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Articles commonly state examples of carbon sinks are plants, soils, and oceans (e.g., link). My understanding is that carbon sinks must directly take up carbon from the atmosphere. Therefore, other organisms such as herbivores and carnivores are not considered carbon sinks (I understand their roles as carbon sources) although they store much carbon. Is this interpretation true?

If so, what is the main mechanism that soils (independent of plants) directly absorb carbon from the atmosphere? It looks as if most articles simply say that soils contain much carbon from plant organic matters, which is a carbon store rather than a carbon sink.

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According to Wikipedia:

A carbon sink is a natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period.

The key point here is the "indefinite period" part. For example, although my body stores some kg of carbon, I can't qualify as a carbon sink because most of it is likely to be released again in some decades.

On the other hand, soils store large amounts of carbon, and as the soils keep developing that amount tends to grow, making them an effective carbon sink. The mechanism consists mostly on accumulating organic matter, as you say, but as long the amount of that organic matter keeps growing, soil works as a carbon sink.

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