The soil-to-air cycle of carbon dioxide or soil respiration is a major source of CO2 emission. The soil is likely to respire even faster in future as temperatures rise due to global warming. Plants photosynthesize during the day and give you oxygen from CO2. At night a different metabolic cycle takes place which results in the release of CO2. This along with microbial respiration and chemical processes that lead to oxidation of soil minerals are the ways in which the soil respires. The total emission due to soil respiration is about 98 billion in a year. This number has risen about 15% in the last 20 years.
If soil respiration is increasing, thanks to increasing temperatures, it is very important to put the changes in context.
You’ll do well to remember that temperature-induced change in atmospheric CO2 is much smaller than changes brought about by humans burning fossil fuel.