The Earth has a fixed amount of carbon, nitrogen and water. Nothing new arrives. Everything alive is built from atoms that were part of something else before — and the systems that move them round are biogeochemical cycles. Three words unlock this whole topic: store, sink and source.
📚 What you need to know
Biogeochemical cycles circulate the chemical elements that life needs, so they stay available to organisms.
The three you should know are the carbon, nitrogen and hydrological (water) cycles.
A store holds an element in equilibrium: total input equals total output.
A sink is where an element accumulates: total input is greater than total output.
A source releases an element: total output is greater than total input.
Stores, sinks and sources can be natural or artificial, and the same place can change role over time.
Residence time is the average time an atom stays in a store.
Why cycles exist at all
Energy arrives from the Sun continuously and leaves as heat, so it never needs recycling. Matter has no such supply. The carbon in your body was once in the air, before that in a plant, and before that in something that died. If elements were not returned, the supply available to living things would run out and new organisms could not be built.
That is why decomposition matters so much, and why anything that interrupts a cycle — burning fossil fuels, clearing forests, draining wetlands — has effects far beyond the place it happens.
Store, sink or source?
These three words describe the same kind of thing — a part of the cycle where an element sits. What separates them is the balance between what goes in and what comes out.
Nothing about the place itself decides the label. Measure what goes in, measure what comes out, then decide.
Term
The balance
Example
Store
Input equals output, so the amount held stays roughly constant
A mature forest, where carbon taken in by photosynthesis is matched by carbon released by respiration and decay
Sink
Input is greater than output, so the element builds up
A young growing forest, or the ancient ecosystems that slowly became fossil fuels
Source
Output is greater than input, so the element is released
A volcanic eruption, or a forest being burned or cleared
Notice that a forest can be all three, at different points in its life. Young and growing: a sink. Mature and steady: a store. On fire: a source. If an exam question gives you data about inputs and outputs, use the data — do not label something a sink just because it sounds like one.
Residence time
Residence time is the average length of time an atom stays in a store before moving on. It varies enormously, and that variation is exactly why human activity is such a problem.
Each step along the axis is ten times longer than the one before, otherwise the fossil fuel bar would be about a kilometre wide.
The point of residence time: fossil fuels were a store that took hundreds of millions of years to fill. We are emptying it in a couple of centuries. The cycle is not broken — it has simply been sped up in one direction.
How humans disrupt cycles
Burning fossil fuels moves carbon from a very long-term store into the atmosphere in seconds.
Deforestation removes the trees that absorb carbon dioxide, and releases the carbon they held.
Urbanisation covers soil with concrete, changing how water and nutrients move through the land.
Agriculture adds nitrogen and phosphorus far faster than natural processes do, and disturbs soil so stored carbon is released.
In each case the result is the same shape of problem: a flow that used to be slow becomes fast, the balance between input and output breaks, and something that was a store turns into a source.
Worked examples
WORKED EXAMPLE
A peat bog absorbs 5.2 tonnes of carbon per hectare each year and releases 3.1 tonnes. State whether it is a store, sink or source, and justify your answer.
Step 1: compare input and outputinput 5.2 > output 3.1Step 2: find the difference5.2 − 3.1 = 2.1 tonnes per hectare per year accumulatingIt is a sinkStep 3: justify
Input exceeds output, so carbon builds up in the bog over time.
Always quote the two numbers in the justification. “It absorbs a lot” is not a justification.
WORKED EXAMPLE
The same bog is drained for farmland. Explain how its role in the carbon cycle changes.
Step 1: what draining does
Removing the water lets oxygen into the peat, so decomposers can break down organic matter that had been preserved for thousands of years.
Step 2: the effect on the balance
Decomposition releases carbon dioxide, so output now exceeds input.
The bog changes from a sink to a sourceUse the phrase “from a sink to a source”. It shows you understand that the label depends on the balance, not the place.
💡 Exam tip
Decide store, sink or source by comparing input with output, using the numbers you are given.
Say net accumulation for a sink and net release for a source. Those are the mark scheme words.
Remember the same place can change role, and that human activity is usually what changes it.
Use residence time to explain why releasing fossil carbon matters so much more than releasing carbon from a leaf.
Learn one example of a natural and one of an artificial sink or source. Both types are examinable.
⚠ Common mix-up
Store and sink treated as the same word. A store is balanced; a sink is gaining.
Deciding by the name of the place rather than by the input and output data given.
Saying carbon is “used up”. Elements are not destroyed — they move between stores.
Confusing residence time with the size of a store. A store can be large and fast-turnover, or small and slow.
Thinking cycles apply only to carbon. Nitrogen and water cycle in exactly the same way.
Forgetting decomposition. Without decomposers, nutrients stay locked in dead material and the cycle stalls.
Up next: Carbon Stores and Flows — the full carbon cycle, what every arrow on the diagram represents, and how a forest switches between sink, store and source.
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