I have grave doubts about some of the currently proposed geosequestering techniques. However there are other ways.
One candidate that may work:
Grow plankton somewhere sunny (in shallow brackish water, sewage, or whatever). Let it die (let the water evaporate). Move the dead algae to land you want to improve. Char and bury.

Plankton (algae) grows at astounding rates. In geological history, it was often responsible for variations in CO2 levels. It could be again.