Overstory #256 - Biochar

Simply put, biochar is the carbon-rich product obtained when biomass, such as wood, manure or leaves, is heated in a closed container with little or no available air. In more technical terms, biochar is produced by so-called thermal decomposition of organic material under limited supply of oxygen (O₂), and at relatively low temperatures (<700°C). This process often mirrors the production of charcoal, which is one of the most ancient industrial technologies developed by mankind – if not the oldest (Harris, 1999). However, it distinguishes itself from charcoal and similar materials that are discussed below by the fact that biochar is produced with the intent to be applied to soil as a means of improving soil productivity, carbon (C) storage, or filtration of percolating soil water. The production process, together with the intended use, typically forms the basis for its classification and naming convention.

In contrast to the organic C-rich biochar, burning biomass in a fire creates ash, which mainly contains minerals such as calcium (Ca) or magnesium (Mg) and inorganic carbonates. Also, in most fires, a small portion of the vegetation is only partially burned in areas of limited O₂ supply, with a portion remaining as char (Kuhlbusch and Crutzen, 1995).

The basis for the strong recent interest in biochar is twofold. First, the discovery that biochar-type substances are the explanation for high amounts of organic C (Glaser et al, 2001) and sustained fertility in Amazonian Dark Earths locally known as Terra Preta de Indio (Lehmann et al, 2003a). Justifiably or not, biochar has, as a consequence, been frequently connected to soil management practised by ancient Amerindian populations before the arrival of Europeans, and to the development of complex civilizations in the Amazon region (Petersen et al, 2001). This proposed association has found widespread support through the appealing notion of indigenous wisdom rediscovered. Irrespective of such assumptions, fundamental scientific research of Terra Preta has also yielded important basic information on the functioning of soils, in general, and on the effects of biochar, in particular (Lehmann, 2009).

Second, over the past five years, unequivocal proof has become available showing that biochar is not only more stable than any other amendment to soil (see Chap. 11, Lehmann and Joseph 2009), and that it increases nutrient availability beyond a fertilizer effect (see Chap. 5, Lehmann and Joseph 2009; Lehmann, 2009), but that these basic properties of stability and capacity to hold nutrients are fundamentally more effective than those of other organic matter in soil. This means that biochar is not merely another type of compost or manure that improves soil properties, but is much more efficient at enhancing soil quality than any other organic soil amendment. And this ability is rooted in specific chemical and physical properties, such as the high charge density (Liang et al, 2006), that result in much greater nutrient retention (Lehmann et al, 2003b), and its particulate nature (Skjemstad et al, 1996; Lehmann et al, 2005) in combination with a specific chemical structure (Baldock and Smernik, 2002) that provides much greater resistance to microbial decay than other soil organic matter (Shindo, 1991; Cheng et al, 2008).These and similar investigations have helped to make a convincing case for biochar as a significant tool for environmental management.They have provided the breakthrough that has brought already existing – yet either specialized or regionally limited – biochar applications and isolated research efforts to a new level.This book is a testament to these expanding activities and their results to date.

The Big Picture

Four complementary and often synergistic objectives may motivate biochar applications for environmental management: soil improvement (for improved productivity as well as reduced pollution); waste management; climate change mitigation; and energy production, which individually or in combination must have either a social or a financial benefit or both. As a result, very different biochar systems emerge on different scales (see Chap. 9, Lehmann and Joseph 2009). These systems may require different production systems that do or do not produce energy in addition to biochar, and range from small household units to large bioenergy power plants (see Chap. 8, Lehmann and Joseph 2009). The following sections provide a brief introduction into the broad areas that motivate implementation of biochar.

Biochar as a soil amendment

Soil improvement is not a luxury but a necessity in many regions of the world. Lack of food security is especially common in sub- Saharan Africa and South Asia, with malnutrition in 32 and 22 per cent of the total population, respectively (FAO, 2006).While malnutrition decreased in many countries worldwide from 1990–1992 to 2001–2003, many nations in Asia, Africa or Latin America have seen increases (FAO, 2006). The ‘Green Revolution’ initiated by Nobel Laureate Norman Borlaug at the International Centre for Maize and Wheat Improvement (CIMMYT) in Mexico during the 1940s had great success in increasing agricultural productivity in Latin America and Asia.These successes were mainly based on better agricultural technology, such as improved crop varieties, irrigation, and input of fertilizers and pesticides. Sustainable soil management has only recently been demanded to create a ‘Doubly Green Revolution’ that includes conservation technologies (Tilman, 1998; Conway, 1999). Biochar provides great opportunities to turn the Green Revolution into sustainable agroecosystem practice. Good returns on ever more expensive inputs such as fertilizers rely on appropriate levels of soil organic matter, which can be secured by biochar soil management for the long term (Kimetu et al, 2008; Steiner et al, 2007).

