Carbon Sequestration

The potential of biochar to sequester atmospheric C for centuries is certainly one of its most attractive qualities. As global anthropogenic C emissions continue to increase, C sequestration using biochar employs photosynthesis to draw C from the atmosphere, and pyrolysis to convert that photosynthetically sequestered C into forms that are mostly not biologically degradable. Fossil fuel-derived energy and most biofuels are regarded as carbon-positive, due to the positive net emis­sions of C from their production and use. Carbon-neutral biofuels are those that result in no net emissions of C resulting from their production and use. Pyrolysis energy production combined with biochar incorporation into soil has been described as what may be the only carbon-negative energy production system known (Mathews, 2008). Carbon-negative energy (Figure 25.2) sequesters more C from the atmosphere than is released through its production and use. If biochar is pro­duced from waste feedstocks that would otherwise be microbially degraded and contribute to anthropogenic C emissions, then it is possible to sequester a significant portion of anthropogenic C emissions through pyrolysis (Lehmann et al., 2006). While estimates of biochar C sequestration potential vary between studies depend­ing on methodological details, recent studies reported 1.65 GtC/y or 19% of anthropogenic carbon dioxide emis­sions could be offset (Lee and Day, 2013; Lee et al., 2013). When combined with projected adoptions of renewable energy systems by the year 2100, it has been estimated that through the pyrolysis of agricultural residues, silvi­cultural residues, organic waste from industry, and urban waste, 5.5—9.5 GtC/y is achievable (Lehmann et al., 2006). This would exceed current fossil fuel emis­sions and thereby represent a potential remediation, as opposed to a conservation tool. Others have recently projected the possibility of up to 15.6 GtC/y (Smith et al., 2013). While the C sequestration potential of biochar is currently appealing, it must be deployed carefully in order to minimize the risk of damaging the soil, due to the irreversibility of biochar application to soil (Sohi et al., 2008).