The carbon cycle — five named processes
Photosynthesis removes from the atmosphere and fixes the carbon into glucose. Respiration returns it — from plants, animals and decomposers alike. Feeding passes carbon compounds along the food chain without touching the air. Decomposition releases as decomposers respire on dead matter. Combustion of fossil fuels adds carbon locked away for millions of years. Name the process, never "carbon moves around".
Decomposers — the recyclers in both cycles
Decomposers are saprotrophic bacteria and fungi that feed on dead organisms and waste (fallen leaves, faeces), never on living prey. Their own respiration releases to the atmosphere; their breakdown of proteins and urea releases ammonium compounds to the soil. Without them, carbon and nitrogen stay locked in dead matter and no nutrients return for new plant growth. They act on every trophic level, so they sit outside the pyramid.
(Extended) Nitrogen cycle — four roles
Nitrogen gas is 78% of the air but too unreactive to use directly. Nitrogen-fixing bacteria convert atmospheric into ammonium compounds (free-living, or in legume root nodules). Decomposers turn proteins and urea into ammonium. Nitrifying bacteria convert ammonium to nitrites then nitrates. Denitrifying bacteria return nitrate to . Roots absorb nitrate by active transport to build amino acids, proteins (including enzymes) and DNA.
Drawn from real examiner reports.
Decomposers ≠ carnivores
A food pyramid shows the trophic levels of living organisms; decomposers are not another layer stacked on top of the final consumer, and they do not hunt living prey. They are saprotrophs feeding on dead matter and waste from every level, returning and ammonium compounds to the abiotic environment. Draw them as a separate box linked to all levels.
Flagged s23 P11 Q37; P13 Q38 — decomposers called "carnivores" or placed at the top of the pyramid.
Eutrophication kills by oxygen, not light
The algal bloom does block light, and the plants below do die — but that is the middle of the chain, not the cause of the fish deaths. Aerobic decomposers then feed on the dead algae and plants, and it is their respiration that strips dissolved oxygen from the water, so the fish suffocate. "The algae blocked the light" or "less photosynthesis" alone earns nothing.
Flagged w22 P43 Q5b ii; s22 P41 Q4a iii — low $\text{O}_2$ blamed on light-blocking or reduced photosynthesis, with no decomposer respiration.
(Extended) Denitrifying bacteria REMOVE
Denitrifying bacteria convert soil nitrate back into nitrogen gas, so they subtract usable nitrogen from the soil and lower its fertility — they never add it. They are favoured by waterlogged, low-oxygen soils, where water fills the air spaces between the soil particles. Read the prefix as removal: de-nitrifying takes the nitrate away.
No dedicated nitrogen-cycle misconception appears in the three sittings distilled in the shared digest; grounded in the mark-scheme process definitions rather than a cited quote — flagged for human review.
(Extended) Nitrogen-fixing ≠ nitrifying
Nitrogen-fixing bacteria act on atmospheric nitrogen gas, capturing it as ammonium compounds and adding brand-new nitrogen to the soil. Nitrifying bacteria never touch the atmosphere — they convert ammonium that is already in the soil into nitrites and then nitrates, the form roots absorb. One is the entry point; the other is an internal soil step.
Respiration ≠ decomposition
Every living organism respires and releases — plants, animals and decomposers — so "only plants release carbon dioxide" is wrong. Decomposition is the separate process of decomposers breaking down dead organisms and waste; a living plant releasing is respiring, not decomposing. A cycle diagram needs both arrows, drawn separately.
Combustion is not part of the balance
Photosynthesis on one side, respiration and decomposition on the other, roughly balance in the natural cycle. Combustion of fossil fuels sits outside that balance, moving carbon from a store locked away for millions of years into the atmosphere faster than photosynthesis can take it back — which is why atmospheric concentration is rising.
Deforestation: say "less photosynthesis"
Clearing trees removes photosynthesis, so less is taken out of the air and its concentration rises — the land is a smaller carbon sink. Vague answers about "oxygen levels changing" miss the credited point: name reduced photosynthesis and reduced uptake as the mechanism.
Flagged s22 P31 Q8c; P32 Q4c — candidates wrote "oxygen levels change" instead of naming the decrease in photosynthesis.
The five carbon-cycle arrows
Atmosphere plants is photosynthesis. Organisms atmosphere is respiration. Plants animals is feeding. Dead matter atmosphere is decomposition. Fossil fuels atmosphere is combustion.
Read both ends of the arrow first
Before naming any process on a cycle diagram, say what sits at the tail of the arrow and what sits at the head. Only then pick the process that moves material in exactly that direction — it is the quickest guard against naming the reverse process by mistake.
(Extended) The four nitrogen arrows
Atmospheric soil compounds is nitrogen fixation. Dead matter ammonium is decomposition. Ammonium nitrate is nitrification. Nitrate is denitrification. Name the bacterial group too.
Never answer "recycling"
A question asking for the process wants a specific name — photosynthesis, decomposition, nitrification — not "the nitrogen cycle" or "nutrients are recycled". Vague process words score zero even when the underlying biology is understood.
Nutrients (carbon, nitrogen) are not used up and lost — they are continuously recycled between living organisms and the abiotic (non-living) environment. Two named cycles are required at Cambridge 0610:
Name the FIVE processes of the carbon cycle and state whether each ADDS or REMOVES carbon dioxide from the atmosphere.
(a) Name three processes of the carbon cycle that release carbon dioxide into the atmosphere. [3]
(b) Name the process of the carbon cycle that removes carbon dioxide from the atmosphere. [1]
