There is an upper limit to the amount of CO2 that soil absorbs - it's clearly not infinite. There is an equilibrium in the Earth's natural Carbon cycle.
Once a soil matures (a process that might take many decades), the amount of CO2 retained vs emitted reaches equilibrium.
Some thoughts on finite CO2-absorbing capacity:
- If these forests are sustainably logged, we can take some of the carbon out and sequester it in buildings (or even just bury it), allowing the forest to regrow and absorb more carbon.
- I have a theory that presently discussed global climate change solution only needs to buy us a maximum of 100 years, and by then our tech for dealing with silly problems like excess carbon will have almost surely advanced beyond recognition. Given rates of progress in chemistry, physics, and materials science over the last two centuries, this isn't crazy.
Once a soil matures (a process that might take many decades), the amount of CO2 retained vs emitted reaches equilibrium.