[bjelkeman-again]

Many Dutch greenhouses have a CHP power plant, where the heat goes to the greenhouse, the CO2 to the plants and the power to the lamps or to the electricity network. They are swapping the heating to ground heat pumps and the electricity to renewables, so not sure where the CO2 comes from in the future other than of course a higher ambient level.

[genewitch]

take the part of the plants that isn't used for anything else and ferment it with yeast. Free CO2. I'm not sure how old this trick is, as i used it a decade and a half ago with 2-3 liter bottles with holes drilled in the caps and a bubbler to remove solids to provide CO2 for a heavily planted aquarium bootstrap.

My theory here is that running all the stems, stalks, and roots that you don't use from stuff like annual food crops through a wood chipper or something should macerate the cell walls enough to allow the yeast to feast, at least.