ReviewAgricultural bio-char production, renewable energy generation and farm carbon sequestration in Western Australia: Certainty, uncertainty and risk
Introduction
Working Group III, in their contribution to the IPCC's Fourth Assessment Report (AR4) stated the high agreement and much evidence that soil restoration and land use change mitigation measures can be implemented immediately by using existing technologies. Working Group III also stated the high agreement and much evidence that soil carbon sequestration is the mechanism responsible for most climate change mitigation potential (Paustian et al., 1997). The IPCC's AR4 Synthesis Report confirmed that effective carbon-price signals can mobilise environmentally effective mitigation options in the agriculture and forestry sectors, including as improved land management practices that maintain soil carbon density and for soil carbon sequestration. However, to be able to successfully utilise soil carbon mitigation incentives, farmers will need to use iterative management processes that balance economic carbon sequestration benefits with conventional production co-benefits, and attitudes to risk (Intergovernmental Panel on Climate Change, 2000).
Decreasing the financial risks of farming in this period of relative climate policy uncertainty requires feasibility studies of synergies between conventional productivity and climate change mitigation and adaptation measures. Similarly, reducing farm investment risk in a changing climate will entail the greater use of monitoring to inform management practices that increase farm ecosystem stability and resilience to climate stress (Griffiths et al., 2000, Tobor-Kaplon et al., 2005, Harle et al., 2006, Brussaard et al., 2007). Therefore, sequestering carbon in agricultural soils is one such possible synergy that creates additional property rights for farmers, retains land values by soil conservation, and may improve conventional yields by modulating soil ecosystem variability (Klein et al., 2007, Milne et al., 2007).
There is considerable interest in finding reliable methods of sequestering carbon in agricultural soils to both reduce farm investment risk and cut atmospheric greenhouse gas concentrations, in a timeframe suitable to investors. Increasing the levels of soil organic carbon (SOC) by conventional agricultural management can take many years and involves significant uncertainty in regards to the resultant carbon fluxes (Denman et al., 2007). A report by the National Carbon Accounting System (NCAS) authored by Valzano et al. (2005), focussed on the impact of tillage on changes in SOC density in Australia. The report found that low tillage and stubble retention management practices only had an effect on SOC density up to depths of 30 cm in areas with mean annual temperatures between 12.8 and 17.4 °C and an average annual rainfall above 650 mm (Valzano et al., 2005). In Australian research plots that did show significant differences of SOC densities between using minimum disturbance methods and conventional tillage, the results have been modest. Farms using direct drill, retained stubble and moderate grazing production methods were found to have densities of around 57 t ha−1 up to 30 cm of depth, while nearby heavily grazed farms using multiple crop tillage (with either tyned or disc implements), had SOC densities of 43 t ha−1 up to 30 cm soil depths (Valzano et al., 2005). A study by Wright et al. (2007) on SOC and nitrogen levels over 20 years of various tillage regimes, found the no-tillage practices only increased SOC, dissolved organic carbon and total nitrogen by 28, 18 and 33% respectively, when compared to conventional tillage (Wright et al., 2007). While the benefit of using minimum tillage methods are clear for retaining natural SOC densities, sequestering sufficient volumes of SOC for carbon markets will likely require new approaches to purposefully add SOC to enhance existing carbon sinks.
The conversion of biomass to long-lived soil carbon species results in a long-term carbon sink, as the biomass removes atmospheric carbon dioxide through photosynthesis. Bio-char carbon species range in complexity from graphite-like carbon to high molecular weight aromatic rings, which are known to persist in soil for thousands to millions of years (Graetz and Skjemstad, 2003). Unlike fossil fuels, biomass is a renewable source of carbon and using it to produce bio-char can release energy with virtually no sulphur or mercury and very little nitrogen and ash waste (Antal and Gronli, 2003). Thus, producing bio-char from farm wood-waste appears to be one promising method of achieving greater levels of certainty and flexibility for integrating carbon sequestration accounting and renewable energy generation into conventional agricultural production (Lehmann, 2007). However, there remain large uncertainties of the effects of how bio-char applications to soil affect the surrounding ecology, and the productivity of particular crops in specific soil types and climates. This paper aims to reduce investment uncertainty for agriculturalists looking to diversify into converting biomass to bio-char and energy, with a special focus on experiences in Western Australia.
Section snippets
Bio-char production and feedstock
Worldwide, 41 million tonnes (t) of bio-char (charcoal) is estimated to be produced annually for cooking and industrial purposes (Food and Agriculture Organization of the United Nations, 2006) as cited in Lehmann et al. (2006). Conventional low efficiency production can result in losses of 80–90% of biomass weight (wet basis) and most of the energy content of the original biomass (Antal et al., 1996, Okello et al., 2001). If not produced according to sensible environmental parameters, the
Bio-char and agricultural suitability
At the local scale, soil organic carbon levels shape agro-ecosystem function and influence soil fertility and physical properties, such as aggregate stability, water holding capacity and cation exchange capacity (CEC) (Milne et al., 2007). The ability of soils to retain nutrients in cation form that are available to plants can be increased using bio-char. The CEC of the bio-char itself can also be improved by producing the bio-char at higher temperatures (700–800 °C), although this is at the
Bio-char and alternative biomass products and services
The integration of bio-char soil improver production and renewable energy generation in the form of biofuels, electricity and heat is a promising new industry (Lehmann et al., 2006). Producing bio-char and energy from wastes may both reduce waste disposal costs and provide cost-effective energy services that can be used by agricultural industries (Marris, 2006). In contrast to other renewable energy technologies, biomass can be used to produce a number of liquid, solid and gaseous fuels (
Bio-char production and greenhouse gas emissions
There is a major role for biomass conversion technologies in the mitigation of climate change through soil sequestration (Milne et al., 2007). Globally, up to 12% of all anthropogenic land use change emissions can be offset annually in soils if slash-and-burn agriculture is replaced by slash-and-char systems (Lehmann et al., 2006). Despite the lack of reliable information, an estimated 29.1 × 106 ha of global secondary forests are exposed to slash-and-burn clearing annually, which represents an
Conclusion
Producing bio-char from farm or forestry waste provides an impressive list of potential co-benefits, including the generation of renewable electricity, liquid biofuels, gas biofuels, activated carbon, eucalyptus oil, large amounts of heat or low-pressure steam, and the potential of a net withdrawal of carbon dioxide from the atmosphere. With the introduction of new policies and initiatives, the sum profitability of these various production streams is likely to improve, especially if they are
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