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Spec mapping: AQA 7037, Paper 1 (Physical), §3.1.1 — the processes that sequester carbon and those that release it, and the changing balance between sources and sinks under natural and human influence. This depth lesson assumes the store/flux framework of the previous lesson and pushes into the productivity hierarchy (GPP→NPP→NEP→NBP), soil and peatland carbon dynamics, the permafrost carbon feedback, anthropogenic release pathways and the carbonate chemistry of ocean acidification. AOs exercised: AO1 (precise process and chemistry knowledge; quantified stores and fluxes), AO2 (explaining why sinks can flip to sources; how temperature sensitivity drives feedback), AO3 (calculating NPP, % shares of emissions, sequestration potential). Synoptic links run to Ecosystems under stress (NPP, biome productivity), Hazards and Coasts (blue carbon, reef loss).
Carbon sequestration is the transfer of carbon from the atmosphere into a sink (vegetation, soil, ocean, rock); carbon release is the reverse. Whether atmospheric CO₂ rises, falls or holds steady is the net of these opposing fluxes, summed across the biosphere. The central depth idea is that the same store can be a sink or a source depending on conditions — a forest, a peat bog, a permafrost soil — so the balance is dynamic and, critically, temperature-sensitive, which is what couples the carbon cycle to climate change through feedback.
Ecosystem carbon balance is built up through a nested sequence of fluxes, each subtracting a loss term.
GPP is the total carbon fixed by photosynthesis per unit area per time (gC/m²/yr or GtC/yr globally).
NPP subtracts autotrophic respiration (Ra, the plants' own respiration):
NPP=GPP−Ra.
Around 50% of GPP is respired by plants, so global terrestrial NPP ≈ 60 GtC/yr. NPP is the carbon available to build biomass and feed the rest of the ecosystem — effectively the productivity that supports all higher trophic levels (a direct link to Ecosystems).
NEP further subtracts heterotrophic respiration (Rh, decomposition by microbes, fungi and animals):
NEP=NPP−Rh=GPP−Ra−Rh.
If NEP > 0 the ecosystem is a net sink; if NEP < 0 it is a net source. NEP is small and finely balanced — the difference between two large fluxes — which is exactly why warming-driven changes in Rh can tip an ecosystem from sink to source.
NBP additionally subtracts losses from disturbance — fire, logging, windthrow, pests, harvest:
NBP=NEP−(disturbance losses).
NBP is the true long-term, large-scale net exchange with the atmosphere, and it is what determines whether a region is genuinely sequestering carbon over decades. A forest with positive NEP in an average year can have negative NBP once a major fire year is included.
flowchart TB
GPP[GPP ~120 GtC/yr
total photosynthesis] -->|minus autotrophic respiration Ra| NPP[NPP ~60 GtC/yr]
NPP -->|minus heterotrophic respiration Rh| NEP[NEP
small net flux]
NEP -->|minus disturbance: fire, logging, pests| NBP[NBP
true long-term sink/source]
A temperate forest fixes 2,400 gC/m²/yr (GPP). Autotrophic respiration is 55% of GPP; heterotrophic respiration returns 900 gC/m²/yr; an average fire year removes 120 gC/m²/yr.
Manipulate.
Ra=0.55×2400=1320 gC/m2/yr. NPP=2400−1320=1080 gC/m2/yr. NEP=1080−900=180 gC/m2/yr (>0⇒net sink). NBP=180−120=60 gC/m2/yr.
Explain. The forest is a modest net sink (NBP = 60), but its sink strength has been cut by two-thirds once disturbance is counted — and a single severe fire year (say 300 gC/m²/yr) would make NBP negative, flipping it to a source.
Evaluate. NEP is the small residual of much larger fluxes (GPP 2400, Ra 1320, Rh 900), so a modest percentage rise in respiration under warming produces a large proportional fall in NEP. This sensitivity — not the absolute numbers — is the key insight, and stating it is an AO3 evaluation point.
Soils are the largest terrestrial store: ~1,500–2,400 GtC in the top ~2 m, roughly three times the atmospheric store.
Carbon in: litterfall (dead leaves, wood, roots), root exudates (carbon-rich compounds released by living roots), and root turnover delivering carbon at depth.
Carbon out: decomposition (microbial/heterotrophic respiration releasing CO₂); erosion (loss of carbon-rich topsoil to water and wind); and leaching of dissolved organic carbon (DOC) into rivers.
Decomposition roughly doubles for each 10 °C rise in temperature (a Q10 of ~2):
Rh(T)=Rh,ref×Q10(T−Tref)/10.
This temperature sensitivity is the engine of the soil-carbon feedback: warming → faster decomposition → more CO₂ released → more warming. Because the soil store is so large, even a small fractional release is significant.
Exam application: "Warmer → faster decomposition → more CO₂ → more warming" is a textbook positive feedback and a near-guaranteed component of feedback-mechanism answers. The size of the soil store (3× the atmosphere) is what makes it dangerous.
Formation. Peat accumulates where waterlogging, acidity and oxygen-poverty slow decomposition below the rate of organic-matter production, so partly decayed plant material builds up.
