6.2 - Biological processes and the fast carbon cycle

Land uptake and respiration

Terrestrial plants draw carbon from the atmosphere to build organic matter, but living organisms also return carbon through respiration.

Primary producers, such as green plants, use sunlight to combine carbon dioxide and water into carbohydrates during photosynthesis. Carbon becomes part of leaves, roots and woody tissue. Photosynthesis transfers matter and converts solar energy into chemical energy; it does not create carbon.

Plants themselves respire, using organic molecules to release energy for cellular work and returning CO₂ to the atmosphere. Consumers obtain carbon by eating plants or other animals, and release some during respiration. Growth retains another fraction in biomass; waste and death transfer some to soil organic matter. The pathway is therefore atmosphere → producer → consumer → atmosphere or soil, with several branches rather than one conveyor belt.

Gross primary productivity (GPP) is total carbon fixed by producers. Net primary productivity (NPP) subtracts producers' own respiration and represents biomass production potentially available to consumers or litter. A whole ecosystem's carbon balance also depends on respiration by animals and decomposers, fires and transfers out of its boundary. High NPP alone does not prove a permanent net carbon sink.

For an original woodland example, GPP is 12 tonnes C per hectare per year and plant respiration is 5. NPP is $12-5=7$ tonnes C per hectare per year. If animals and decomposers return another 6, the ecosystem retains 1 before accounting for fire, harvest or lateral transfers. This illustrates why “the plants absorb 12” is not a net-storage claim.

Dead organic matter and soil storage

Dead plant and animal material can either remain in a soil store or be broken down and returned to the atmosphere.

Leaves, roots, animal remains and waste add organic matter to soil. Bacteria and fungi decompose it, obtaining energy and nutrients. Under oxygenated conditions, their respiration releases CO₂. Some compounds decompose rapidly; some are protected within soil aggregates or associated with minerals and can remain much longer. “Several years” is a useful biological timescale for much litter turnover, but it is not the lifetime of all soil carbon.

Decomposition depends on temperature, water, oxygen and the material itself. Warm, moist, aerated conditions usually allow faster microbial activity. Very dry soil limits organisms' activity, while waterlogging restricts oxygen and can slow aerobic decay. In cold or waterlogged environments, plant inputs may exceed breakdown for long periods, allowing peat to accumulate. Oxygen-poor decomposition can also produce methane, so a slowly decaying store is not automatically free of greenhouse-gas release.

Soil storage grows only when carbon inputs exceed outputs. A productive ecosystem can have a small accumulating soil store if decomposition is equally rapid. Disturbance that exposes soil organic matter to oxygen, or removes vegetation inputs, can reverse the balance. The store's size is the accumulated history of these flows, rather than a direct indicator of one year's photosynthesis.

Carbon atoms can re-enter plants after decomposition releases CO₂, but the energy originally captured from sunlight is progressively dissipated as heat. Carbon cycles; energy flows through the ecosystem. Confusing these two processes hides why fresh solar input is needed for continuing biological production.

Photosynthesis and sinking ocean carbon

Marine primary producers fix dissolved carbon in sunlit surface waters, and some of that carbon is transferred below the surface layer.