This is due the fact that, plants have the ability to use atmospheric carbon do oxide and convert it into organic compounds by the help of process called "photosynthesis". Now these organic forms of carbon I.e sugars are stored by plants which are then consumed by animals. As animals die, their bodies are decomposed by the scavengers and the organic carbon returns to the environment in inorganic form.

Inorganic carbonates can not be used by plants and thus can't be processed. Decomposers can not feed on the combined form of CO₂ present in the atmosphere