The Nitrogen Cycle

Nitrogen is needed to make amino acids, proteins, nucleic acids (DNA and RNA) and ATP. The atmosphere is about 78% nitrogen gas (N₂), but plants and animals cannot use it directly because the triple bond is very strong. Plants take up nitrogen from the soil as nitrate ions (NO₃⁻) or ammonium ions; animals get it by eating plants or other animals. Bacteria carry out the key steps.1. Nitrogen fixation: N₂ is converted to ammonia (NH₃), which forms ammonium ions (NH₄⁺) in the soil. This is done by nitrogen-fixing bacteria, such as Rhizobium in root nodules and free-living Azotobacter. Lightning and the industrial Haber process also fix nitrogen.2. Ammonification: decomposers (saprotrophic bacteria and fungi) break down proteins and nucleic acids in dead organisms, faeces and urea, releasing ammonium ions.3. Nitrification: Nitrosomonas oxidises ammonium ions to nitrite ions (NO₂⁻), then Nitrobacter oxidises nitrites to nitrates (NO₃⁻).4. Assimilation: plants absorb nitrate by active transport into root hair cells and use it to make amino acids and nucleotides. Animals eat plants, passing the nitrogen along the food chain.5. Denitrification: in anaerobic soil, denitrifying bacteria convert nitrates back into nitrogen gas, which returns to the air.Fixation and denitrification link the soil to the atmosphere; ammonification and nitrification recycle nitrogen within the soil.