I have recently been able to discount the use of pottery as an important active factor in the Amazon. It simply does not show up in terra mulato. That made the simple earthen kiln as the best possible explanation. Here in the Cameroon we have a field length earthen kiln produced and then lit. It is a good bet that the earth collapses behind the burn front helping smother the produced char.
Corn is obviously much more bulky but the same method could well apply. I still think that building a vertical stack with the root balls forming the outer shell is likely to be much more effective for corn.
The important point though is that biochar is a living indigenous practice in this part of West Africa.
The Batibe technique was described to us as to work as follows: before the planting season, farmers collect big piles of elephant grass or any other type of savannah grass, which they spread out over their fields to dry it. After the grass has dried, they pile it so as to make long strips, on which they will grow their crops. Then they cover the big rows of grass with a layer of mud, which they leave to dry again. After the mud has dried and hardened, they open one part of the strip and set fire to the grass contained in this "container". The fire travels slowly through this "kiln", providing a low oxygen environment, and chars all the biomass. After this operation, they crush the mud layer, and the char beneath it. They repeat the effort several times to create layers of char and crushed mud. This then becomes their soil bed, on which they start planting crops when they rains arrive. The rains turn this soil layer into an apparently fertile soil. To our own amazement, the farmers of our workshop in Kendem immediately understood the biochar concept, because of their knowledge of this Batibe technique.