From atmosphere to protein and back
Nitrogen (N₂) makes up 78% of the atmosphere, but its triple bond makes it extremely stable and unusable by most organisms. The nitrogen cycle converts atmospheric N₂ into reactive forms (ammonia, nitrates) that plants and animals can use, then returns it to the atmosphere.
Nitrogen Fixation: Specialised bacteria (Rhizobium in legume root nodules, Azotobacter in soil, cyanobacteria in water) convert N₂ → NH₃ (ammonia). Lightning also fixes small amounts of nitrogen. This is the only natural way to "break" the N₂ triple bond.
Nitrification: Soil bacteria (Nitrosomonas, Nitrobacter) convert NH₃ → NO₂⁻ (nitrite) → NO₃⁻ (nitrate). Nitrates are the form most easily absorbed by plant roots.
Assimilation: Plants absorb nitrates through roots and use them to build amino acids, proteins, and DNA. Animals obtain nitrogen by eating plants or other animals.
Ammonification: When organisms die, decomposer bacteria break down proteins and return nitrogen to the soil as ammonia (NH₃). Urine and faeces also contribute ammonia.
Denitrification: Anaerobic bacteria (Pseudomonas) in waterlogged soils convert nitrates back to N₂ gas, returning nitrogen to the atmosphere and completing the cycle.
Why farmers grow legumes: Legumes (peas, lentils, chickpeas, soybeans) have Rhizobium bacteria in their root nodules that fix atmospheric nitrogen into the soil. Rotating crops with legumes naturally fertilises the soil — a practice used in Indian agriculture for thousands of years.
The Haber-Bosch process
In 1909, German chemist Fritz Haber developed a process to synthesise ammonia from atmospheric nitrogen and hydrogen. Carl Bosch scaled it up industrially. The Haber-Bosch process now produces ~150 million tonnes of synthetic fertiliser per year, feeding roughly half the world's population. It is arguably the most important chemical process in human history.
Environmental cost: The Haber-Bosch process consumes ~1–2% of global energy and produces significant CO₂. Excess synthetic fertiliser runs off into rivers and lakes, causing eutrophication. Nitrogen oxides (NOₓ) from fertilisers are also potent greenhouse gases.
Photosynthesis and respiration
The oxygen cycle is tightly linked to the carbon cycle. Oxygen (O₂) makes up 21% of the atmosphere and is continuously cycled through photosynthesis and respiration.
🌿 Photosynthesis (produces O₂)
6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂
Plants, algae, and cyanobacteria split water molecules using sunlight, releasing oxygen as a byproduct. Earth's oxygen-rich atmosphere is entirely the result of billions of years of photosynthesis.
🫁 Respiration (consumes O₂)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
All aerobic organisms (including plants at night) consume oxygen to release energy from glucose. This returns CO₂ to the atmosphere, completing the cycle.
🌊 Oceans as oxygen producers
Phytoplankton (microscopic marine algae) produce approximately 50–80% of Earth's oxygen through photosynthesis — far more than all land forests combined. The Amazon rainforest produces about 20% of terrestrial oxygen but recycles most of it through its own respiration. Protecting ocean health is therefore critical for maintaining atmospheric oxygen levels.
International environmental agreements
| Agreement | Year | Focus | Key Outcome |
|---|---|---|---|
| Montreal Protocol | 1987 | Ozone layer | Phased out CFCs globally; ozone layer is recovering. Most successful environmental treaty. |
| Kyoto Protocol | 1997 | Greenhouse gases | First binding targets for developed nations to reduce CO₂ emissions. Limited success. |
| Paris Agreement | 2015 | Climate change | 196 countries committed to limit warming to 1.5–2°C above pre-industrial levels. India committed to net-zero by 2070. |
| Kunming-Montreal | 2022 | Biodiversity | 30×30 target: protect 30% of Earth's land and oceans by 2030 to halt biodiversity loss. |