Biochar.

It lasts for at least a couple thousand years, and is considered a long-lasting soil amendment.

The presence of biochar creates habitats for those microbes and conserves nutrients, making the soil fertile, even in areas like the Amazon where rainfall normally washes away nutrient accumulations. It can be made with processes that sequesters carbon, both in the charring stage (via gassifier designs optimized towards sequestering) and during the inoculation stage where it can capture greenhouse gases emitted by a compost pile.