ($NH_3$). This conversion involves breaking the tough triple bond of molecular nitrogen, allowing for the formation of ammonia, which is a more accessible nitrogen compound for plants.

Once ammonia is produced, various microorganisms in the soil further convert it into other nitrogen compounds that are easier for plants to utilize. Thus, plants indirectly obtain their nitrogen from the air through the action of these beneficial microorganisms in the soil and within the roots of certain plants.

Other Sources of Nitrogen Fixation

It is also worth noting that nitrogen fixation can occur through natural phenomena such as lightning and high-energy solar radiation, which can split nitrogen molecules and convert them into usable forms. However, the amount of nitrogen fixed by these methods is negligible compared to that fixed by diazotrophs in the soil.

Conclusion

The fixation of nitrogen is a critical process for agriculture and ecosystems, as it converts atmospheric nitrogen into forms that are accessible to plants. As noted by John Postgate in his book, Nitrogen Fixation, this process is fundamental to world agriculture and is facilitated primarily by biological activities in the soil.

The biological nitrogen cycle relies heavily on soil-inhabiting and root-inhabiting bacteria and archaea, which efficiently break apart nitrogen molecules from the air, enabling plants to thrive.