A strategy based around biochar thus differs from the more established proposition for sequestration of carbon in soil, where the objective is to increase the equilibrium level of active soil organic matter, which broadly requires that rate at which carbon—in the form of organic resource or wastes—is forced through the soil system to be permanently increased to a higher rate. Despite the large size of the global soil carbon pool (1500 GtC) the potential for this strategy to accumulate carbon per unit area is limited in absolute quantitative terms. This is because intensively managed lands account for a small part of the global soil carbon pool, and incremental enhancements account for trivial amounts of carbon, and because the capacity for any soil to stabilize labile carbon has fundamental limits. In addition, storage is delivered over a lengthy period of time (usually decades), and this annual rate diminishes as equilibrium is approached. The increased flow of carbon into the soil must be maintained after equilibration to avoid reversal of soil storage, making the diversion of resource into the soil a permanent commitment. Increasing the quantity and turnover of carbon in soil, in the form of organic matter, seems certain to provide crop-related benefits (Janzen, 2006 and Lal et al., 2004). This is therefore a desirable strategy where sufficient organic resource exists, but should be weighed against a more efficient strategy where adopted explicitly for carbon sequestration, especially in fertile soils.
The cycling of black carbon produced during wildfire provides a natural analog for a biospheric intervention based on biochar. Wildfire is currently the largest source of black carbon globally, a small proportion of above-ground biomass (about 1%) being incompletely combusted and returned to the soil as char of various forms. The extent and frequency of wildfire in many systems means that this pathway may already provide a terrestrial net sink for about 0.05–0.2 Gt yr− 1 atmospheric CO2–C (Kuhlbush, 1998). Increasing recognition for the global significance of this flux arises in part, from development of measurements that discriminate black carbon from other soil carbon. These seem to indicate much larger amounts of black carbon in soil than has been assumed in global stock estimates, or than has been allowed for in soil models. This may affect, among other things, the response of the global soil pool to climate change, black carbon being much more stable than the typical components of soil carbon (Lehmann et al., 2008). Interpretation of black carbon measurements is, however, complicated by some uncertainty over their efficacy and also their capacity to discriminate charcoal from other forms of black carbon, specifically those arising from anthropogenic activity—deliberate vegetation burning, wood fuel, combustion of coal and oil.
The possible indirect effects of biomass stabilization on radiative forcing have to be considered. Soot from biomass burning is implicated in an acceleration of polar ice melt, but conversely in facilitating cloud formation and “global dimming” (McConnell et al., 2007 and Ramanathan and Carmichael, 2008). The production of biochar under controlled conditions should be clean, but the means to control methods of production are unclear. Biochar in soil also visibly darkens soil color, especially in soils that are already low in organic matter, and a relationship between soil color and occurrence of low temperature wildfire has been demonstrated (Ketterings and Bigham, 2000 and Oguntunde et al., 2008). As dark soils absorb more solar energy they may, depending on water content and plant cover, display higher soil temperatures (Krull et al., 2004). This would potentially accelerate cycling of nutrients and beneficially extend growing season in temperate regions, and in Japan it is a traditional farming practice to apply charcoal to accelerate snow melt. The study of Oguntunde et al. (2008) showed a one-third reduction in soil albedo in char-enriched soils from historic charcoal making sites. On a large spatial scale, the application of biochar could potentially reduce the albedo of the Earth's surface, whereas increasing surface albedo has been proposed as a possible mitigation measure for climate forcing (Crutzen, 2006).
1.4. Scenarios for the production and deployment of biochar
Producing charcoal using traditional kilns liberates greenhouse gases, particularly methane and nitrous oxide, and conserves relatively small propositions of carbon in the feedstock (FAO, 1985) and wastes the heat energy product. Apart from being associated with deforestation, sequestration of carbon into charcoal using unmodified, traditional methods may therefore not, depending on the source and ordinary fate of feedstock, provide climate change mitigation.
Controlled pyrolysis stabilizes some carbon in solid form but also captures energy-rich liquids and gases which can be used to drive the pyrolysis reactions or used elsewhere. Although energy is retained in solid char the amount of energy liberated from the pyrolyzed feedstock may be higher, per mass of feedstock carbon, than in combustion. Pyrolysis could therefore be more efficient in terms of carbon emissions (CO2 MJ− 1), and production of biochar carry greater abatement potential than biomass combustion, provided there is an overall adequate supply of feedstock, and storage for the biochar product is available.
Although it has previously been proposed that entire valleys might be dedicated to provide storage for carbon stabilized as biochar (Seifritz, 1993), applying biochar to agricultural soil is proposed for three reasons: (1) only the soil seems to have a capacity sufficient to accommodate biochar at the scale relevant to the long-term mitigation of climate change, (2) there is potential for biochar to enhance soil function for agricultural productivity and thus offset the opportunity cost associated with its residual energy value, and (3) the possible suppression of methane and nitrous oxide release would increase the value of biochar as a means to offset agricultural greenhouse gas emissions. The impact of biochar on existing and future levels of non-biochar soil carbon should also be considered in this context.
Ideally biochar will provide reliable agronomic benefits and command a value in crop production that precludes combustion for energy, with or without a value placed on sequestration of the carbon that it contains. In this evaluation of value, allowance has to be made for the cost of acquisition and incorporation of biochar into soil. A value can also be assigned by producers and upstream food processors to the marketing potential of low-carbon or “carbon-neutral” food products produced in systems that deploy biochar. It is expected that a growing understanding of the relationship between feedstock, the manipulation of the pyrolysis process, and the function of biochar in soil will ultimately enable biochar to be “engineered” to provide the balance of benefits most appropriate to a particular system. The value of the energy captured in pyrolysis must also exceed the price for the alternative use of the feedstock, unless it is genuinely a waste, in which case the normal cost of disposal can be added to the value of the energy. It should be recognized that the price of feedstock depends on demand, however, and from a market perspective wastes may cease to be wastes once demand as novel feedstock exceeds their rate of production within a relevant catchment area.