The term 'Biochar', or more appropriately 'biocarbon', refers to all products made from the process of pyrolysis that decomposes organic materials at temperatures generally between 350 and 500°C in the absence of oxygen or with limited oxygen. During pyrolysis, an average of 50% of feedstock carbon content is converted to char; however, this percentage varies by feedstock and pyrolysis conditions. Due to the environmental and agronomic values of biochar, production of biochar from agricultural wastes (e.g., green plant material, feedlot manure, poultry litter, wood waste, bagasse, etc.) has recently been trialled in many countries (Figure 3). Although various types of feedstock can be used to produce biochar (Figure 4), not all chars produced from different feedstock have similar characteristics.
A range of environmental benefits can be obtained from biochar: reduction in greenhouse gas emissions; reduction in nutrient leaching; improvement in soil structure; water retention; and higher crop productivity. In NZ, most soils have > 3% carbon; however, pH can range from around 4.5 to 6.5, so liming is often required. The application of biochar could reduce lime use as well as provide other benefits. NZ soils also have a finite ability to store nitrogen, and nitrogen-saturated soils create the risk of nitrogen leaching into waterways. Soils with high carbon:nitrogen ratios usually have a greater capacity to store nitrogen and thereby reduce nitrous oxide emissions and nitrate leaching. Locking carbon in soil through the application of biochar (70-80% carbon level) seems a novel idea to reduce both greenhouse gas emissions and nitrate leaching. Preliminary results indicate that biochar amendments to soil appear to decrease emissions of nitrous oxide as well as methane, which is a greenhouse gas 23 times more potent than CO2. In greenhouse and field experiments in Colombia, nitrous oxide emissions were reduced by 80% and methane emissions were completely suppressed with biochar additions to a forage grass stand (Lehmann and Rondon 2006).
Biochar can influence the global carbon cycle in two main ways. If biochar is produced from material that would otherwise have oxidized in the short to medium term, and the resultant carbon-rich char can be placed in an environment in which it is protected from oxidation, it may provide a means to sequester carbon that would otherwise have entered the atmosphere as a greenhouse gas (Woolf 2008). In addition, the gaseous and liquid products of pyrolysis may be used as a fuel that can offset the use of fossil fuels (Lehman 2007). Biochar can potentially be used as a soil amendment for improving the quality of agricultural soils (Glaser et al. 2002a, 2002b; Lehmann et al. 2003). For example, Chan et al. (2007) observed that while there were significant changes in soil quality, including increases in pH, organic carbon, and exchangeable cations as well as reduction in tensile strength at higher rates of biochar application (>50 t/ha), long-term field experiments are required to confirm and quantify the eventual long-term benefits from biochar use.
Beneficial effects of biochar in terms of increased crop yield and improved soil quality have been reported (e.g., Iswaran et al. 1980; Glaser et al. 2002a, 2002b). However, review of previous research showed a huge range of biochar application rates (0.5-135 t/ha of biochar) as well as a huge range of plant responses (29-324%) (Glaser et al. 2002a). Recently, the use of poultry litter biochar applied at 10 t/ha as soil amendments on an Australian hard-setting soil showed significant increases in the dry matter yield of radishes. These yield increases were largely due to the ability of this biochar to increase nutrient availability, particularly nitrogen.
It must be noted that in much of the research undertaken so far, the properties of the biochar used in the investigations were not reported. Biochar can be produced from a range of organic materials and under different conditions resulting in products of varying properties (Baldock and Smernik 2002; Nguyen et al. 2004; Guerrero et al. 2005). However, little research has been published elucidating the mechanisms responsible for the reported benefits of the biochar on crop growth, production, and soil quality, and such understanding is essential for the development both of agricultural markets for biochar and of technology for the production of biochar products with improved quality and value. Although research on biochar is still in its infancy, and the various benefits that it can offer to the environment are yet to be fully demonstrated through long term field trials, the effectiveness of the use of 'agricultural waste' as a potential source for alternative energy needs greater appreciation by regulatory authorities for managing agricultural wastes.
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