Why can most plants not use atmospheric nitrogen gas directly?

Most plants cannot use atmospheric nitrogen gas ($N_2$) directly because they lack the specific enzymes required to break the extremely strong triple bond that holds the nitrogen atoms together. Because of this high bond energy, molecular nitrogen is relatively inert and cannot be easily assimilated by plants in its gaseous form.123

To utilize nitrogen, plants rely on a process called nitrogen fixation, where the gas is converted into reactive, inorganic compounds such as ammonia or nitrates. This essential transformation is primarily performed by specialized nitrogen-fixing bacteria and blue-green algae, which possess the necessary biological machinery to break the nitrogen bonds. Once these bacteria convert the nitrogen into usable forms, plants can assimilate the nutrients through their roots to support their growth and development.14356