The Nitrogen Cycle Step by Step
Part of The Nitrogen Cycle · GCSE GCSE Biology revision
This deep dive covers The Nitrogen Cycle Step by Step within The Nitrogen Cycle for GCSE Biology. The nitrogen cycle: nitrogen-fixing, nitrifying, denitrifying bacteria, ammonification, and the role of legumes It is section 3 of 14 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 3 of 14
Practice
15 questions
Recall
15 flashcards
⚙️ The Nitrogen Cycle Step by Step
Step 1: Nitrogen Fixation
Nitrogen-fixing bacteria convert unreactive atmospheric nitrogen gas (N₂) into ammonia (NH₃). This process is called nitrogen fixation. Some nitrogen-fixing bacteria, such as Rhizobium, live inside swellings called root nodules on the roots of legume plants (peas, beans, clover). This is a mutualistic relationship: the bacteria get sugars from the plant; the plant gets fixed nitrogen. Other nitrogen-fixing bacteria live freely in the soil. Lightning can also fix small amounts of nitrogen by providing the energy to break the N₂ bond — but this is a minor pathway compared to bacterial fixation.
Step 2: Nitrification
Nitrifying bacteria in the soil carry out a two-stage conversion:
- Ammonia (NH₃) → nitrites (NO₂⁻)
- Nitrites (NO₂⁻) → nitrates (NO₃⁻)
Nitrifying bacteria need oxygen — they are aerobic organisms. This is why well-aerated, well-drained soils tend to have higher nitrate concentrations than waterlogged soils. The end product, nitrates, is the form plants can absorb through their roots.
Step 3: Plant Uptake
Plants absorb nitrates from soil water through their root hair cells using active transport (against the concentration gradient, requiring energy). Inside the plant, nitrates are used to build amino acids, which are assembled into proteins. Nitrogen is also essential for making DNA and chlorophyll. Animals obtain their nitrogen by eating plants (or other animals).
Step 4: Ammonification (Decomposition)
When organisms die or produce waste (urea, faeces), decomposers — bacteria and fungi — break down the nitrogen-containing organic molecules (proteins, nucleic acids) into ammonia. This process is called ammonification. The ammonia is released into the soil, where nitrifying bacteria can then convert it to nitrates — completing the inner loop of the cycle.
Step 5: Denitrification
Denitrifying bacteria run the cycle in reverse: they convert nitrates (NO₃⁻) back into nitrogen gas (N₂), which is released into the atmosphere. These bacteria are anaerobic — they thrive in waterlogged, oxygen-poor conditions such as compacted or flooded soils. From a farmer's perspective, denitrifying bacteria are the "enemy" — they remove valuable nitrates from the soil. This is why waterlogged fields have poor crop growth.