By the end of this lesson
- Describe the stages of the nitrogen cycle.
- Explain the roles of nitrogen-fixing, nitrifying and denitrifying bacteria and decomposers.
- Explain how soil conditions affect the cycle.
BIG QUESTION
Why can plants surrounded by nitrogen gas still be short of nitrogen?
Most organisms cannot use nitrogen gas; bacteria must first convert it into ammonium or nitrate.
Why nitrogen must be converted
About 78% of air is nitrogen gas (N₂), but plants and animals cannot use it directly. Plants absorb nitrate ionsMineral ions that supply plants with nitrogen to make amino acids and then proteins; a lack causes stunted growth and often yellowing, especially of older leaves. (and some ammonium ions) from the soil to make amino acids and proteins.
Animals obtain nitrogen compounds by eating plants or other animals.
Nitrogen fixation
nitrogen-fixing bacteriaBacteria, free in the soil or in root nodules of legumes, that convert nitrogen gas into ammonium compounds that plants can use. convert nitrogen gas into ammonium compounds. Some live free in the soil; others live in root noduleA swelling on the roots of a legume, such as a pea or clover plant, that contains nitrogen-fixing bacteria. of legumes such as peas, beans and clover, where the plant supplies sugars and the bacteria supply nitrogen compounds.
Lightning and the industrial Haber process also fix nitrogen; this is extension context.
Decomposition and nitrification
When organisms die or excrete, decomposerAn organism, such as a bacterium or fungus, that breaks down dead organic matter and waste, returning nutrients to the soil. break down proteins and urea and release ammonium ions (ammonification).
nitrifying bacteriaSoil bacteria that convert ammonium ions into nitrites and then into nitrates, which plants can absorb. convert ammonium ions into nitrite and then nitrate ions. Nitrification needs oxygen, so it is favoured in well-aerated soils.
Denitrification
denitrifying bacteriaBacteria in waterlogged, anaerobic soil that convert nitrates into nitrogen gas, reducing soil fertility. convert nitrate ions back into nitrogen gas. They are most active in waterlogged, oxygen-poor soils.
Denitrification reduces soil fertility, which is why farmers drain and plough soils to keep them aerated.
Bacteria in the nitrogen cycle
Learn the starting substance and product for each group.
| Organism group | Conversion | Conditions favouring it |
|---|---|---|
| Nitrogen-fixing bacteria | Nitrogen gas → ammonium compounds. | Root nodules of legumes or free in soil. |
| Decomposers | Proteins and urea → ammonium ions. | Moist, warm soil with dead material. |
| Nitrifying bacteria | Ammonium → nitrite → nitrate. | Well-aerated (oxygen-rich) soil. |
| Denitrifying bacteria | Nitrate → nitrogen gas. | Waterlogged, oxygen-poor soil. |
Language that earns precision
- nitrogen fixation
- Conversion of nitrogen gas into nitrogen compounds such as ammonium.
- nitrification
- Conversion of ammonium ions to nitrite and then nitrate by bacteria.
- denitrification
- Conversion of nitrate ions to nitrogen gas by bacteria.
- root nodule
- A swelling on legume roots containing nitrogen-fixing bacteria.
Common misconceptions
Claim: Plants absorb nitrogen gas from the air through their leaves.
Correction: Plants absorb nitrate (or ammonium) ions from the soil through roots.
Claim: Nitrifying and nitrogen-fixing bacteria do the same job.
Correction: Fixing bacteria convert N₂ to ammonium; nitrifying bacteria convert ammonium to nitrate.
Claim: Denitrifying bacteria help plants.
Correction: They remove nitrate from the soil, reducing the nitrogen available to plants.
Match the answer to the command word
Name the bacteria and give the substance converted and the product.
Link ploughing or drainage to oxygen, which favours nitrifying and inhibits denitrifying bacteria.
Recall, apply, then justify
Describe the role of nitrifying bacteria in the nitrogen cycle.
Hint
Starting ion and final ion.
Suggested marking points
- Convert ammonium ions (to nitrite and then) to nitrate ions.
- Nitrate can then be absorbed by plant roots.
Explain why a waterlogged field may have low levels of nitrate in the soil.
Hint
Oxygen affects two groups of bacteria.
Suggested marking points
- Waterlogged soil is low in oxygen.
- Denitrifying bacteria are more active and convert nitrate to nitrogen gas.
- Nitrifying bacteria need oxygen, so less nitrate is made.
Farmers sometimes grow clover in a field one year and then plough it into the soil before planting wheat. Explain how this increases wheat yield.
Planning hint
Root nodules, decomposition and nitrification.
What a strong design includes
- Clover is a legume with root nodules containing nitrogen-fixing bacteria.
- These bacteria convert nitrogen gas into ammonium compounds used by the clover.
- Ploughing adds clover proteins to the soil.
- Decomposers convert the proteins into ammonium ions.
- Nitrifying bacteria convert ammonium into nitrate.
- Wheat absorbs the nitrate to make proteins, so it grows more.
- Ploughing also aerates the soil, favouring nitrification.