The current best in class methodology for biochar aims for 100+ years. There are some folks saying there are pathways to make biochar last 1000+ years.
The nice thing is biochar is relatively inert. It just sits there in soil, holding onto water, making space for organisms to grow, but isn't "food", so it doesn't get eaten up and turned back into carbon dioxide. So it's a win for farmers and carbon removal.
Puro methodology: https://biochar.groups.io/g/main/attachment/32853/2/Puro.ear...
The nice thing is biochar is relatively inert. It just sits there in soil, holding onto water, making space for organisms to grow, but isn't "food", so it doesn't get eaten up and turned back into carbon dioxide. So it's a win for farmers and carbon removal.