Stores, sinks and sources at Higher Level: The same three ideas as SL, with different examples: the Congo peatlands, Canada's 2023 fires and the ocean sink. HL adds the nitrogen cycle, so you can use both cycles in one answer.
Practise this as you read
- Define each term by the balance of flows in and out.
- Give a store, sink and source for carbon and nitrogen.
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Think of a bath: Every store in a cycle has flows in and flows out, like a bath with the tap on and the plug out. Whether it fills, empties or stays level decides what we call it.
The points to remember
- A store (storage) holds an element for a time: carbon in the air, in soil, in trees.
- A store is in equilibrium when what flows in equals what flows out.
- So a store in equilibrium stays the same size over time.
- Store, sink or source depends on the balance of flows, not on how big the store is.
Remember it as: Same in, same out: a steady store.
Real example: nitrogen gas has made up about 78% of the air for millions of years. Bacteria take some out of the air and other bacteria put about as much back, so the atmosphere is a nitrogen store in equilibrium.
Stores and flows again: Storages are boxes and flows are arrows in a system diagram (see 1.2.2 and 1.2.3).
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When the tap runs faster than the plug drains, the bath fills. A store that keeps gaining is a sink.
The points to remember
- A sink takes in more of an element than it releases: a net accumulation.
- So a sink grows over time.
- Carbon sinks: growing plants (photosynthesis greater than respiration), peat bogs, the oceans.
- Carbon sinks slow global warming by taking carbon dioxide out of the air.
- Say 'more in than out': 'it absorbs carbon' on its own does not show a sink.
Remember it as: Sink soaks: more in than out.
Real example: the peatlands of the Congo Basin, mapped only in 2017, hold about 30 billion tonnes of carbon, built up over thousands of years because waterlogged plant remains hardly rot. While they stay wet, they keep gaining carbon: a sink.
Not just 'absorbs': A growing forest both takes in and gives out carbon. It is a sink only because it takes in MORE than it gives out.
When the plug drains faster than the tap fills, the bath empties. A store that keeps losing is a source.
The points to remember
- A source releases more of an element than it takes in: a net release.
- So a source shrinks over time, and adds the element to other stores such as the air.
- Carbon sources: burning fossil fuels, respiration, burning or clearing forests, drained peat.
- A sink can become a source when people disturb it: a drained bog, a burned forest.
- Say 'more out than in': 'it releases carbon' on its own does not show a source.
Remember it as: Source sends: more out than in.
Sink
- More in than out
- Net accumulation
- The store grows
Source
- More out than in
- Net release
- The store shrinks
Real example: in 2023, record wildfires burned across Canada and released about 480 million tonnes of carbon into the air: a burning forest is a carbon source.
Sinks can switch: When a question describes a disturbance, such as draining, burning or clearing, ask whether the balance flips from 'more in' to 'more out'. (Forests as sinks and sources in detail: 2.3.6.)
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Exam data often give the flows in and out as numbers. The label (store, sink or source) comes from a simple subtraction.
The points to remember
- Find what flows in and what flows out, in the same units (e.g. tonnes per year).
- Work out the net: in minus out.
- Net above zero: a sink. Net zero: a store in equilibrium. Net below zero: a source.
- Quote the numbers: 'takes in 12, releases 7, so a net gain of 5 a year: a sink'.
- A clearly labelled diagram (boxes and arrows sized by the flows) can show the same thing.
| Flow (billion tonnes of carbon a year) | Amount |
|---|---|
| Carbon dissolving into the oceans | about 80 |
| Carbon released from the oceans | about 77 |
| Net: in minus out | about +3: a sink |
Real example: in the 2010s the oceans took in about 3 billion tonnes more carbon each year than they released, so the ocean as a whole was a sink (Global Carbon Project; more on 2.3.9).
Same time, same units: Compare flows over the same period and in the same units. A big store is not a sink unless it is gaining.
At HL you can be asked to use both cycles. The three words mean the same thing in the nitrogen cycle.
The points to remember
- The atmosphere is the main nitrogen store: 78% of the air is nitrogen gas.
- Sinks: plants absorbing nitrates from the soil; the Haber process fixing nitrogen into fertiliser.
- Sources: decomposers releasing ammonium; denitrifying bacteria returning nitrogen gas to the air.
- Carbon to compare: store = air, soils, fossil fuels; sink = plants; source = respiration, combustion.
Store
- Carbon: atmosphere, soils, fossil fuels
- Nitrogen: the atmosphere
Sink
- Carbon: plants in photosynthesis
- Nitrogen: plants taking up nitrates; the Haber process
Source
- Carbon: respiration; burning fossil fuels
- Nitrogen: decomposers; denitrifying bacteria
Real example: the Haber process now takes about 120 million tonnes of nitrogen out of the air each year, more than all the natural fixation on land: a huge new nitrogen sink for the air's nitrogen (more on 2.3.23).
Both cycles, every word: For a 'both cycles' question, give a store, a sink and a source for carbon AND for nitrogen, after the three definitions.
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How this comes up: Paper 2, Section B (b): distinguish the three terms using both the carbon and nitrogen cycles [7].
Biogeochemical cycles move elements between the air, living things, soils and rocks.
Using both the carbon cycle and the nitrogen cycle, distinguish between stores, sinks and sources.
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