You are here
💡 Air we cannot use directly
Almost four-fifths of the air is nitrogen, yet plants cannot use it as a gas. A team of bacteria must first turn it into forms life can absorb.
What is the nitrogen cycle?
- Atmospheric nitrogen (N₂) is non-reactive, so nitrogen-fixing bacteria like Rhizobium (root nodules) and Azotobacter convert it to ammonia (fixation).
- Nitrifying bacteria ( Nitrosomonas , Nitrobacter ) convert ammonia to nitrite then nitrate (nitrification); plants assimilate these, and animals get nitrogen by eating plants.
- Decomposers return nitrogen as ammonia (ammonification), and denitrifying bacteria ( Pseudomonas ) convert nitrates back to N₂ gas (denitrification), completing the cycle; lightning also fixes some nitrogen.
NCERT Question 7 — Explain the processes involved in
Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?
View Answer →
📚 Ready to Go Beyond
During lightning strikes, a little atmospheric nitrogen is fixed into nitrogen oxides. Today most nitrogen is fixed artificially by the Haber-Bosch process (early 1900s), making ammonia from atmospheric nitrogen. This ‘Bread from Air’ enabled India’s Green Revolution and feeds billions, but it is energy-intensive (about 1–2% of global energy) and the overuse of fertilisers has degraded soil and water.
Important Definitions
- Nitrogen cycle — the overall movement of nitrogen between air, soil, water and organisms.
- Nitrogen fixation — the conversion of atmospheric N 2 into ammonia (NH 3 ) by bacteria such as Rhizobium and Azotobacter , or by lightning.
- Nitrification — the conversion of ammonia into nitrite (NO 2 − ) by Nitrosomonas , and then into nitrate (NO 3 − ) by Nitrobacter .
- Assimilation — the uptake of nitrogen compounds from the soil by plants; animals get their nitrogen by eating plants or other animals.
- Ammonification — the breakdown of dead organic matter by decomposers, returning nitrogen compounds like ammonia to the soil.
- Denitrification — the conversion of some nitrates back into nitrogen gas by denitrifying bacteria such as Pseudomonas .
