

I don’t think it’s that the plants are controlling the ratio. I think it’s that more C14 is being made all the time. And it only gets mixed into plants when they are living. Specifically it looks like C14 based CO2 is made in the atmosphere and then consumed by plants.






I don’t think the burial matters. It’s not random atoms that are turned into C14. It’s specifically nitrogen. And I think those interactions mainly happen in the mid atmosphere with cosmic rays. So it’s the atmosphere providing the shielding.
The Wikipedia page for this explains it pretty well, especially the Physical and Chemical Details section.