The primary reason atmospheric nitrogen is inaccessible to most organisms lies in its molecular structure. Nitrogen gas (N2) consists of two nitrogen atoms that are bonded by a very strong triple bond. This triple bond makes it incredibly stable and inert, meaning that most living organisms, including plants, cannot utilize it directly.
While nitrogen is an essential nutrient for living organisms—playing a critical role in amino acids, nucleic acids, and other biological molecules—plants are unable to absorb nitrogen in its gaseous form. They need it to be in the form of ammonia (NH3) or nitrates (NO3), which are more chemically reactive and can be taken up through their roots.
This inability to use atmospheric nitrogen directly drives the need for nitrogen-fixing processes. Certain bacteria and archaea can break the triple bond in N2 and convert it into forms that plants can utilize. Without these nitrogen-fixing bacteria, the nitrogen cycle would be severely limited, affecting the entire ecosystem.