In the Walstad method the decomposing soil (humus) produces bioavailable carbon similar to the carbon in Flourish Excel in addition to CO2 gas. Many microbes do produce CO2 gas while breaking bonds associated with decomposition but I would consider that as an extra. I wouldn't worry about your airstone as you're likely diffusing more atmospheric CO2 into the tank than letting CO2 gas escape and water movement is important with planted tanks. Keep in mind the soil you picked isn't going to break down all that much anyways - peat is pretty much inert underwater and for the most part will not break down at all.
Last edited by ReluctantHippy; 02-21-2013 at 03:58 PM.
Reason: Added "underwater"