“Our preliminary calculations take all of these carbon costs into account and suggest that the energy balance for various feedstocks, such as corn or switchgrass, is very favorable, with approximately 3–9 kg C energy yield for every kg C energy invested, even with the proposed use of biochar as a carbon sink instead of an energy source (Gaunt and Lehmann unpublished data). Comparable ratios for ethanol currently amount to 0.7–2.2 kg C (kg C)-1 (Pimentel and Patzek 2005; Metzger 2006) and, for biomass burning, to 10–13 kg C (kg C)-1 (willow; Keoleian and Volk 2005), with the caveat that the latter produces only heat, not liquid fuel. This means that pyrolysis produces 3–9 times more energy than is invested in generating the energy. At the same time, about half of the carbon can be sequestered in soil. Such a carbon-negative technology would lead to a net withdrawal of CO2 from the atmosphere, while producing and consuming energy.”

“A substantial reduction in the phosphorus mobility of animal manures may be achievable by directly pyrolyzing the manures (He et al. 2000). This would not only reduce the volume and weight of the manures that need to be disposed of, but could presumably also convert the soluble inorganic phosphate contained in manure into adsorbed phosphate in biochar. The properties and behavior of charred manure in soil need more attention, especially with respect to phosphorus dynamics.”

“Pyrolysis appears to offer additional opportunities to decrease greenhouse gas emissions, namely through the ability of biochar to scrub CO2, nitrous oxides, and sulfur dioxide from flue gas (Day et al. 2005; Figure 1). The CO2 is precipitated onto the biochar surfaces during an exothermic process (Lee et al. 2003). Such a procedure could be used to reduce net emissions by fossil fuels, for example in conjunction with coal firing. At the same time, the precipitate creates a highly nitrogen-rich biochar that could be used instead of nitrogen fertilizer additions (Day et al. 2005). Such benefits would need to be more fully investigated.”

“Compared to the limited amount of CO2 that can be removed from the atmosphere by other land-based sequestration strategies, such as notillage or afforestation (Jackson and Schlesinger 2004), a biochar sink has the advantage of easy accountability and multiple other environmental benefits.”

"However, as discussed previously, biochar applications do not always benefit mycorrhizal fungi (see Table 1). In these situations, one could argue that biochar, via any of our proposed mechanisms, reduces formation of mycorrhiza, e.g. by decreasing nutrient availability or creating unfavourable nutrient ratios in soils (Wallstedt et al. 2002)."

Mycorrhizal responses to biochar in soil – concepts and mechanisms Daniel D. Warnock & Johannes Lehmann & Thomas W. Kuyper & Matthias C. Rillig
http://www.css.cornell.edu/faculty/lehmann/publ/PlantSoil%20300,%209-20,%202007,%20Warnock.pdf