L. Reijnders (2009): Are forestation, bio-char and landfilled biomass adequate offsets for the climate effects of burning fossil fuels?, Energy Policy Volume 37, Issue 8, Pages 2839-2841.
Forestation and landfilling purpose-grown biomass are not adequate offsets for the CO2 emission from burning fossil fuels. Their permanence is insufficiently guaranteed and landfilling purpose-grown biomass may even be counterproductive.
As to permanence, bio-char may do better than forests or landfilled biomass, but there are major uncertainties about net greenhouse gas emissions linked to the bio-char life cycle, which necessitate suspension of judgement about the adequacy of bio-char addition to soils as an offset for CO2 emissions from burning fossil fuels. If you want to read the full paper and don't have a subscription to the journal contact the author directly. Earthscan have published a book on biochar entitled Biochar for Environmental Management. See here.
There is a UK Biochar Research Centre at the University of Edinburgh.