As mentioned earlier, two important factors in bio­char production are the type of feedstock used and pyrolysis conditions as they affect the physical and chemical properties of the biochar that is produced. Depending on the origin of the feedstock (e. g. cellulose, lignin, lingocellulose, hemicellulose) the chemical and structural composition of pyrolyzed biochar can change, and thus when biochar is used as a soil amendment, its behavior, function and fate in soils could be different. For instance, Winsley (2007) showed that when wood — based feedstocks are pyrolyzed, coarse and resistant bio­chars were generated with nearly 80% carbon contents, because the rigid ligninolytic nature of the source mate­rial is still retained in the biochar residue. The following sections discuss the chemical, physical, and biological properties of soil amended with biochar with particular focus on how the various properties influence the soil — biochar interactions and consequently improve the soil’s biological health and crop productivity.

Chemical and Physical

In an effort to offset anthropogenic C emissions to the atmosphere, a number of geoengineering technologies have been proposed, which can be divided into the

two broad categories of solar radiation management and CO2 removal, with C sequestration through biochar incorporation into soil in the latter (Vaughan and Lenton,

2011) . What distinguishes biochar CO2 sequestration (Figure 25.1) from competing CO2 removal technologies, such as ocean iron fertilization and CO2 geological injec­tion, are two factors. First, biochar C sequestration relies on photosynthesizing plants to draw CO2 from the atmo­sphere and is stored in the form of charcoal, whereas methods such as CO2 geological injection rely on rela­tively new and untested storage methods, including me­chanically forcing supercritical CO2 into depleted fossil fuel reservoirs or deep sea sediments (Vaughan and Lenton, 2011). Second, charcoal storage in soils is already ubiquitous and the clearest challenges in the short term involve optimizing economic profitability, rather than projected efficacy (Spokas et al., 2012a).

C sequestration through pyrolysis for biochar pro­duction is a result of the conversion of C forms present in the feedstock into recalcitrant forms present in biochar. Pyrolysis consists of a succession of changes in chemical structural composition as temperature in­creases. Cellulose and lignin are degraded as volatile compounds are driven off between 250 °C and 350 °C, followed by lateral growth and coalescence of polyaro­matic graphene sheets, and culminating with carboniza­tion, constituted by the expulsion of the majority of non-C atoms above 600 °C (Verheijen et al., 2010). Addi­tionally, as pyrolysis temperature and residence time increase, H/C and O/C ratios of biochar decrease and
aromaticity, the extent to which aromatic rings are con­nected, increases, resulting in greater recalcitrance against degradation (Kookana et al., 2011). Recent research suggests that biochars intended for both soil improvement and C sequestration should possess recal­citrant carbon of >15%, O/C ratios of <0.4, H/C ratios of <0.6, polyaromatic hydrocarbon contents below back­ground soil values, and a surface area of >100 m2 g (Schimmelpfennig and Glaser, 2012). As pyrolysis tem­peratures increase, biochar specific surface area and microporosity analogously increase. Biochar chemical and physical properties are due to both the composition of the feedstock and the extent of the alterations under­gone during pyrolysis (Kookana et al., 2011). The diver­sity of biochar chemical and physical qualities achievable through varied pyrolysis conditions and feedstocks is reflected in the diversity of effects on soil biota that may be achievable. Biochar is sterile when produced, yet has been observed to have beneficial effects on soil microbes that play essential roles in nutrient cycling. These effects are complex; however, they are linked to the chemical and physical properties of the biochar employed, which can serve as both a source of nutrients and as a habitat for soil microbes (Lehmann and Rondon, 2006; Lehmann et al., 2011).

Microbiological Effects and Synergisms

Literature focused on the effects of biochar on soil biota is currently sparse relative to biochar chemical

and physical effects on soil (Lehmann et al., 2011). Pub­lished research fails to adequately address the diverse spectrum of biochars within a single study, typically evaluating the effects of a small number of biochars on microbial and root abundance for a small number of soil types. Depending on the feedstock from which bio­char is produced and the pyrolysis conditions involved in the feedstock conversion, biotoxic substances may persist through or be generated during pyrolysis. Stan­dardized methods, including germination rates of various crops and degree of earthworm avoidance, have recently been proposed in order to assess biochar toxicity to avoid inadvertent detrimental effects on crop production and the environment (Busch et al., 2012; Rogovska et al., 2012).

The addition of biochar to compost systems and the resultant effects on both the microbial community dy­namics and final compost quality have been evaluated, and the potential for synergistic effects and increased soil C stability are known (Fischer and Glaser, 2012). While the aromatic core of biochar has been observed to remain unchanged during the composting process with manure, C and plant-available nutrients are drawn into its pores and adhere to its surfaces, elevating the cation exchange capacity (CEC) and acid neutralizing capacity, and enhancing the functionalization of biochar surfaces, although the mechanisms responsible warrant additional research (Prost et al., 2013). Biochar has been observed to affect mycorrhizal symbioses, yet the mech­anisms responsible are still being determined (Warnock et al., 2007) and recently, it has been proposed that soil nitrogen may act as a switch controlling the proliferation of mycorrhizae and the subsequent oxidation of fresh biochar surfaces (LeCroy et al., 2013). Compared to compost, biochar is a much more stable soil amendment and the addition of biochar to composts has been shown to dramatically increase compost stability (Bolan et al.,

2012) .