I believe it takes a very long time for activated carbon (charcoal) to break down in the soil. There are many examples of archeologists finding charcoal in sites dating back tens of thousands of years, so aside from its CEC, it must remain very stable. Of course, if it is ground up into dust, it would likely break down faster, just from an erosion standpoint. If you put it in your garden soil, I doubt it would harm anything, and may do some good, considering its CEC. In creating terrariums, it is often placed as the bottom strata before adding other soil components, which is said to prevent the soil from "going sour", and that would indicate it certainly does no harm. I have a surplus of carbon that I've accumulated over the years from buying new filters or some other package deals, because, personally, I don't use carbon in my filters unless I'm trying to remove something specific from the water, such as tannins, medications or possibly after household pest control chemicals have been sprayed, in case some may have become airborne and I have some species that might be sensitive to it (such as crustaceans). When I received a couple of used canister filters in a deal, they had bags of carbon in them; I used them as substrate filler in my aquascape.Whenever I clean my filter, I do it next to some plants to give them fertilizer. Does used activated carbon have any value to garden/regular plants such as breaking down in the soil or anything like that?