Specifically in Africa, the Green Revolution has not had sufficient success (Evenson and Gollin, 2003), to a significant extent due to high costs of agrochemicals (Sanchez, 2002), among other reasons (Evenson and Gollin, 2003). Biochar provides a unique opportunity to improve soil fertility and nutrient-use efficiency using locally available and renewable materials in a sustainable way. Adoption of biochar management does not require new resources, but makes more efficient and more environmentally conscious use of existing resources. Farmers in resource-constrained agroecosystems are able to convert organic residues and biomass fuels into biochar without compromising energy yield while delivering rapid return on investment (see Chap. 9, Lehmann and Joseph 2009).

In both industrialized and developing countries, soil loss and degradation is occurring at unprecedented rates (Stocking, 2003; IAASTD, 2008), with profound consequences for soil ecosystem properties (Matson et al, 1997). In many regions, loss in soil productivity occurs despite intensive use of agrochemicals, concurrent with adverse environmental impact on soil and water resources (Foley et al, 2005; Robertson and Swinton, 2005). Biochar is able to play a major role in expanding options for sustainable soil management by improving upon existing best management practices, not only to improve soil productivity (see Chaps. 5 and 12, Lehmann and Joseph), but also to decrease environmental impact on soil and water resources (see Chaps. 15 and 16, Lehmann and Joseph 2009). Biochar should therefore not be seen as an alternative to existing soil management, but as a valuable addition that facilitates the development of sustainable land use: creating a truly green ‘Biochar Revolution’.

Biochar to manage wastes

Managing animal and crop wastes from agriculture poses a significant environmental burden that leads to pollution of ground and surface waters (Carpenter et al, 1998; Matteson and Jenkins, 2007). These wastes as well as other by-products are usable resources for pyrolysis bioenergy (Bridgwater et al, 1999; Bridgwater, 2003). Not only can energy be obtained in the process of charring, but the volume and especially weight of the waste material is significantly reduced (see Chap. 8, Lehmann and Joseph 2009), which is an important aspect, for example, in managing livestock wastes (Cantrell et al, 2007). Similar opportunities exist for green urban wastes or certain clean industrial wastes such as those from paper mills (see Chap. 9, Lehmann and Joseph 2009; Demirbas, 2002). At times, many of these waste or organic by-products offer economic opportunities, with a significant reliable source of feedstock generated at a single point location (Matteson and Jenkins, 2007). Costs and revenues associated with accepting wastes and by-products are, however, subject to market development and are difficult to predict. In addition, appropriate management of organic wastes can help in the mitigation of climate change indirectly by:

• • decreasing methane emissions from landfill; 
  • reducing industrial energy use and emissions due to recycling and waste reduction; 
  • recovering energy from waste; 
  • enhancing C sequestration in forests due to decreased demand for virgin paper; and 
  • decreasing energy used in long-distance transport of waste (Ackerman, 2000).

Strict quality controls have to be applied for biochar, particularly for those produced from waste, but also from other feedstocks. Pathogens that may pose challenges to direct soil application of animal manures (Bicudo and Goyal, 2003) or sewage sludge (Westrell et al, 2004) are removed by pyrolysis, which typically operates above 350°C and is thus a valuable alternative to direct soil application. Contents of heavy metals can be a concern in sewage sludge and some specific industrial wastes, and should be avoided. However, biochar applications are, in contrast to manure or compost applications, not primarily a fertilizer, which has to be applied annually. Due to the longevity of biochar in soil, accumulation of heavy metals by repeated and regular applications over long periods of time that can occur for other soil additions may not occur with biochar.

