BiologyISC Class 12 · Ecosystem

Decomposition in an Ecosystem: ISC Class 12 Biology

Updated 15 October 2026 · Fact-checked

Decomposition is the breakdown of complex organic matter (detritus) into inorganic substances such as carbon dioxide, water and nutrients. Its steps are fragmentation by detritivores, leaching, catabolism by bacterial and fungal enzymes, humification and mineralisation. It is fastest in warm, moist, aerobic conditions and with detritus rich in nitrogen and sugars, and slowest with lignin and chitin.

Understand Decomposition

Detritus and decomposers. Dead plant remains (leaves, bark, flowers) and dead animal remains, including faecal matter, form detritus, the raw material of decomposition. Decomposers (mainly bacteria and fungi) break complex organic matter into inorganic substances: carbon dioxide, water and nutrients. Earthworms are 'farmers' friends' because they break down organic matter and loosen soil.

Steps of decomposition (all act simultaneously on detritus).
1. Fragmentation: detritivores such as earthworms break detritus into smaller particles. This increases surface area, so leaching and microbial catabolism are faster.
2. Leaching: rainwater carries water-soluble inorganic nutrients down through the soil layers, where they settle out as salts that plants cannot use.
3. Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
4. Humification (in soil): accumulation of humus, a dark-coloured, amorphous substance that is highly resistant to microbial action and decomposes extremely slowly. Being colloidal, it serves as a reservoir of nutrients.
5. Mineralisation: some microbes degrade humus further and release inorganic nutrients.

Flow chart: detritus → (fragmentation by detritivores) → small particles → (leaching: soluble nutrients sink; catabolism: enzymes break down) → humus (humification) → inorganic nutrients (mineralisation) → absorbed by plants.

Factors controlling the rate.

Factor	Faster decomposition	Slower decomposition
Chemical composition of detritus	rich in nitrogen and water-soluble substances such as sugars	rich in lignin and chitin
Temperature	warm	low temperature
Soil moisture and oxygen	moist and aerobic (decomposition is largely oxygen-requiring)	dry, or waterlogged and anaerobic

Temperature and soil moisture are the most important climatic factors, acting through their effect on soil microbes. Low temperature and anaerobiosis inhibit decomposition, so organic matter builds up.

Why decomposition matters. It recycles nutrients (without it, nutrients would stay locked in dead bodies and producers would starve), clears dead matter, forms humus that improves soil structure and water-holding capacity, and is the start of the detritus food chain. On land, more of the energy passes along the detritus food chain than along the grazing food chain.

Forest versus desert. A forest has abundant vegetation and so abundant detritus, with warmth and moisture that favour microbes; a desert has very little organic matter and too little moisture, so decomposition is slight.

Link to productivity (often tested with decomposition). Gross primary productivity (GPP) − respiration (R) = net primary productivity (NPP). NPP is the biomass available to heterotrophs: herbivores and decomposers.

Litter-bag arithmetic. Percentage mass lost = (initial − final) ÷ initial × 100; accumulation = litter input − litter decomposed.

Diagram checklist (decomposition cycle). Detritus at the top → arrow 'fragmentation (detritivores, e.g. earthworm)' → 'catabolism (bacteria, fungi)' → 'humification → humus' → 'mineralisation → inorganic nutrients' → back to plants; a side arrow 'leaching → unavailable salts' going down into the soil.

Decomposition: key facts

Five steps: 1.Fragmentation by detritivores (earthworms)
2.Leaching of soluble nutrients
3.Catabolism by bacterial and fungal enzymes
4.Humification (humus forms)
5.Mineralisation (inorganic nutrients released); They act simultaneously; fragmentation comes first.
Rate rule: Faster detritus rich in nitrogen and sugars
Faster warm, moist, aerobic soil
Slower detritus rich in lignin and chitin
Slower cold, dry or waterlogged (anaerobic) soil; Composition plus climate.
Productivity: $NPP = GPP - R$; So GPP = NPP + R.
Litter mass loss: $% lost = \frac{m _{ini t ia l} - m _{f ina l}}{m _{ini t ia l}} \times 100$; Used in litter-bag experiments.

