[dice]

I still think Terra Preta the product is mainly hype.

The conclusions of the research report were that the charcoal
from forest fires does provide a substrate for bacteria, but there
is also nitrogen drawdown from the carbon in it, with this effect
being the least pronounced in coniferous forest soils (which does
not describe the soils of the Amazon Basin). The Wikipedia article
on Terra Preta soils mentioned that there was practically zero
unbound nitrogen in it, because the carbon content was so high.

Plants need nitrogen to grow, so where does it come from in a soil
with that much carbon in it? My guess would be that it washes down
out of decaying organic matter in the layer of dead leaves and other
vegetation (and dead bugs, birds, lizards, animals, etc, plus all of their
droppings) on top of the soil. Without that thick layer of rapidly
decaying humus on top, the carbon-rich Terra Preta soils would be
poor soils to try to grow crops in. They would be severely nitrogen
deficient.

(I think the potshards are just archaeological artifacts, basically,
something left behind where they lived by every human civilization
since pots were invented. Finding them where people lived and
farmed for thousands of years does not seem to me especially
related to some function that the shards may have had in the soil.)