I've been wondering if anyone has burned the bottom layer of a hugel, smothered it to stop the burn aka beginning to create biochar and after it's quenched and allowed to rest completed the addition of logs, limbs etc. This seems to make sense to me, but I'm hoping someone's tried it.
I would add the biochar layer above the hugel base wood. While it seems like a sound idea, the ideal wood for a huge base is decomposing wood (already rotting) which would not make the best biochar.
While biochar does increase moisture retention, it would be better used as a micro organism inducer which means the best use would be closer to the top rather than deep in the mound.
Biochar is most useful when in the top 16 inches of soil.
I have one mound with biochar throughout as the result of the previous owners home burning down and the resultant scraping and piling of the remains after the fire was put out.
This mound has better growth in the upper layers but the bottom of the "junk mound" stays relatively dryer, and there is nothing growing well around the base of this mound.
I need to mention that at this time there are still other items from the old home, bricks, concrete blocks, some steel and lots of charred wood (the biochar) piled in as only a bulldozer can do.
We have plans to get around to a dismantle of this mound to remove the "Junk" and leave all the tree materials but this will be a long time in the getting around to it stage.
The mound right now grows a lot of native grasses, fungi, ferns and moss. Some rabbits have made it their home so we are in no hurry to disturb the critters occupying this mound.
We have added some top layers of branches, leaves, grass clippings in order to be able to grow some bunny friendly plants there to keep them from looking to our gardens for supper.
Eventually this mound will be used for growing pumpkins, melons and perhaps some squashes.
Our other mounds were constructed with lots of rotting hardwoods and some fresh sumac trunks, leaves were packed in between layers of wood then the cover of humus enhanced soil mixed with mushroomcompost and biochar.
These mounds grow awesome passion fruit, squashes, muscadines, and potatoes. They need very little water, even in the dry season (august) when the temps are at their highest here.
I've used a lot of partially burned wood from some slash piles left behind by the previous land owner's logging of our property. While this isn't exactly biochar, I think it is awfully close. In my experience the beds with the charred wood seem to not be settling as much as the other beds, though performance is about the same.