Why doesn't the obvious thing, i.e., making charcoal, work? You can call it "biochar" if you want. A big pile runs the risk of catching fire, but if it's mixed with soil I'd think it won't burn. Is there some slow oxidation process to worry about? I'd think that charcoal briquettes, pencil leads, and soot would all last essentially forever.<p>Plus, you can harness the pretty-high-grade heat energy extracted during the charcoal-making, to run heat engines or for other uses. So it's basically a way to use biology to get some solar power, and to sequester carbon at the same time.<p>If you're talking about only the charcoal-making, then this is prehistoric technology, and if you throw heat engines into the mix then you're at maybe an 1880s tech level. Seems easy?<p>I guess the "giant pile of frozen vegetables" method is even simpler in some ways (pipes being the only tech), but it also seems less stable, and it doesn't return the non-carbon nutrients to the soil.<p>What am I missing?