Apologies for weirdness.
Of the various legal methods of disposing of a human corpse, which one ensures the nutrients which compose the body get back into the wider ecosystem the fastest?
Unless I've overlooked something (which is likely) the answer would seem to boil down to:
Burial, in which case soil microbes have relatively free access to the body but will initially restrict their nutrients to the sub-soil ecosystem.
Cremation, in which case most of the nutrients are converted into heat and light energy by the furnace, although this energy, and the remaining nutrients, are then made available to the wider environment much more rapidly.