What are you gonna do about all the nitrogen etc which the plants need? Are there good ways to reextract these nutrients from dead plant material without releasing loads of carbon at the same time?
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.
You calculate the cost of manufacturing hydrogen from water to feed into a the Haber-Bosch process to produce ammonia. All you are doing is replacing the existing steam reformer with an electrolysis plant.
But the what if is what if you can take a further step and directly create amino acids instead of ammonia. You go why do that. The answer is an acre of solar panels produces 25-50 times more energy than corn.
This. It's widely underappreciated how much more efficient solar panels are than plants at harvesting sunlight.
Forget amino acids, those are hard; if we could even just create sugar directly from electrical energy we could save a shit ton of corn being grown and turned into HFCS.
Off the top of my head, for a given amount of wood biomass, you can get about a 70% ratio of product to fuel if you use a high-efficiency wood fire to cook the wood itself.
Then you can take that carbon, bury it in decommissioned open pit mines, or use it as a soil additive (biochar), where it will sequester the carbon for thousands of years and act as a fertilizer.
You could also pair the biochar with a fast-growing swamp tree (willow?), re-incorporating the char into the areas around the willow plantation to create a sort of artificial peat bog which could also be useful for water storage and filtration.