Decomposition and its steps

What happens to a fallen leaf?

In autumn a tree drops its leaves. By the next spring, most of them have disappeared. Where did the leaves go? They were decomposed. The decomposers and the detritivores broke them down, and the nutrients returned to the soil.

The raw material is detritus: dead leaves, bark, flowers, the dead remains of animals and the faecal matter. Decomposition is a long chain of small steps, and NCERT names five.

The five steps

1. Fragmentation. The detritivores, such as the earthworm, break the detritus into smaller particles. More surface is then open to the microbes.
2. Leaching. Water carries the water-soluble inorganic nutrients down into the soil. There they may be precipitated as salts that the plants cannot use.
3. Catabolism. The enzymes of bacteria and fungi break the detritus into simpler inorganic substances.
4. Humification. A dark, amorphous substance, humus, builds up. It is highly resistant to the action of microbes, and it decays at an extremely slow rate. It is colloidal, so it is a reservoir of nutrients.
5. Mineralisation. Some microbes break the humus down and release inorganic nutrients.

Fragmentation, leaching and catabolism go on at the same time on the detritus. Humification and mineralisation happen in the soil.

What controls the rate?

NCERT gives two sets of factors.

The chemical composition of the detritus.

• Decomposition is slower when the detritus is rich in lignin and chitin.
• Decomposition is faster when the detritus is rich in nitrogen and in water-soluble substances such as sugars.

The climate. The important factors are temperature and soil moisture.

• A warm and moist environment favours decomposition.
• A low temperature and anaerobiosis (no oxygen) slow it down. Dead matter then builds up.

Decomposition is largely an oxygen-requiring process. This is why waterlogged soils and cold places, such as bogs, store a lot of undecayed organic matter.

A model of the litter that is left

The next part is not in the NEET syllabus, but A-Level and IB use it. A simple model says that each year the litter loses a share of what is left. Then the mass falls in a smooth, bending curve:

Here $m_0$ is the starting mass, $t$ is the time and $k$ is the rate. A larger $k$ means a faster decay. The half-life is the time for half of the litter to go, and it is $\ln 2 / k$, which is $0.693 / k$.

You should find that a warmer place and a lower lignin share both raise $k$, so the litter goes faster, as NCERT says.

Try it in Python

The program follows $100\ \text{g}$ of litter for six years.

The second program matches events to the five steps.

Practise with new numbers

Common mistakes

• Putting the steps in a strict order. Fragmentation, leaching and catabolism go on together.
• Saying that humus decays fast. It is highly resistant and decays very slowly. It stores nutrients.
• Confusing leaching with catabolism. Leaching is done by water and washes soluble nutrients away. Catabolism is done by microbial enzymes.
• Thinking that cold, wet soil speeds up decay. Low temperature and anaerobiosis slow it.

Remember these

• Detritus: dead leaves, bark, flowers, dead animals and faecal matter.
• Steps: fragmentation, leaching, catabolism, humification, mineralisation.
• Slower with lignin and chitin. Faster with nitrogen and sugars. Warm, moist and oxygen rich conditions favour it.
• Humus: dark, amorphous, colloidal, very slow to decay, a reservoir of nutrients.
• Model (not in NEET): $m_t = m_0 e^{-kt}$, half-life $0.693/k$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the five steps of decomposition.

2. What does an earthworm do in decomposition?

3. Why is humus called a reservoir of nutrients?

4. Which decomposes faster, a leaf rich in lignin or a leaf rich in nitrogen?

5. Why do bogs collect so much dead plant matter?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 12, Ecosystem, section on decomposition.
• OpenStax Biology 2e (2018), chapter 46, Ecosystems, section on biogeochemical cycles.
• Campbell Biology, 12th edition (2021), Pearson, chapter 55, Ecosystems and Energy, section on decomposition.