If, for example, a plant takes up let’s say one million CO2 molecules in its life - and then you eat this plant, you will emit these (same) one million CO2 molecules by breathing.
If nobody would eat the plant (e.g. a corncob) and the plant would rot on the compost pile: This plant would ALSO emit these (same) one million CO2 molecules back to the nature by rotting.
So a plant can only store CO2 over a certain time period. But the CO2 is still there - within the plant; It does not disappear. At the latest when the plant decomposes it starts emitting CO2.
Below the line plants do not reduce CO2, if the amount of growing plant-matter and the amount of harvested (or rotting) plant-matter are about the same.
How can you find out, if the world wide vegetation is emitting or taking up CO2 below the line? (I ask you)