Faster	detritus rich in nitrogen and sugars
Faster	warm, moist, aerobic soil
Slower	detritus rich in lignin and chitin
Slower	cold, dry or waterlogged (anaerobic) soil

How to answer Decomposition questions

ISC sets labelled decomposition diagrams, the order and meaning of the steps, factors affecting the rate, forest versus desert comparisons, and GPP-NPP sums alongside.

1Name fragmentation as the first step and give the organism (earthworm, a detritivore).
2Define each step in one line with what happens and who does it.
3For 'importance', say what would happen without decomposition: nutrients locked in dead matter, no recycling, soil infertile.
4For rate questions, name both the chemical factor (lignin, chitin versus nitrogen, sugars) and the climatic factor (temperature, moisture, oxygen).
5In productivity sums, write NPP = GPP − R first; rearrange for GPP when NPP and R are given.
6For litter-bag data, compute percentage loss and link it to climate or litter quality.

Decomposition in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Composting speed

Suppose two school compost pits are filled at the same time: pit A with chopped vegetable peels kept moist and turned weekly; pit B with whole dry sugarcane stalks and coconut husks, left untouched and packed tight.

Predict which pit gives compost first and explain using decomposition.

Try it first, then show the solutionSolution

Pit A: chopping is like fragmentation, giving more surface for microbes; peels are rich in nitrogen and soluble sugars.
Moisture and turning keep it moist and aerobic, so microbial catabolism is fast.
Pit B: whole, lignin-rich stalks and husks resist microbial attack; dry, packed material lacks moisture and oxygen.
So pit A forms humus-rich compost in weeks; pit B takes many months.

Answer: Pit A, because fragmented, nitrogen-rich material kept moist and aerated decomposes fastest.

Memory hook: Chop, wet, turn: compost in a month.

Tricks to remember Decomposition

Short memory aids. Say each one aloud once, then use it on a question the same day.

F-L-C-H-M
Farmers Like Clean Healthy Meadows: Fragmentation, Leaching, Catabolism, Humification, Mineralisation.
Use it when: Order of steps.
Lignin is a lock
Lignin and chitin lock nutrients in; nitrogen and sugars open the door.
Use it when: Chemical factors.
Warm, wet, windy (airy)
Warm, moist and aerobic soils decompose fastest.
Use it when: Climatic factors.

Decomposition: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Five steps · 3 marks · Short answerBoard level

Describe fragmentation, leaching and catabolism, naming the organisms or agents involved in each.

Try it first, then show the solutionSolution

Fragmentation: detritivores such as earthworms break detritus into smaller particles (1 mark).
Leaching: water-soluble inorganic nutrients move down into the soil and precipitate as unavailable salts (1 mark).
Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances (1 mark).

Answer: Earthworms fragment; water leaches; microbial enzymes catabolise.

Example 2: Humus · 2 marks · Short answerBoard level

What is humus? Why is it called a reservoir of nutrients?

Try it first, then show the solutionSolution

Humus is a dark-coloured, amorphous substance formed by humification; it is highly resistant to microbial action and decomposes very slowly (1 mark).
Being colloidal, it holds nutrients and releases them slowly by mineralisation, so it acts as a nutrient reservoir (1 mark).

Answer: Dark amorphous colloid; holds and slowly releases nutrients.

Example 3: Litter bags · 3 marks · Short answerHarder board level

Equal 50 g bags of leaf litter are buried for a year in a tropical rainforest and in a cold desert. The forest bag weighs 12 g and the desert bag 41 g at the end. Calculate the percentage mass lost in each and explain the difference.

Try it first, then show the solutionSolution

Forest: (50 − 12)/50 × 100 = 76% lost (1 mark).
Desert: (50 − 41)/50 × 100 = 18% lost (1 mark).
The rainforest is warm and moist, favouring microbes and detritivores; the cold desert is cold and dry, inhibiting them (1 mark).

