Which is the largest emitter of Carbon dioxide in the atmosphere?

Prior to humans, it was volcanoes. But humans currently emit over 100 times as much as volcanoes do on an annual average basis, and this is partially because volcanic activity has been low in the recent past.

It also matters how we view the issue. If we view man's contributions in total verses natures contributions in total, nature produces 94% of the total CO2 produced. Some of man's 6% would belong to nature had we not burned the wood or ate the growth before it decayed.