'Up to one-tenth of the missing carbon may have leached into ground water, but Kirk says the majority is likely to have been lost as carbon dioxide into the atmosphere. This is likely to be due to plant matter and organic material decomposing at a faster rate as temperatures rise.
More worryingly, soil sinks are predicted to release their carbon at an even faster rate as temperatures increase, giving rise to a feedback loop.'
So, the earth is releasing CO2 on its own, but it is because of a feedback loop likely caused by global warming.
Also from TFA:
'Models had predicted that the rate of carbon released from terrestrial sinks would eventually outpace the rate of absorption of carbon dioxide, says Kirk, but it was thought this would not happen for another 10 to 50 years. "We've shown that it's happening rather faster than that." '
In other words, the feedback loop will become self susaining in 10 to 50 years. If humans can stop global warming before that time, than the carbon dioxide will be reabsorped, and the feedback loop will not occur. We still have 10 to 50 years of hope. After that, we're screwed.