Answer: 76% versus 18%; warmth and moisture speed decomposition.

Example 4: Why fragmentation first · 3 marks · Short answerHarder board level

Why is fragmentation important for the later steps of decomposition? What would happen in a forest if decomposition stopped?

Try it first, then show the solutionSolution

Breaking detritus into small particles greatly increases its surface area, so leaching and microbial catabolism act faster and organic matter becomes available to microbes (1½ marks).
Without decomposition, dead leaves and bodies would pile up, nutrients would stay locked in them (1 mark),
and the soil would lose fertility, so producers and the whole food web would decline (½ mark).

Answer: More surface for microbes; without decomposition nutrients are not recycled and the ecosystem fails.

Example 5: Productivity from data · 3 marks · Short answerHarder board level

A forest has an NPP of 7000 kcal m⁻² yr⁻¹, and its plants use 9500 kcal m⁻² yr⁻¹ in respiration. Calculate the GPP. What fraction of NPP finally reaches decomposers, and why?

Try it first, then show the solutionSolution

NPP = GPP − R, so GPP = NPP + R = 7000 + 9500 = 16 500 kcal m⁻² yr⁻¹ (2 marks).
Ultimately almost all of the NPP not lost as heat by consumers reaches decomposers, because every dead plant, animal and faecal mass becomes detritus; in forests most energy flows through the detritus food chain (1 mark).

Answer: GPP = 16 500 kcal m⁻² yr⁻¹; most of the remaining energy enters the detritus food chain.

Example 6: Peat versus compost · 4 marks · Short answerHOTS / competency

Suppose litter input is 600 g m⁻² yr⁻¹ in a tropical forest, of which 540 g decomposes, and 200 g m⁻² yr⁻¹ in a tundra, of which only 40 g decomposes. (a) Calculate yearly accumulation in each. (b) Explain the difference using two factors. (c) Predict the effect on soil organic matter over centuries.

Try it first, then show the solutionSolution

(a) Forest: 600 − 540 = 60 g m⁻² yr⁻¹; tundra: 200 − 40 = 160 g m⁻² yr⁻¹ (1 mark).
(b) Low temperature in the tundra slows microbial activity; waterlogged, anaerobic soils inhibit the largely oxygen-requiring process (2 marks).
(c) Tundra soils accumulate thick organic layers (peat), while tropical soils hold little organic matter because it is recycled quickly (1 mark).

Answer: 60 versus 160 g m⁻² yr⁻¹; cold and anaerobic conditions cause build-up in the tundra.

Where students lose marks in Decomposition

The slips examiners see most often, and what to write instead.

Mistake: Naming a later step as the first
In 2024 examiners reported catabolism or leaching named instead of fragmentation as the first step.
Do this: Fragmentation by detritivores comes first.
Usually costs: 1 mark.
Mistake: Wrong productivity equation
Examiners (2024) saw GPP = NPP − R instead of NPP = GPP − R.
Do this: NPP = GPP − R, so GPP = NPP + R.
Usually costs: 1 to 2 marks.
Mistake: Repeating the question instead of explaining importance
Examiners (2024) noted answers that repeated 'breakdown of organic matter' without saying why fragmentation or decomposition matters.
Do this: Say what it enables: more surface for microbes, nutrient recycling.
Usually costs: 1 mark.
Mistake: Ignoring climate in forest-desert comparisons
Examiners (2024) found candidates unaware of the climatic and vegetation differences between forest and desert.
Do this: Forest: abundant detritus, warm and moist; desert: little detritus, dry.
Usually costs: 1 mark.

Exam tips

Learn the five steps in order with one organism or agent each.
Give both a chemical and a climatic factor in rate answers.
Use 'detritivores' for earthworms and 'decomposers' for bacteria and fungi.
Write units (kcal m⁻² yr⁻¹) in productivity sums.
Mention that decomposition is largely an oxygen-requiring process.

