Microorganisms in the Nitrogen Cycle

Nitrogen is needed to make amino acids, proteins and nucleic acids. The air is 78% nitrogen gas (N₂), but plants and animals cannot use it directly. Microorganisms carry out the key steps that convert nitrogen between its different forms.1. Nitrogen fixation: N₂ is reduced to ammonia (NH₃), which forms ammonium ions (NH₄⁺), by the enzyme nitrogenase.
- Rhizobium bacteria live in the root nodules of legumes such as peas, beans and clover. This is mutualism: the plant receives ammonium, and the bacteria receive carbohydrates.
- Free-living bacteria in the soil, such as Azotobacter, also fix nitrogen.2. Ammonification: decomposers (saprobiontic bacteria and fungi) break down proteins and other nitrogen compounds in dead organisms, faeces and urine, releasing ammonium ions.3. Nitrification: nitrifying bacteria oxidise ammonium ions to nitrate in two steps. This needs oxygen, so it happens in well-aerated soils.NH₄⁺ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter)Plants absorb nitrate by active transport into their roots and use it to make amino acids.4. Denitrification: in waterlogged, anaerobic soils, denitrifying bacteria convert nitrate back into nitrogen gas, which is lost to the air. This reduces soil fertility.Farmers encourage the useful steps by ploughing and draining fields to aerate the soil, which favours nitrification and prevents denitrification, and by growing legumes in crop rotation.