The efficient thing to do is render them into charcoal, yes.
Biochar is a great soil amendment, and doesn't oxidize over decades or even centuries, depending. Putting it back in the mines is an option, if we ever need to stop rebuilding topsoil, which is itself getting urgent.
Hemp is compostable, though because it's so tough, shredding and waiting for it to compost trades (vertical) space & time for far less energy use than biocharification unless it's waste heat from a different process.
> Biomass sources used in BECCS include agricultural residues & waste, forestry residue & waste, industrial & municipal wastes, and energy crops specifically grown for use as fuel. Current BECCS projects capture CO2 from ethanol bio-refinery plants and municipal solid waste (MSW) recycling center.
> A variety of challenges must be faced to ensure that biomass-based carbon capture is feasible and carbon neutral. Biomass stocks require availability of water and fertilizer inputs, which themselves exist at a nexus of environmental challenges in terms of resource disruption, conflict, and fertilizer runoff.
If you keep taking hemp off a field without leaving some down, you'll probably need fertilizer (see: KNF, JADAM,) and/or soil amendments to be able to rotate something else through; though it's true that hemp grows without fertilizer.
> A second major challenge is logistical: bulky biomass products require transportation to geographical features that enable sequestration. [27]
Composting oxidizes a lot of the carbon. And since you have to replant, harvest and fertilise everyyear it is a lot more intensive than forestry where you do that ever 30 (15-100) years
Biochar is a great soil amendment, and doesn't oxidize over decades or even centuries, depending. Putting it back in the mines is an option, if we ever need to stop rebuilding topsoil, which is itself getting urgent.