I am trying to figure out the difference between a carbon sink and a carbon reservoir.
The definition for a carbon sink from Wikipedia is
carbon sink is a natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period.
And the definition for carbon reservoir is
Carbon-storing natural feature (such as a forest or the land mass) that exchanges carbon with other reservoirs.
My textbook says that mature forests are carbon reservoirs but they are not carbon sinks. So what is the difference between a carbon sink and a carbon reservoir on earth?