Carbon Sinks and Sources

An ecosystem is a net carbon sink when it takes up more carbon than it releases over a specified period. It is a net carbon source when releases exceed uptake. The classification depends on the balance of carbon flows across a defined boundary, not simply on how much vegetation or stored carbon the ecosystem contains.

The carbon balance that determines sink or source status

In terrestrial ecosystems, photosynthesis removes carbon dioxide from the atmosphere, while respiration, decomposition and combustion return carbon to it. Sink status depends on the balance after all relevant gains and losses are counted.

• Gross primary productivity (GPP) is the total rate at which producers form organic matter through photosynthesis.
• Net primary productivity (NPP) equals GPP minus the producers' respiratory losses. It represents production available for plant growth and consumption by other organisms.
• Positive NPP alone does not establish a net ecosystem sink because animals and decomposers also respire.
• Net ecosystem production subtracts respiration by consumers and decomposers from NPP. A fuller carbon budget must also account for losses such as fire, harvesting and carbon transported across the ecosystem boundary.

What strengthens a sink or turns it into a source

An ecosystem accumulates carbon when photosynthetic inputs exceed respiratory and other losses. Both biological activity and environmental conditions influence this balance.

• Vegetation growth can increase carbon uptake, but water availability, nutrients, temperature and light constrain photosynthesis.
• Carbon retained in wood, roots and soil organic matter contributes to storage; slow decomposition can allow substantial accumulation, as in waterlogged peatlands.
• Drought can reduce photosynthesis, while warming can increase respiration and decomposition where moisture and other conditions permit.
• Deforestation, fire and drainage can release stored carbon and reduce future uptake, potentially changing an ecosystem from a sink into a source.
• Regrowth and ecosystem restoration can rebuild carbon stocks, but subsequent disturbances can reverse these gains.

Why carbon stocks and the assessment period matter

A carbon stock is the quantity stored at a particular time; a net sink describes a flow over time. A carbon-rich forest can therefore become a source if its losses exceed its uptake.

• Sink or source status must specify the time period and spatial boundary because seasonal variation, disturbances and recovery can change the result.
• Longer-term assessment is necessary to distinguish sustained accumulation from temporary uptake; preserving an existing stock also prevents emissions even when additional uptake is small.