Decomposition is how dead plants and animals are broken down by bacteria and fungi, returning carbon dioxide to the air and mineral ions to the soil. It appears in carbon cycle questions and in experiments comparing how fast material rots.
You should already be able to trace carbon through a supplied system. This lesson adds the detail of what decomposers release. The overall topic is in nutrient cycles and populations.
How does recycling work, step by step?
- Material dies or is shed. Leaves, waste and bodies become dead organic matter.
- Decomposers digest it. Bacteria and fungi release enzymes onto the material and absorb the soluble products.
- Decomposers respire. Some absorbed compounds are used in respiration, so carbon dioxide is released.
- Mineral ions are released. Nitrate and other ions from proteins and other compounds enter the soil water.
- Plants absorb the ions. Roots take up the minerals, which are used to make new tissue, and the cycle continues.
Without step 4 the soil would slowly run out of minerals that plants need. Decomposition is what lets matter be reused.
Which factors change the rate?
Decomposers are living, so their activity depends on temperature, water and oxygen. Warm, moist, well-aerated conditions give faster decay. Cold, dry or oxygen-poor conditions slow it.
Worked example
A student puts 20 g of dry leaves into each of three mesh bags and buries them in identical soil, at three temperatures, for four weeks. The data are invented for practice.
| Temperature | Mass at start (g) | Mass after 4 weeks (g) |
|---|---|---|
| 15 °C | 20 | 16 |
| 30 °C | 20 | 9 |
| 45 °C | 20 | 17 |
Question: Calculate the percentage mass lost at each temperature and explain the pattern.
Step 1, mass lost: 15 °C: 20 − 16 = 4 g. 30 °C: 20 − 9 = 11 g. 45 °C: 20 − 17 = 3 g.
Step 2, percentage lost: (mass lost ÷ start) × 100. 15 °C: 4/20 × 100 = 20%. 30 °C: 11/20 × 100 = 55%. 45 °C: 3/20 × 100 = 15%.
Step 3, describe: loss rises from 20% to 55% between 15 °C and 30 °C, then falls to 15% at 45 °C.
Step 4, explain: between 15 °C and 30 °C, decomposer enzymes and respiration speed up, so more leaf material is digested. At 45 °C the enzymes of these decomposers are likely to denature, or the organisms die, so decomposition slows.
The mass lost is carbon and mineral matter leaving the leaf: carbon mostly as carbon dioxide from respiration.
What mistake should I watch for?
Mistaken answer: “Higher temperature always makes decomposition faster, so 45 °C should have lost the most.”
The student applies a rule without checking the data. The table shows the rate falling at 45 °C, so the evidence says there is an optimum.
Corrected: “Decomposition is fastest at 30 °C, which is closer to the optimum. At 45 °C the enzymes are likely denatured, so less material is broken down.”
Always make sure your rule fits the highest and lowest values in the data before you commit.
Check yourself
1. Name two groups of organisms that act as decomposers.
Show answer
Bacteria and fungi.
2. A bag starts with 50 g of leaves and has 40 g left after a month. Calculate the percentage lost.
Show answer
Lost = 50 − 40 = 10 g. 10/50 × 100 = 20%.
3. Suggest why nutrients would run out faster in a field where all the crop is removed and nothing is returned.
Show answer
The plants took up mineral ions while they grew. If the crop is removed, there is no dead matter for decomposers to break down, so the ions are not returned to the soil and the soil becomes depleted.
Where does this lead next?
Next, look at how living numbers change, in interpreting population growth phases. Bring your ability to name a cause, because that comes up again in limiting factors.
If explanations like the one for 45 °C are hard to phrase, online one-to-one Biology tuition gives you a teacher to rehearse them with.