# Carbon cycle & biogenic carbon

The fast biological carbon cycle, the distinction between biogenic and fossil carbon, and why the carbon-neutrality assumption depends on timing, counterfactual and permanence.

Why 'biogenic carbon is neutral' is an accounting convention rather than a physical fact.

Source: https://en.bioecon.ru/docs/foundations/carbon-cycle-basics/
Updated: 2026-08-24



Photosynthesis fixes atmospheric CO₂ into organic matter; respiration, decay and combustion return it. The fluxes are large — terrestrial photosynthesis takes up carbon on the order of a hundred-odd gigatonnes a year, with a comparable return — against an atmospheric stock that is itself a few hundred gigatonnes larger than it was before industrialisation. Two features of that picture do the work for the bioeconomy: the biological cycle is fast, and it is close to balanced.

## Biogenic versus fossil carbon

Chemically, a carbon atom from a plant and a carbon atom from crude oil are the same atom. The distinction is about the reservoir and the timescale it was resident in. **Fossil carbon** has been out of the atmosphere for geological time; burning it is a one-way transfer that adds to the active carbon stock. **Biogenic carbon** was in the atmosphere recently, was fixed by a living organism, and would have returned to the atmosphere on a timescale of years to centuries regardless. Releasing it returns carbon that was already circulating.

That is the entire physical basis of the "bio-based is better" argument, and it is a real one. It is also narrower than it is usually made to sound.

## Where the neutrality assumption breaks

**Timing.** Carbon neutrality over a full rotation is not neutrality now. A forest burned today and regrown over sixty years releases its carbon immediately and recovers it slowly — a *carbon debt* with a payback period. Whether that matters depends on whether the target is cumulative emissions or a near-term peak.

**The counterfactual.** The right question is never "was this carbon biogenic?" but "what would have happened otherwise?" Biomass diverted from a use where it would have been sequestered, or grown on land that was storing carbon as forest or peat, carries that loss. Land-use change, including indirect displacement of production elsewhere, is where most of the contested emissions in bioenergy accounting sit.

**Permanence.** Carbon held in a product is only removed for as long as the product lasts. A structural timber holds it for decades; a fuel holds it for minutes. Soil carbon is reversible under a change of management. Durability is a property of the destination, not of the origin.

**The rest of the balance sheet.** Cultivation, harvest, transport and conversion have their own emissions, often fossil, and nitrogen fertiliser releases nitrous oxide — a far more potent greenhouse gas than CO₂. A bio-based product can carry a larger footprint than the fossil one it displaces.

## How it is handled in practice

Life-cycle assessment conventions typically account biogenic uptake and release separately from fossil emissions, so that biogenic carbon nets out over the assessed period unless something prevents it — land-use change, delayed regrowth, or storage in a durable product, which is credited only where the storage is real and its duration stated. Different standards make different choices here, and the choice is not cosmetic: it can decide whether a product appears beneficial.

The practical conclusion: "biogenic" describes where a carbon atom came from. It does not by itself say what its release does to the climate, and only a full accounting — timing, counterfactual, permanence and the fossil inputs alongside — answers that.