It really depends on the local conditions. A log on the surface, in an area with things that will eat it (e.g. fungi, bacteria, termites, and other decomposers)

It really depends on the local conditions. A log on the surface, in an area with things that will eat it (e.g. fungi, bacteria, termites, and other decomposers) will fairly readily give up its carbon. If it gets covered, and spends time in a cold, anaerobic environment, there will be nothing to return it to the atmosphere. Decomposition can eventually stall out entirely, allowing it (given enough time and compression) to form coal or oil. I'm very interested in wood gas pyrolosis, a process that pulls burnable fuel from wood and produces almost pure carbon as a byproduct. The gas could be used to generate electricity or heat, and the carbon could be buried in old mines. I suspect the economics just aren't there for it yet, (or the process doesn't scale to the numbers we'd need for large scale de-carbonization).