[dice]

I've read that wood ash (and probably charcoal, too) improves
drainage in heavy clay soils, probably by binding some
of the fine clay particles into larger structures. The earthworms
will tell you, however, that a little ash goes a long way in
a heavy clay soil (radically changes the pH). Less-soluble charcoal may have less drastic effects on pH while still
providing improved drainage.

I don't know enough about soil microbes to have an opinion
about how charcoal would be used by them, other than
to note the possible carbon-nitrogen issue.