Biochar additions.. could store 29 billion tons of CO2-equivalent..
"Advocates calculate that if biochar additions were applied at this rate on just 10 percent of the world's cropland (about 150 million hectares), this method could store 29 billion tons of CO2-equivalent, offsetting nearly all the emissions from fossil fuel burning."
Sara J. Scherr and Sajal Sthapit. "Farming and Land Use to Cool the Planet," State of the World 2009. 2009, 36