Yeah biochar is very interesting. We definitely shouldn't massively start using it in the short term though - its effects are not so well understood. Most "knowledge" is based on terra preta, Amazonian dark soil which is hundreds of years old and man-made with charcoal, pottery shards, bones and other organic matter. There have been field trials but many of them by biochar organizations/companies who might not be completely objective and once we put it in the soil we can't really take it out anymore.

I think the evidence suggests that turning waste into biochar and applying it to fields at a local scale has a lot of potential. Growing trees for the purpose of biochar might be a different story (this has many of the downsides of biofuel production).