It could be spread over a vast area. Also, often enough the surface had been lowered by agricultural abuse and consequential erosion.
That's possible. Possibly buried shallow in the forests where the trees are grown. And we probably would use degraded land for the forests so filling "shallow canyons" that are a result of our bad land practices could be a fill site.
Fifty years ago I traveled through the area east of Paris and observed farmers growing their firewood on top of their fence posts. They planted trees on the fence line and once the trunks were large enough attached barbwire to them.
Then the cut off the growth above the post to fuel their heating and cooking fires. They used species of trees which, when topped, put up new growth in the form of several twigs/limbs. The farmers thinned the shoots to three or four, let those grow large enough, and then cut that new growth for firewood.
They timed their cutting so that each year they cut a portion of their fence tops, working their way around their fields, and enjoying a sustainable supply of wood.
I can see a carbon sequestering forest operating in the same way. A mobile, solar powered, automated "charcoal factory" that cuts trees once they're achieved the end of their rapid growth phase then turns them into charcoal, and buries the charcoal. A factory that might work for five or six years before returning to the same location.
And a second automated device that followed along, about a year behind, thinning the shoots that come from the stump down to three or so.
Automated, it could simply sit on days when there was not enough sunshine to power operation.
I've done a manual version of this. I helped plant two eucalyptus wood lots. After five years we started cutting about 25% of each grove each year. Year nine we were cutting 'stump growth'.
We've got over used, "burned out" fields in the South where cotton has stripped the land of nutrients. We're shipping trains of NYC sewage to the South for disposal. Seems like there could be an opportunity to put a few pieces together and put at least some carbon back underground.