Cambridge O Level Biology · Syllabus 5090 · Relationships of Organisms with One Another and with the Environment
Nitrogen Cycle
What is Nitrogen Cycle?
The circulation of nitrogen between the atmosphere, the soil and living organisms. Nitrogen gas is made available in compounds by nitrogen-fixing bacteria and by lightning; decomposition of nitrogen-containing material forms ammonium compounds; nitrifying bacteria convert these toward nitrate ions; plant roots absorb nitrate ions and use the nitrogen to make amino acids and proteins; feeding transfers protein to animals; death and excretion return nitrogen-containing material for decomposition; and denitrifying bacteria convert nitrate compounds back to nitrogen gas.
This definition is part of the Relationships of Organisms with One Another and with the Environment chapter in Cambridge O Level Biology.
Nitrogen Cycle in context
Ecosystems run on one input and two very different accounting systems. Light energy from the Sun enters through photosynthesis, passes along food chains by feeding, and is transferred to the environment as thermal energy at every step — so energy flows through an ecosystem and is never recycled. The atoms carried with it are different: carbon and nitrogen move from air to producer to consumer to decomposer and back to the air, so matter cycles and is used again. Everything else in this topic — short food chains, ecological pyramids, the carbon and nitrogen cycles, eutrophication, conservation — is a consequence of those two sentences.
Questions students ask about Nitrogen Cycle
Do I need to learn the names of the bacteria in the nitrogen cycle?
No. Nitrogen-fixing, nitrifying and denitrifying bacteria are named for what they do, and those descriptive names are what the syllabus asks for. What you must get right is the direction each drives the nitrogen: fixation and nitrification make it available to plants, and denitrification returns it to the atmosphere.