Nitrogen: plenty, but hard to use
Nitrogen makes up about 78% of the air you breathe. Every living thing needs nitrogen to build proteins and DNA. But here’s the problem: plants and animals can’t use nitrogen gas (N₂) straight from the air. The two nitrogen atoms are bonded so tightly that breaking them takes a lot of energy.
The nitrogen cycle describes how nitrogen is changed into usable forms, passed through living things and returned to the air.
Steps of the nitrogen cycle
| Step | What happens | Who does it |
|---|---|---|
| 1. Nitrogen fixation | N₂ gas → ammonia (NH₃) | Bacteria, lightning, factories |
| 2. Nitrification | Ammonia → nitrites → nitrates | Nitrifying bacteria in soil |
| 3. Assimilation | Plants absorb nitrates through roots and build proteins | Plants |
| 4. Eating | Animals get nitrogen by eating plants or other animals | Animals |
| 5. Ammonification | Dead things and wastes → ammonia | Decomposers |
| 6. Denitrification | Nitrates → N₂ gas, back to the air | Denitrifying bacteria |
Nitrogen fixation
Three things can "fix" nitrogen, turning N₂ into usable forms:
Bacteria
Nitrogen-fixing bacteria live in soil and water. Some live in bumps called nodules on the roots of legumes like peas, beans, clover, alfalfa and soybeans. The bacteria give the plant nitrogen; the plant gives the bacteria sugar. Both benefit.
That’s why farmers often plant legumes in rotation with other crops. The legumes add nitrogen back to the soil naturally.
Lightning
The huge energy of a lightning bolt breaks N₂ apart. The nitrogen combines with oxygen and falls to the ground in rain. This fixes a small amount.
Industry
In the early 1900s, German chemists Fritz Haber and Carl Bosch invented a way to make ammonia from nitrogen in the air. The Haber–Bosch process is used to make fertilizer. It now feeds billions of people: about half the nitrogen in your body may have come from a fertilizer factory!
Nitrification
Ammonia in soil can be toxic to plants in large amounts. Nitrifying bacteria change it in two steps:
ammonia → nitrites → nitrates
Most plants absorb nitrogen mainly as nitrates.
Through the food web
Plants use nitrates to build proteins and DNA. Animals get nitrogen by eating plants or other animals. Your hair, muscles and fingernails are made of proteins that contain nitrogen.
Back to the soil and air
When living things die or produce wastes (like urine and manure), decomposers break them down and release ammonia, a process called ammonification. This ammonia can be nitrified again.
Denitrifying bacteria, which live in wet soils with little oxygen, change nitrates back into nitrogen gas, completing the cycle.
Humans and the nitrogen cycle
Humans now add more usable nitrogen to the environment than all natural processes combined, mostly through fertilizers and burning fuels.
| Problem | Cause | Effect |
|---|---|---|
| Algal blooms and dead zones | Fertilizer runs off fields into lakes and oceans | Algae grow out of control, die and decompose, using up oxygen. Fish die. Lake Erie has frequent algal blooms. |
| Water pollution | Nitrates seep into groundwater | Unsafe drinking water, especially for babies |
| Air pollution | Car engines and power plants make nitrogen oxides | Smog and acid rain |
| Climate change | Fertilized soils release nitrous oxide (N₂O) | N₂O is a powerful greenhouse gas |
Solutions
- Use just the right amount of fertilizer, at the right time.
- Plant buffer strips of grass and trees along streams to catch runoff.
- Rotate crops with legumes.
- Protect wetlands, which remove extra nitrogen.
Common mistakes
- Thinking plants take nitrogen from the air. Most rely on bacteria or fertilizer.
- Thinking all bacteria in the cycle do the same job. Different bacteria fix, nitrify and denitrify.
- Thinking more fertilizer is always better. Extra runs off and pollutes water.
Try it yourself
- Dig up a clover or bean plant (with permission) and look for root nodules.
- Draw the nitrogen cycle as a circle, labelling each step and who does it.
- Grow bean seedlings in two pots: plain sand and sand with a little fertilizer. Compare growth after three weeks.
Quick recap
- Nitrogen is 78% of air, but most living things can’t use N₂ directly.
- Nitrogen fixation by bacteria, lightning and industry makes it usable.
- Nitrification makes nitrates; plants absorb them.
- Decomposers and denitrifying bacteria return nitrogen to the soil and air.
- Too much fertilizer causes algal blooms, water pollution and greenhouse gases.