I suspect the Amazonians didn't set out to make bio-char on purpose. They found bio-char with broken clay pottery. This leads me to believe that the production of pottery was the main reason for the massive amounts of wood burnt in concentrated areas.
Charcoal is what you want to turn into bio-char. Ashes is what you don't want. Wood goes through stages while being burned. Ash is the final stage and has little surface area for any microbial habitat.
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