NITROGEN is essential for life — it is a key component of:
AMINO ACIDS — the building blocks of proteins (enzymes, structural proteins, haemoglobin).
NUCLEOTIDES — the building blocks of DNA and RNA.
CHLOROPHYLL — the photosynthetic pigment in plants.
Nitrogen makes up approximately 78% of the atmosphere — but as N₂ gas, which is EXTREMELY UNREACTIVE. Plants and animals CANNOT use N₂ directly.
Nitrogen must first be FIXED — converted into a usable form (ammonia or nitrates) — before organisms can use it.
The nitrogen cycle describes how nitrogen moves between the atmosphere, soil, plants, animals and decomposers.
The Key Processes
NITROGEN FIXATION — converting N₂ to ammonia (NH₃):
CARRIED OUT BY: nitrogen-fixing bacteria.
IN SOIL: free-living bacteria (e.g. Azotobacter) fix N₂ in the soil.
IN ROOT NODULES: mutualistic bacteria (Rhizobium) live in the root nodules of LEGUMES (peas, beans, clover, soybeans). They fix N₂, providing the plant with nitrates. The plant provides the bacteria with glucose.
LIGHTNING: very high energy lightning can also fix small amounts of N₂.
NITRIFICATION — converting ammonia to nitrates:
Ammonia (NH₃) in the soil is converted to NITRITES then NITRATES (NO₃⁻) by NITRIFYING BACTERIA.
Plants can absorb nitrates through their roots.
ABSORPTION — plants absorb nitrates from soil → use them to make amino acids → proteins.
DENITRIFYING BACTERIA convert nitrates → N₂ gas, which returns to the atmosphere.
Occurs mainly in WATERLOGGED (anaerobic) soils — these bacteria don't need oxygen.
Reduces soil fertility.
Human Impact on the Nitrogen Cycle
FERTILISERS:
Farmers add ARTIFICIAL FERTILISERS (ammonium nitrate, etc.) or ORGANIC FERTILISERS (manure, compost) to replace nitrates removed by harvesting crops.
Excess fertiliser can be washed from fields into rivers and lakes by rain — a process called LEACHING.
EUTROPHICATION — the consequence of nitrate leaching:
1. Excess nitrates enter a river or lake.
2. Algae grow rapidly (algal bloom) — covering the water surface.
3. Light cannot penetrate to underwater plants — they die.
4. Dead plants and algae are decomposed by bacteria.
5. Decomposing bacteria use up all the OXYGEN in the water (aerobic decomposition).
6. Oxygen concentration falls → fish and other aquatic animals suffocate and die.
NITROGEN OXIDES from vehicle exhausts and power stations:
Fall as ACID RAIN — damages vegetation and acidifies rivers and lakes.
⚠️ Common Mistake
DENITRIFYING bacteria convert NITRATES BACK TO N₂ — they REDUCE soil nitrogen and make it less fertile. NITRIFYING bacteria convert AMMONIA TO NITRATES — they INCREASE soil nitrogen availability. These are opposite processes by different bacteria. Denitrification is favoured in WATERLOGGED (anaerobic) soils.