IMO-injecting CO2 isn't needed in the soil based systems. Your system will create some CO2 on its own naturally as organics decomp.
Usually, you don't inject CO2 in the natural systems, you can, but its not needed. The Co2 can help the plants grow faster if you have enough light-otherwise you might end up with an algae farm.
The soil based natural planted systems are intended to be low tech-with low-mod lights and limited water changes. Once the NPT's has matured and balanced-they can provide most everything they need on their own.