Hugelkulture doesn't involve burying the plants very deeply though - I suspect that it's shallow enough that CO2 that's released during decomposition can easily

Hugelkulture doesn't involve burying the plants very deeply though - I suspect that it's shallow enough that CO2 that's released during decomposition can easily make it back to the surface. Most the CO2 from fossil fuels comes from ancient bogs, where plants were prevented from decomposing for long enough that they could eventually turn into materials like coal and petroleum. In essence, they were a result of taking plant matter _out_ of the biosphere, which is sort of the opposite of hugelkultur's goal.