I did a little searching on ' "carbon-nitrogen ratio" AND charcoal',
without finding much. I get the impression from a report
of an Australian study that there is a problem with carbon
loss as well as nitrogen loss in some heavily farmed
soils, but crop rotation and leaving stubble in place
on top of the soil seems to mitigate carbon loss.
So far I don't see how adding charcoal would
help soil, any more than adding pea gravel would
help. (Wood ash is a whole different story, due
the potassium and pH effects.)
This one excellent document on composting did
say that charcoal is very hard for the microorganisms
that operate on organic compostable materials
to break down, practically inert in a compost
pile:
http://www.clemson.edu/psapublishing/mg/chapter2.pdf