What is meant by "nitrogen fixation"? State two ways in which nitrogen gas of the atmosphere can be 'fixed' in nature to get nitrogen compounds in the soil.

Nitrogen fixation is the process of converting atmospheric nitrogen gas (N₂), which makes up 78% of Earth's atmosphere but cannot be directly used by most living organisms, into nitrogen compounds such as ammonia (NH₃), nitrates (NO₃⁻), or nitrites (NO₂⁻) that plants can absorb and utilize for synthesizing proteins, nucleic acids, and other essential biomolecules. Although nitrogen is abundant in the atmosphere, the strong triple bond between nitrogen atoms makes N₂ extremely unreactive and unusable by plants and animals in its gaseous form, creating a paradox where the most abundant element in air is often the limiting nutrient for plant growth.

Two natural methods fix atmospheric nitrogen into soil compounds. First, biological nitrogen fixation occurs through specialized microorganisms, particularly nitrogen-fixing bacteria like Rhizobium species that live symbiotically in root nodules of leguminous plants (peas, beans, clover, alfalfa) and free-living bacteria like Azotobacter and cyanobacteria (blue-green algae) that fix nitrogen independently. These microorganisms possess the enzyme nitrogenase that breaks the strong N≡N bond and converts nitrogen gas into ammonia, which can be used by plants or further converted to nitrates by nitrifying bacteria in soil. Second, atmospheric nitrogen fixation happens during lightning strikes when the enormous electrical energy breaks nitrogen and oxygen molecules apart, allowing them to recombine into nitrogen oxides (NOₓ) that dissolve in rainwater forming dilute nitric acid, which falls to soil as nitrates. While biological fixation contributes approximately 90% of natural nitrogen fixation globally, lightning accounts for a smaller but significant portion, together sustaining the nitrogen cycle essential for all life on Earth.