There is a fixed amount of carbon, nitrogen and water on Earth. Nothing new arrives. The only reason life keeps going is that these elements get used, released and used again — that is what a biogeochemical cycle is. Before you learn any individual cycle, get comfortable with three words: store, sink and source.
📘 What you need to know
Biogeochemical cycles circulate the chemical elements that life needs.
The main ones for ESS are the carbon, nitrogen and hydrological cycles.
A store holds an element in equilibrium — total input equals total output.
A sink gains more than it loses, so the element builds up (net accumulation).
A source loses more than it gains, so the element is released (net release).
Stores, sinks and sources can be natural or artificial, and the same place can change role.
Human activity — burning fossil fuels, deforestation, farming, urbanisation — disrupts these cycles.
Why anything has to cycle at all
Energy is delivered fresh every morning by the Sun. Matter is not. The carbon in your body has been round the cycle countless times already, and there is no supply of new atoms waiting somewhere. So if elements were not recycled, life would simply run out of building material.
That is the reason decomposers matter so much. They are the step that puts elements back into circulation instead of letting them sit locked in dead bodies forever.
Keep the two halves of the topic straight in your head: energy flows through an ecosystem and leaves, matter cycles round and stays. Every biogeochemical cycle is an example of the second one.
Store, sink or source?
These three words all describe a place where an element sits. What separates them is nothing to do with size — it is entirely about whether the inputs and the outputs balance.
Red bars are inputs, green bars are outputs. Compare the two lengths and the label writes itself — that is genuinely all there is to it.
Stores
Also called storages or reservoirs. An element sits here for a while, but as much goes out as comes in, so the amount held stays about the same. Oceans, the atmosphere, soil and living organisms are all stores. The ocean, for example, dissolves carbon dioxide from the air and releases roughly the same amount back.
Sinks
A sink is gaining. More of the element goes in than comes out, so it piles up over time. Fossil fuel deposits are the classic example — carbon from ancient organisms accumulated underground for millions of years.
Sources
A source is losing. More goes out than comes in, so the element is released into the cycle. Volcanic eruptions release large amounts of carbon dioxide into the atmosphere, which makes them a natural carbon source.
All three can be natural or artificial. A peat bog is a natural sink; a landfill is an artificial one. A volcano is a natural source; a coal-fired power station is an artificial one. Exams like this distinction, so have an example of each ready.
Residence time
One more useful idea before the carbon cycle proper. Residence time is the average length of time an atom stays in a particular store before moving on.
Carbon in a leaf: weeks to months.
Carbon in a tree trunk: decades to centuries.
Carbon in fossil fuels, if humans leave it alone: hundreds of millions of years.
That last line is the whole environmental problem in one sentence. Mining and burning takes carbon that was going to sit still for hundreds of millions of years and moves it in seconds.
WORKED EXAMPLE
A peat bog absorbs 480 g of carbon per square metre each year and releases 310 g m−2 yr−1. State whether it is acting as a store, a sink or a source, and calculate the net change.
Step 1: Compare input with output480 in, 310 out → input is greaterStep 2: Name the roleMore coming in than going out means carbon accumulatesStep 3: Find the net change480 − 310 = 170A sink, gaining 170 g m−² yr−¹Always give the role and the number if the question asks for both
WORKED EXAMPLE
The bog above is drained for farmland. Measurements now show 90 g m−2 yr−1 absorbed and 640 g m−2 yr−1 released. Explain what has happened.
Step 1: Compare again640 out is far greater than 90 inStep 2: Net change640 − 90 = 550 released each yearStep 3: ExplainDraining lets oxygen into the peat, so decomposition speeds upIt has flipped from a sink to a source
💡 Exam tip
Decide store, sink or source by comparing input with output, never by how big the reservoir is.
Use the phrases net accumulation (sink) and net release (source) — they are the mark-scheme wording.
If asked for the net change, subtract and give the units. Do not stop at naming the role.
Have one natural and one artificial example ready for each of the three terms.
Mention residence time when a question asks why burning fossil fuels is different from ordinary respiration.
⚠ Common mix-up
Assuming a big reservoir must be a “store”. The ocean is huge, but if it starts absorbing more than it releases it is behaving as a sink.
Using sink and source as opposites of “big” and “small”. They describe direction of net movement, not amount held.
Saying elements are used up. Elements are recycled — they change form, they do not disappear.
Forgetting a place can change role. A young forest, a mature forest and a burning forest are sink, store and source in turn.
Confusing residence time with total amount stored. A small store can hold carbon for a very long time.
Up next: Carbon Stores and Flows — the full carbon cycle, and how to work out what every arrow on the diagram is doing.
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