Big picture: Nitrogen is essential for all living organisms (found in amino acids, proteins, DNA). Although 78% of the atmosphere is N2, most organisms cannot use it directly — it must first be fixed into reactive forms.
- Nitrogen fixation
- Conversion of atmospheric N2 into ammonia (NH3) or ammonium (NH4+) by nitrogen-fixing bacteria or lightning. This makes nitrogen available to plants.
- Nitrification
- Conversion of ammonium (NH4+) to nitrite (NO2-) and then nitrate (NO3-) by nitrifying bacteria (Nitrosomonas and Nitrobacter). Occurs in aerobic conditions.
- Assimilation
- The uptake of nitrate (NO3-) or ammonium (NH4+) by plants through their roots, incorporating nitrogen into organic molecules.
- Ammonification
- The decomposition of organic nitrogen (dead organisms, waste) back into ammonium (NH4+) by decomposing bacteria and fungi.
- Denitrification
- Conversion of nitrate (NO3-) back to N2 or N2O gas by denitrifying bacteria in anaerobic (waterlogged) conditions. Returns nitrogen to the atmosphere.
Types of nitrogen-fixing organisms
- Rhizobium bacteria — live in root nodules of leguminous plants (mutualistic symbiosis)
- Cyanobacteria (blue-green algae) — fix nitrogen in aquatic environments and soil crusts
- Free-living soil bacteria (e.g., Azotobacter) — fix nitrogen independently
- Lightning — converts small amounts of N₂ to NO₃⁻ in the atmosphere
Key concept: Biological nitrogen fixation is the largest natural source of reactive nitrogen. Legume-Rhizobium symbiosis is a classic example of mutualism: the bacteria receive sugars from the plant, and the plant receives usable nitrogen.
IB exam tip: Draw and label the nitrogen cycle showing all five processes: fixation, nitrification, assimilation, ammonification, and denitrification. Include the bacteria responsible for each step.
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Key concept: The Haber process (developed 1909) industrially converts atmospheric N2 into ammonia (NH3) using high temperature and pressure. It has doubled the amount of reactive nitrogen entering ecosystems globally.
The Haber process
- Reaction: N2 + 3H2 → 2NH3 (requires ~450°C, ~200 atm, iron catalyst)
- Uses natural gas (methane) as hydrogen source — energy-intensive
- Produces ammonia for fertilisers, explosives, and industrial chemicals
- Currently produces ~150 million tonnes of ammonia per year
Consequences of the Haber process
- Enabled the Green Revolution — feeding billions through synthetic fertilisers
- More than doubled global food production
- Massively increased reactive nitrogen in the environment
- Accounts for ~1–2% of global energy consumption
- Human nitrogen fixation now exceeds natural fixation
Key concept: The planetary boundary for nitrogen has been exceeded. Human activities have more than doubled the amount of reactive nitrogen entering the biosphere, with severe consequences for ecosystems.
Consequences of excess reactive nitrogen
- Eutrophication of freshwater and marine ecosystems
- Dead zones in coastal waters (e.g., Gulf of Mexico)
- Nitrous oxide (N2O) emissions — a potent greenhouse gas
- Acid deposition from nitrogen oxides
- Loss of biodiversity in nitrogen-sensitive ecosystems
- Groundwater contamination with nitrates