Soil and nutrient recycling at Higher Level: The same cycle as SL, with different examples: Finland's slow needle litter, Australia's imported dung beetles, white-rot fungi in an ancient Polish forest, and the Hubbard Brook valley that lost its nitrate when its trees were cut.
Practise this as you read
- Follow each element from the dead leaf back into a living plant.
- Explain every fast or slow rate with its cause.
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Nothing is wasted: The elements in a dead leaf are not lost: the soil breaks the leaf down and hands its elements back to living plants. This is part of the biogeochemical cycles.
The points to remember
- Elements such as carbon, nitrogen, phosphorus and potassium are used again and again: they cycle.
- The soil is where much of this recycling happens, as part of biogeochemical cycles.
- The major input is dead organic matter from plants: leaf litter, dead roots, fallen wood.
- Detritivores break it into small fragments; saprotrophs decompose the fragments.
- Decomposition releases mineral nutrients (nitrate, phosphate, potassium) that plant roots absorb again.
- Without it, nutrients would stay locked in dead matter and plants would run short.
Remember it as: Fall, fragment, decompose, release, absorb.
Real example: in the cold boreal forests of Finland, fallen pine needles take years to break down, so a thick layer of needles builds up on the ground and nutrients return to the trees only slowly.
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The first step is done by detritivores. They eat dead leaves and break them into pieces, which makes the next step faster.
The points to remember
- Detritivores are animals that eat dead organic matter (detritus) inside their bodies.
- Examples: earthworms, woodlice, millipedes, springtails, mites, dung beetles.
- They break litter into small fragments, so it has a much larger surface area for decomposers.
- Their droppings (worm casts) are rich in nutrients; their burrowing mixes litter into the soil.
- They do the first step: they speed decomposition up but do not finish it.
Real example: in Australia, cattle dung once lay on pastures for months, because native dung beetles could not use it. From 1965, scientists brought in dung beetles from Africa and Europe to bury the dung and return its nutrients to the soil.
Detritivore or saprotroph?: Detritivores are animals that eat dead matter and break it into fragments. Saprotrophs are fungi and bacteria that digest it outside their cells. Earthworms are detritivores, not decomposers.
Digesting from the outside: Saprotrophs finish the job. As they feed, they release the elements in the dead matter as simple mineral nutrients.
The points to remember
- Saprotrophs are fungi and bacteria that feed on dead matter.
- They release enzymes onto it, digest it outside their cells, then absorb the products.
- This decomposition turns organic matter back into mineral nutrients, such as nitrate and phosphate.
- As they respire, they release carbon dioxide to the air, so carbon is recycled too.
- What resists decay, a dark crumbly material, builds up as humus.
- Some soil bacteria turn ammonium into nitrate; others fix nitrogen from the air.
Remember it as: Detritivores chop it up; saprotrophs break it down.
Real example: in Poland's Białowieża Forest, one of Europe's last ancient forests, fallen trees are left to rot. White-rot fungi, among the few organisms that can digest wood fully, take decades to return a log's nutrients to the soil.
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Decomposers work fastest where it is warm, moist and airy, and slowest where it is cold, dry or waterlogged. People can also break the cycle.
The points to remember
- Warm soils decompose fast; cold soils slowly.
- Moist soils decompose fast; dry soils slowly; waterlogged soils very slowly (no oxygen).
- Soft leaves rot fast; tough needles and wood rot slowly.
- More detritivores and decomposers means faster recycling.
- The cycle is broken when crops are harvested, forests are cleared or burned, or nutrients are leached away.
Remember it as: Warm, wet and airy: fast. Cold, dry or drowned: slow.
Real example: at Hubbard Brook in the USA, scientists cut down all the trees in one small valley in the 1960s. With no roots to take up nutrients, nitrate was washed out of the soil and the streams carried away many times more than before.
Explain the speed with a reason: Not 'it is hot'. Write 'warm, moist conditions let decomposers grow and respire faster, so nutrients are released sooner'.
How this comes up: Paper 2, Section A: explain a difference between two ecosystems [3]. Each point links a condition to the decomposers, then to the speed of recycling.
Leaf litter on the floor of a rainforest in Borneo decomposes within months. Needle litter in a spruce forest in northern Sweden can take years.
Explain why nutrients are recycled faster in the rainforest soil.
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