Describe the role of microorganisms in the nitrogen cycle, including the names of the specific types of bacteria involved.

Microorganisms are essential in the nitrogen cycle. 1. Decomposers (bacteria and fungi) break down protein in dead organisms or waste, producing ammonium ions (NH₄⁺). 2. Nitrifying bacteria oxidise ammonium to nitrite (NO₂⁻) and then to nitrate (NO₃⁻). 3. Nitrogen‑fixing bacteria (e.g. Rhizobium in legume root nodules, or free‑living Azotobacter) convert atmospheric N₂ into ammonia or ammonium ions, which can be assimilated or further oxidised to nitrate. 4. Denitrifying bacteria reduce nitrate back to N₂ gas, returning nitrogen to the atmosphere.