Practice questions on Decomposition

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Water-soluble inorganic nutrients move into the soil and precipitate as unavailable salts during:
(a) fragmentation (b) leaching (c) humification (d) mineralisation
Hint
Washed down.
Answer
(b) leaching.
Q2 · 2 marks · Short answerBoard level
Distinguish between detritus and detritivores, with an example of each.
Hint
Material versus organism.
Answer
Detritus is dead organic matter, e.g. fallen leaves, bark, dead animals and faeces. Detritivores are organisms that feed on detritus and fragment it, e.g. earthworms.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Leaves rich in lignin decompose slowly.
Reason (R): Lignin is resistant to microbial breakdown, whereas nitrogen and sugars are easily used by microbes.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Chemical composition.
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
Distinguish between humification and mineralisation.
Hint
Product of each.
Answer
Humification is the formation and accumulation of humus, a dark, amorphous, slowly decomposing colloid. Mineralisation is the further degradation of humus by microbes to release inorganic nutrients.
Q5 · 2 marks · Short answerHarder board level
Two 80 g litter bags are placed in a garden: bag P with soft vegetable leaves loses 60 g in six months; bag Q with woody twigs loses 8 g. Calculate the percentage loss and explain.
Hint
Same climate, different chemistry.
Answer
P: 60/80 × 100 = 75%; Q: 8/80 × 100 = 10%. In the same climate, soft leaves rich in nitrogen and sugars decompose fast, while lignin-rich woody twigs resist microbes.
Q6 · 2 marks · Short answerHarder board level
Mangrove plants on a coast fix 18 400 kcal m⁻² yr⁻¹ as gross production and spend a quarter of it on their own respiration. Work out the NPP.
Hint
R = 0.25 × GPP.
Answer
R = 0.25 × 18 400 = 4600; NPP = 18 400 − 4600 = 13 800 kcal m⁻² yr⁻¹.
Q7 · 3 marks · Short answerHOTS / competency
Suppose all earthworms and other detritivores in a farm soil are killed by heavy pesticide use. Predict three effects on decomposition and soil fertility.
Hint
First step lost.
Answer
Fragmentation slows, so litter stays in large pieces with little surface for microbes; catabolism and humus formation slow, so nutrients are released slowly and fertility falls; soil is less loosened and aerated, lowering oxygen for aerobic decomposers and water infiltration.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a diagram shows dead leaves on a forest floor, earthworms breaking them into bits, rain washing soluble salts downwards, fungi growing on the fragments, and a dark layer forming beneath.
(a) Name the first step shown and the organism performing it. (1)
(b) Name the process shown by rain and its effect. (1)
(c) Name the dark layer and the process that releases nutrients from it. (1)
Hint
F-L-C-H-M.
Answer
(a) Fragmentation by earthworms (detritivores). (b) Leaching: water-soluble inorganic nutrients move down and precipitate as unavailable salts. (c) Humus (formed by humification); nutrients are released by mineralisation.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.First step of decomposition?Show answer
Fragmentation.
2.Which step forms humus?Show answer
Humification.
3.Two substances that slow decomposition?Show answer
Lignin and chitin.
4.Two most important climatic factors in decomposition?Show answer
Temperature and soil moisture.
5.What releases nutrients from humus?Show answer
Mineralisation.
6.GPP 15 000, R 6000 (kcal m⁻² yr⁻¹). NPP?Show answer
9000 kcal m⁻² yr⁻¹.

Decomposition: frequently asked questions

What are the steps of decomposition?

Fragmentation, leaching, catabolism, humification and mineralisation, which act simultaneously on detritus.

Why is decomposition faster in tropical forests?

Warm, moist and aerobic conditions favour soil microbes and detritivores, and there is abundant detritus.

What is humus?

A dark, amorphous, colloidal substance formed during decomposition; it resists microbes, decomposes slowly and stores nutrients.

What slows decomposition?

Detritus rich in lignin and chitin, low temperature, dryness and anaerobic (waterlogged) conditions.

Why are earthworms called farmers' friends?

They fragment organic matter, speeding decomposition, and loosen the soil.