Living things need nitrogen to build proteins and DNA. Air is about 78% nitrogen gas, so it seems easy. But nitrogen gas (N₂) has two atoms held by a very strong triple bond, and plants and animals cannot break it. This is where the Bacteria come in. Compare it with the The Carbon Cycle.
Key idea
Nitrogen-fixing bacteria in soil and in the root nodules of plants such as peas, beans and clover turn N₂ into ammonium and nitrates that roots can absorb. Animals get nitrogen by eating plants. Decomposers return nitrogen from dead matter and wastes to the soil, and denitrifying bacteria return it to the air.
Why it works
Breaking the triple bond takes a special enzyme that only certain microbes, mostly bacteria, have. A bean plant gives its root-nodule bacteria sugar; the bacteria give the plant usable nitrogen. That mutualism is why farmers plant beans or clover to enrich soil. Lightning fixes a small amount of nitrogen too.
Worked example: Why the nodules?
Two bean plants grow in nitrogen-poor soil. One has root nodules with bacteria; one has none. Which grows better, and why?
- Neither plant can use nitrogen gas from the air directly.
- The bacteria in the nodules fix N₂ into ammonium the plant can use.
- The plant without nodules depends on the little nitrogen already in the soil.
Answer: The plant with nodules, because its bacteria supply usable nitrogen.
Worked example: Back to the soil
A rabbit dies in a meadow. How does its nitrogen get back to the grass?
- The rabbit's proteins hold nitrogen.
- Decomposers break the proteins down and release ammonium into the soil.
- Grass roots absorb it (often after other bacteria turn it into nitrate).
Answer: Decomposition returns it to the soil as ammonium; roots take it up.
Common mistake
Students think plants take nitrogen from the air through their leaves, the way they take in carbon dioxide. Leaves can't use N₂. Plants get nitrogen through their roots, as ammonium or nitrate, thanks to bacteria.