Decomposers & nutrient cycling

The big idea: When an organism dies, or produces waste, the carbon, nitrogen, phosphorus and other nutrients locked inside it are not lost.

Decomposers feed on this dead organic matter and break it down, releasing the nutrients back into the soil as simple inorganic substances.

Producers (plants and algae) can then absorb those nutrients and use them again — so the same atoms are recycled round the ecosystem, over and over.

Decomposer

An organism that feeds on dead organic matter (dead bodies and waste) and breaks it down, releasing nutrients back to the environment. Saprotrophs and detritivores are both decomposers.

Saprotroph

A decomposer (most bacteria and fungi) that secretes digestive enzymes ONTO dead matter and absorbs the small soluble products — it digests externally, outside its body.

Detritivore

An animal (such as an earthworm or woodlouse) that ingests pieces of dead organic matter and digests them internally, inside its gut.

Nutrient cycling

The repeated movement of nutrients (such as carbon, nitrogen and phosphorus) between living organisms and the non-living environment, so the same atoms are used again and again.

Inorganic nutrient

A simple mineral substance (such as nitrate or phosphate) that producers can absorb and use to build their own organic molecules.

Why decomposers matter so much: The supply of nutrients on Earth is finite — no new nitrogen or phosphorus arrives from space.

Decomposers are the only thing that unlocks the nutrients trapped in dead matter and returns them to the soil. Without them, dead bodies would pile up and producers would soon run out of raw materials.

Decomposers come in two kinds, and they feed in different ways.

Detritivores are animals that swallow chunks of dead matter and digest them inside their gut. Saprotrophs (mostly bacteria and fungi) never swallow the dead matter — they release enzymes onto it and digest it outside their bodies, then absorb the small soluble products.

External digestion

Digestion that happens OUTSIDE the organism's body. The saprotroph secretes enzymes onto the dead matter, the enzymes break it down, and the soluble products are absorbed.

Internal digestion

Digestion that happens INSIDE the organism's body, in a gut. A detritivore ingests dead matter and digests it internally.

Litter

The layer of dead leaves, twigs and other dead organic matter lying on top of the soil, before it is broken down.

The shared job: recycling nutrients: Detritivores and saprotrophs feed in different ways, but they do the same job.

Both feed on dead organic matter and break it down, releasing inorganic nutrients (such as nitrate and phosphate) back into the soil.

That shared role — recycling nutrients from dead matter — is what makes them both decomposers.

A memory hook: Detritivore swallows then Digests Down inside. A saprotroph Secretes and Soaks up from outside.

Both do the same job — return nutrients to the soil — they just get there by different routes.

Reading a nutrient-cycle diagram: In a nutrient-cycle diagram (such as a Gersmehl diagram) the boxes are stores of nutrients — for example a soil store, a litter store and a biomass (living) store — and the arrows show nutrients moving (transferring) between the stores.

An arrow into the soil store (decomposition, weathering, rainfall) adds nutrients; an arrow out of the soil store (uptake by roots, leaching, runoff) removes them.

How this is tested: A short Describe question (2 marks) often asks for the role of decomposers in an ecosystem — break down dead matter and release / recycle nutrients to the soil for producers.

Paper 1A multiple-choice items like to Distinguish a detritivore from a saprotroph (internal vs external digestion), or ask for a feature they share (both feed on dead matter and recycle nutrients).

In a data question you may be given a nutrient-cycle (Gersmehl) diagram and asked to State what the arrows represent, or which process reduces the soil nutrient store — read the diagram, don't recall a fact.

✓ Why this scores full marks: A 2-mark Describe needs two distinct ideas: what decomposers break down, and what they release / recycle.

Just saying 'they break down dead things' is one mark — the recycling of nutrients back to producers is the second, easily-missed mark.