Why do plants and animals need nitrogen fixation even though Earth’s atmosphere contains so much nitrogen gas?

Although Earth’s atmosphere is composed of approximately 78 to 79 percent nitrogen gas ($N_2$), this form of nitrogen is relatively inert and cannot be directly utilized by the vast majority of living organisms.12

Nitrogen is a fundamental building block of life, essential for the synthesis of DNA, RNA, amino acids, and proteins. Because plants and animals lack the biological machinery to assimilate atmospheric nitrogen, it must undergo nitrogen fixation—a process that chemically converts the gas into reactive compounds like ammonia and nitrates.12

This critical transformation is primarily performed by nitrogen-fixing bacteria and cyanobacteria, which either live freely in the environment or exist in symbiotic relationships with plants. Once these microorganisms convert the nitrogen into usable inorganic forms, plants can absorb it to support their growth. Animals subsequently obtain the nitrogen they require by consuming those plants or other organisms.321