It's a far better idea to dump the carbon in after it has been used for it's energy and is bound up in a more stable configuration, ie biochar (a.k.a. terra preta or low temp charcoal). Biochar is stable in the soil for thousands of years, and provides a matrix on which soil organisms can thrive. There are also some indications that the biochar induces greater plant growth because the residual bio-oils and aromatic hydrocarbons cause the plants to think that there was a recent fire and this is there opportunity to take over.

But more importantly, and to your point, it is a carbon sink, and if made through gasification produces a carbon negative energy source.