Agriculture and carbon at Higher Level: The same ideas as SL, with different examples: the '4 per 1000' initiative, the Rodale trial, the Holme Fen posts and Drax power station. At HL, expect to judge how far farming can become a carbon sink.
Practise this as you read
- Link every technique to carbon going into or out of the soil.
- Judge the limits: saturation, reversal, methane.
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The same field can go either way: Farmland can store, absorb or release carbon. Most of it sits in the soil organic matter, and farming techniques decide whether that store grows or shrinks.
The points to remember
- A farm's biggest carbon store is its soil organic matter.
- Carbon goes in as crops photosynthesise and leave roots, stubble and manure in the soil.
- Carbon comes out as decomposers respire, and when soil is eroded or burned.
- More in than out: the soil is a sink; more out than in: a source; level: a store.
- The techniques the farmer uses decide which way the balance tips.
Remember it as: Feed the soil and it is a sink; disturb it and it is a source.
Real example: at the 2015 Paris climate talks, France launched the '4 per 1000' initiative. It argued that raising the carbon in farm soils by just 0.4% a year would cancel out a large share of each year's rise in atmospheric CO₂.
Name the soil: In farming questions the carbon store is usually the soil. Say 'soil organic matter' or 'soil carbon', not just 'the farm'.
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Regenerative agriculture aims to put more carbon into the soil each year than comes out.
Techniques that make soil a sink
- Cover crops: plants grown between harvests keep photosynthesising and add roots and leaves.
- No till: not ploughing keeps the soil undisturbed: less oxygen gets in, so decay slows.
- Crop rotation: changing crops each year, with deep roots and legumes, adds more organic matter.
- Adding manure or compost puts carbon straight into the soil.
- Covered, undisturbed soil also erodes less, so its carbon stays put.
Regenerative field
- Cover crop over winter
- No ploughing
- Four-crop rotation with legumes
Conventional field
- Bare soil over winter
- Ploughed every year
- Maize and soya only
Real example: the Rodale Institute in Pennsylvania, USA, has compared farming systems side by side since 1981. Its fields given manure and cover crops hold more soil carbon than its conventional fields.
Link the technique to the carbon: 'Cover crops are good for the soil' is not enough. Say how: they photosynthesise and add organic matter, so carbon builds up in the soil.
Other techniques do the opposite: they speed up decomposition or let the soil blow and wash away.
Techniques that make soil a source
- Heavy tillage: ploughing mixes oxygen into the soil, so decomposers release CO₂ faster.
- Ploughed, bare soil is eroded by wind and rain, and its carbon is lost.
- Monoculture: one crop, often with bare soil between harvests and little residue returned.
- Draining a wetland: waterlogged peat gets air, so it decomposes and releases its carbon.
- Farms also release carbon from machinery fuel, fertiliser factories and methane from cattle and rice.
| Technique | What it does to the soil | Role |
|---|---|---|
| Cover crops, no till, rotation | Adds organic matter, slows decay | Sink |
| Heavy tillage | Lets oxygen in, speeds decay, erosion | Source |
| Monoculture | Bare soil, little residue | Source |
| Draining wetland | Peat decomposes | Source |
Real example: in 1851 an iron post was driven into the peat at Holme Fen, England, level with the ground, before the fen was drained for farming. The ground has since dropped about 4 metres as the drained peat decomposed and blew away.
Say why oxygen matters: Ploughing and draining both let oxygen into the soil. Decomposers need oxygen to respire quickly, so more carbon leaves as CO₂.
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Some crops take decades to grow. With timber, what happens after the harvest matters as much as the growing.
The points to remember
- A timber plantation is a sink while its trees grow, for decades before harvest.
- At harvest, what happens to the wood decides where the carbon goes.
- Long-lived products (buildings, furniture) keep the carbon for decades: a store.
- Short-lived products (paper, fuel) release it within months or at once: a source.
- Replanting after harvest starts a new sink; longer rotations hold more carbon.
Real example: Drax power station in England burns millions of tonnes of wood pellets a year, much of it from North American forests. The carbon in the wood is released at once; new trees take decades to absorb it again.
Timber v burning: Carbon in timber products is released slowly, when the wood finally decays or is burned; burning releases it at once. Say which use you mean.
How this comes up: Paper 1, the final question: to what extent [6]. Arguments for [4 max], against [4 max], and a conclusion [1].
Governments are paying farmers to use regenerative methods to store carbon in their soils.
To what extent can changing farming techniques turn agriculture from a carbon source into a carbon sink?
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