How you do it in the process is correct, IPCC typically does not consider CO2 uptake (nor release of biogenic CO2), and that's why you will not see an effect. If you want the uptake to have an effect, you can add CO2 uptake to your impact category (so, this flow; the factor would be -1 as the CO2 is on the input side. Ideally, make a copy of the category before changing anything). If you do, be certain to also include release of biogenic CO2 in the category. You can also use a 0/1 parameter here so that you can switch the consideration of biogenic C on and off, per calculation. You should mention this modeling in goal and scope of your LCA.
