> I'm also surprised that the vast majority of the output carbon is in the form of CO2 rather than feces.

Not sure if you know, but in the same vein, I was shocked when I learned ~50% of the biomass of trees is carbon (right, we knew that), but also that the carbon came from CO2 the tree respirated. Also makes total sense in hindsight, but still think it's so cool.

The trees literally build their mass from the air and sunlight (and yes, obviously also water and trace minerals from the roots).