Biochar to produce energy

Capturing energy during biochar production and, conversely, using the biochar generated during pyrolysis bioenergy production as a soil amendment is mutually beneficial for securing the production base for generating the biomass (Lehmann, 2007a), as well as for reducing overall emissions (see Chap. 18, Lehmann and Joseph 2009; Gaunt and Lehmann, 2008). Adding biochar to soil instead of using it as a fuel does, indeed, reduce the energy efficiency of pyrolysis bioenergy production; however, the emission reductions associated with biochar additions to soil appear to be greater than the fossil fuel offset in its use as fuel (Gaunt and Lehmann, 2008). A biochar vision is therefore especially effective in offering environmental solutions, rather than solely producing energy.

This appears to be an appropriate approach for bioenergy as a whole. In fact, bioenergy, in general, and pyrolysis, in particular, may contribute significantly to securing a future supply of green energy. However, it will, most likely, not be able to solve the energy crises and satisfy rising global demand for energy on its own. For example, Kim and Dale (2004) estimated the global potential to produce ethanol from crop waste to offset 32 per cent of gasoline consumption at the time of the study. This potential will most likely never be achieved. An assessment of the global potential of bioenergy from forestry yielded a theoretical surplus supply of 71EJ in addition to other wood needs for 2050 (Smeets and Faaij, 2006), in comparison to a worldwide energy consumption of 489EJ in 2005 (EIA, 2007). If economical and ecological constraints were applied, the projection for available wood significantly decreases (Smeets and Faaij, 2006). However, even a fraction of the global potential will be an important contribution to an overall energy solution. On its own, however, it will probably not satisfy future global energy demand.

In regions that rely on biomass energy, as is the case for most of rural Africa as well as large areas in Asia and Latin America, pyrolysis bioenergy provides opportunities for more efficient energy production than wood burning (Demirbas, 2004b). It also widens the options for the types of biomass that can be used for generating energy, going beyond wood to include, for example, crop residues. A main benefit may be that pyrolysis offers clean heat, which is needed to develop cooking technology with lower indoor pollution by smoke (Bhattacharya and Abdul Salam, 2002) than is typically generated during the burning of biomass (Bailis et al, 2005) (see Chap. 20, Lehmann and Joseph 2009).

Biochar to mitigate climate change

Adding biochar to soils has been described as a means of sequestering atmospheric carbon dioxide (CO₂) (Lehmann et al, 2006). For this to represent true sequestration, two requirements have to be met. First, plants have to be grown at the same rate as they are being charred because the actual step from atmospheric CO2 to an organic C form is delivered by photosynthesis in plants. Yet, plant biomass that is formed on an annual basis typically decomposes rapidly. This decomposition releases the CO2 that was fixed by the plants back to the atmosphere. In contrast, transforming this biomass into biochar that decomposes much more slowly diverts C from the rapid biological cycle into a much slower biochar cycle (Lehmann, 2007b). Second, the biochar needs to be truly more stable than the biomass from which it was formed.This seems to be the case and is supported by scientific evidence (see Chap. 11, Lehmann and Joseph 2009).

Several approaches have been taken to provide first estimates of the large-scale potential of biochar sequestration to reduce atmospheric CO2 (Lehmann et al, 2006; Lehmann, 2007b; Laird, 2008), which will need to be vetted against economic (see Chaps. 19 and 20, Lehmann and Joseph 2009) and ecological constraints and extended to include a full emission balance (see Chap. 18, Lehmann and Joseph 2009). Such emission balances require a comparison to a baseline scenario, showing what emissions have been reduced by changing to a system that utilizes biochar sequestration. Until more detailed studies based on concrete locations reach the information density required to extrapolate to the global scale, a simple comparison between global C fluxes may need to suffice to demonstrate the potential of biochar sequestration. Almost four times more organic C is stored in the Earth’s soils than in atmospheric CO2. And every 14 years, the entire atmospheric CO2 has cycled once through the biosphere . Furthermore, the annual uptake of CO2 by plants is eight times greater than today’s anthropogenic CO2 emissions. This means that large amounts of CO2 are cycling between atmosphere and plants on an annual basis and most of the world’s organic C is already stored in soil. Diverting only a small proportion of this large amount of cycling C into a biochar cycle would make a large difference to atmospheric CO2 concentrations, but very little difference to the global soil C storage. Diverting merely 1 per cent of annual net plant uptake into biochar would mitigate almost 10 per cent of current anthropogenic C emissions (see Chap. 18, Lehmann and Joseph 2009). These are important arguments to feed into a policy discussion (see Chap. 22, Lehmann and Joseph 2009).

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