UK peatlands. Blanket and raised bogs are extensive in Scotland, northern England and Wales:
| Threat | Mechanism | Carbon impact |
|---|---|---|
| Drainage (agriculture/forestry) | Lowers water table, oxygenating peat | Aerobic decomposition → CO₂ |
| Burning (e.g. moor management) | Combusts surface peat; damages Sphagnum | Direct CO₂; lost future sequestration |
| Erosion / gullying | Physical removal by water and wind | Particulate + dissolved organic-carbon loss |
| Climate change | Warmer + altered rainfall lowers water tables | Tips bogs from sink to source |
Peatland restoration — rewetting by blocking drainage grips, Sphagnum re-establishment, and ending rotational burning — is one of the most cost-effective domestic carbon-management actions, and it links this lesson directly to the carbon-management strategies covered later.
Permafrost — ground frozen for at least two consecutive years — underlies ~25% of the Northern Hemisphere land surface (notably Siberia, Alaska, northern Canada).
Carbon content: ~1,400–1,700 GtC — roughly twice the current atmospheric carbon — accumulated over millennia as plant material froze before it could fully decay.
The permafrost carbon feedback:
Quantified risk (IPCC AR6): under high-warming SSP5-8.5, near-surface permafrost area could fall by 30–70% by 2100; cumulative release could reach ~150–200 GtC, equivalent to 15–20 years of current anthropogenic emissions. Crucially, this is largely additional to and not captured by most emissions budgets, and it is effectively irreversible on human timescales — points that recur in the feedback and climate lessons.
Burning fossil fuels reverses hundreds of millions of years of slow-cycle burial in hours.
| Source | Approx. share of fossil + cement CO₂ |
|---|---|
| Coal | ~40% |
| Oil | ~32% |
| Natural gas | ~21% |
| Cement | ~4–8% |
| Flaring | ~1% |
Cement releases CO₂ two ways:
Together, cement is ~7–8% of global CO₂ emissions — a "hard-to-abate" sector that is a key target for CCS (later lesson).
Forests hold ~450–650 GtC in biomass; clearing releases it by burning and decay.
The ocean absorbs ~25–30% of anthropogenic CO₂. Dissolving CO₂ shifts the carbonate system:
CO2+H2O⇌H2CO3⇌H++HCO3−⇌2H++CO32−.
The released H⁺ lowers pH and consumes carbonate ions (CO32−), the very ions calcifiers need.
Δ[H+]=10−8.1/10−8.2=100.1≈1.26⇒+26%.
Exam tip: Acidification is a direct chemical consequence of CO₂ uptake, distinct from warming — a reef can be damaged by acidification even where it is not bleaching. Use the logarithmic-pH calculation to show the 26% figure; it is a reliable AO3 discriminator.
The permafrost of Siberia — the world's largest permafrost region, underlying much of the Russian Arctic — is the type-example of a vast, vulnerable carbon store in transition. The active layer (the surface zone that thaws each summer and refreezes each winter) is deepening as the Arctic warms at ~3–4× the global rate, exposing ancient organic carbon (much of it in ice-rich yedoma deposits) to decomposition for the first time in tens of thousands of years.
Two processes make this especially significant. First, where thaw is gradual and well-drained, aerobic decomposition releases CO₂; where it creates waterlogged ground, anaerobic decomposition releases methane, with its far higher short-term GWP. Second, thaw is often abrupt rather than gradual: as ground ice melts, the surface collapses into thermokarst — slumps, craters and lakes — exposing deep carbon rapidly and creating the very waterlogging that favours methane. The dramatic Batagaika "megaslump" in Yakutia, a kilometre-long thaw crater that has grown rapidly since the late twentieth century, is a visible emblem of this abrupt transition. The Siberian case grounds the abstract "1,400–1,700 GtC permafrost store" in a real, observable, accelerating transfer of carbon from a frozen sink to an atmospheric source — and it is the engine of the permafrost feedback developed in the next lesson.
The UK's blanket bogs — in the Flow Country of northern Scotland, the Pennines and Dartmoor — store more carbon than the forests of the UK, France and Germany combined, yet the majority are degraded: drained for agriculture and forestry, burned for grouse-moor management, and eroded into bare peat gullies. Degraded peat is a carbon source: lowering the water table oxygenates the peat, switching it from slow accumulation to active CO₂ release, and exposed peat is also stripped by water and wind as particulate and dissolved organic carbon.
The policy response is one of the UK's most cost-effective carbon actions. Peatland restoration — blocking drainage "grips" to raise the water table, re-vegetating bare peat with Sphagnum, and ending rotational burning — aims to switch the bogs back from source to sink. The Flow Country (the largest blanket bog in Europe) became a UNESCO World Heritage Site in 2024 in recognition of its global carbon and ecological importance. Restoration also delivers flood-management co-benefits (intact bog slows runoff — a link to drainage-basin hydrology) and biodiversity gains, making it a textbook example of a multi-benefit, nature-based carbon strategy that recurs in the carbon-management lesson.
A distinctive AQA emphasis is that the water and carbon cycles are coupled, and sequestration/release is where the coupling is clearest:
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