Fixing nitrogen

The idea

Nitrogen-fixing bacteria, Rhizobium in the root nodules of legumes and Azotobacter in the soil, convert atmospheric N2 into ammonia (NH3); lightning also fixes nitrogen as nitrogen oxides.

Lumps on the roots

Legume plants have small lumps, called root nodules, on their roots. Bacteria live inside them.

What might the bacteria do for the plant?

• Change nitrogen from the air into a usable form
• Make sunlight for the plant
• Stop the roots from growing

Who fixes nitrogen?

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Making nitrogen usable

Converting nitrogen gas into compounds that plants can use is called nitrogen fixation. Two kinds of bacteria do it, and lightning does some of it as well.

Nitrogen-fixing bacteria, Rhizobium in the root nodules of legumes and Azotobacter in the soil, convert atmospheric N₂ into ammonia (NH₃). Lightning also fixes nitrogen as nitrogen oxides.

Notes

Rhizobium (root nodules of legumes) and Azotobacter (soil) convert N₂ into ammonia (NH₃). Lightning also fixes nitrogen as nitrogen oxides.

Check yourself

Which bacteria live in the root nodules of legumes?

Where do Azotobacter bacteria live?

Into what do nitrogen-fixing bacteria convert atmospheric N₂?

Besides bacteria, what also contributes to nitrogen fixation?

• Azotobacter. Azotobacter lives in the soil.
• Pseudomonas. Pseudomonas are denitrifying bacteria.
• Rhizobium — correct. Yes!
• In the soil — correct. Yes!
• In the root nodules of legumes. Rhizobium lives in the root nodules of legumes.
• In the clouds. Azotobacter lives in the soil.
• Nitrate (NO₃⁻). Nitrate is formed later, when Nitrobacter converts nitrite.
• Ammonia (NH₃) — correct. Yes!
• Glucose. Glucose is made by plants in photosynthesis, not by nitrogen-fixing bacteria.
• The ozone layer. The chapter names lightning as the other contributor.
• Moonlight. The chapter names lightning as the other contributor.
• Lightning, forming nitrogen oxides — correct. Yes!