What gas is fixed by plants and is the most abundant in Earth's atmosphere?

• A. Oxygen
• B. Nitrogen
• C. Carbon dioxide
• D. Hydrogen

Answer: (B)

Nitrogen is the gas most abundant in Earth's atmosphere, making up about four-fifths of the air. For living systems to use nitrogen, it must be converted from N2 into usable forms like ammonia or nitrate in a process called nitrogen fixation. This transformation is carried out by certain bacteria, including those living in the roots of leguminous plants and some free-living microbes. Plants don’t fix nitrogen alone, but they rely on these microbes to provide usable nitrogen for building proteins and nucleic acids. So nitrogen fits both parts of the idea: it’s the most abundant atmospheric gas, and it can be fixed into forms plants can use, via a biological partnership.