I wonder the same. This proposal sounds like it is leeching nutrients from the ground and storing it for a long time (on a scale of centuries in the proposal). How do these nutrients cycle back for growing the food that we need? Or, for that matter, for the next round of biomass to freeze?
On a tiny scale I store them via humification in the top soil. In agriculture they manage the humus content of their soil anyway, for example in greenhouses they might have 20% instead of 2% in the surrounding fields.
Someone armed with enough VC money could possibly do that on a really large scale and even monetize it via carbon offset certs and then just throw the C rich output of their giant bioreactor into the bottomless pit.
