Biochars contain very stable forms of carbon (fixed carbon) (Blackwell et al., 2010). The proportion of the original biomass carbon as total solid carbon in biochar ranges from around 5% from gasification technologies to about 35% in slow pyrolysis technologies. Well — managed large-scale biomass power plants can produce biochar of a consistent quality, whereas often small — scale "low-tech" technologies tend to produce very variable quality biochars, and are highly dependent on the homogeneity of the heating regime and the duration of heating. (See Tables 26.3 and 26.4.) A low-cost high — volume supply of sustainable biochar with a high carbon fraction will be needed to generate meaningful climate change mitigation benefits. However, currently

low carbon prices will not provide sufficient commercial incentives to simply apply biochar in soils for mitigation alone. The unacceptably high uncertainties of the direct and indirect influences and residence times of biochars and other organic carbon species in soils, their suitability for carbon markets (Intergovernmental Panel on Climate Change, 2000), and even the commercial incentives of high-volume production are fundamental barriers to widespread biochar use. Therefore, there is a growing need for researchers to quantify the net effect of specific biochars and application methods within niche agroeco­logical systems (particularly grains and livestock) and to verify any stable sequestration of carbon fractions (McHenry, 2011). Furthermore, in terms of farm applica­tion risks, some biochars can contain toxic materials that are controlled by "permissible exposure limit" stan­dards. The levels of these toxic materials in the biochar is dependent on both the biomass feedstock and the bio­char manufacture process, thus no simple permissible exposure limit is available for biochar to date (Blackwell et al., 2009). Thus, the development of a secure and responsible biochar industry will require awareness of safe methods of handling agricultural inputs and will need to be justified economically, and be in­tegrated with existing agricultural production systems (McHenry, 2011).