Plant nutrients are minerals, primarily nitrogen, potassium and phosphorus, in the base form of inorganic compounds or salts. When a plant dies it decomposes a

Plant nutrients are minerals, primarily nitrogen, potassium and phosphorus, in the base form of inorganic compounds or salts. When a plant dies it decomposes and the minerals are released and "recycled". Certainly that's how fossil carbon got to be stored in the first place (by growth of plants) so obviously the availability of nutrients in soils was naturally adequate to support the deposition of carbon. Carbonates are among the most common minerals on earth, particularly as calcium carbonate in its many forms. Calcium carbonate is slightly soluble in water and subject to degradation in acidic environments. While hardly an impervious carbon dioxide store, that's a benefit in that its easy degradation makes it an extremely useful mineral supporting essential infrastructure industries. IOW sequestering carbon as carbonates doesn't imply the CO2 is inaccessible but simply not reduced to an elemental or hydrocarbon form readily usable as an energy source. It's a trade-off between storage in an easier/quicker non-energy form or a protracted/difficult-to-implement energy-source form. Worth considering that in geological time the carbonate vs. carbon storage pools will probably "even out" since both are clearly mutable.