I would add a couple of other points.
As CO2 concentration increases, plants don’t have to work as hard for CO2. So there should be a natural tendency for an increase in the biomass of all plants, which then feedbacks to lowering the CO2 in the atmosphere.
I remember coming to this conclusion when Al Gore was on his world tour. Since then, I have encountered a few papers from scientists making similar claim (Patrick Moore?).
As well, I think we need to consider the length of carbon storage. A wheat field probably cycles its stored carbon in about 2 years. A tree, especially if it is turned into lumber, may keep its carbon stored for several centuries.