Biochar is the product of heating biomass in the absence of or with limited air to above 250ºC, a process called charring or pyrolysis also used for making charcoal (Chapter 3). The material distinguishes itself from charcoal or other carbon (C) products in that it is intended for use as a soil application or broader for environmental management. In some instances, the material properties of biochar may overlap with those of charcoal as an energy carrier, but many types of biochar do not easily burn and charcoals are typically not made to address soil issues (Nomenclature in Box 1.1). An important defi ning feature of biochars, similar to charcoal, is a certain level of organic C forms, called fused aromatic ring structures (Chapter 6). These structures are formed during pyrolysis and are key to biochar properties with respect to mineralization (Chapter 10) or adsorption (Chapter 9). Therefore, biochar is typically enriched in C (Figure 1.1), and even more in phosphorus (P) or other metals such as calcium (Ca) or magnesium (Mg) and sometimes even nitro-

gen (N). The chemical properties of the organic C structure of biochars are fundamentally different from those of the material that the biochar was produced from and depleted in oxygen (O) and hydrogen (H). In contrast, the macro-morphological characteristics of biochars typically resemble those of the starting material, which means that it typically looks the same, apart from its black color. The intended use as a soil amendment also requires that biochars do not contain harmful levels of heavy metals or organic

Figure 1.1 Conversion effi ciency of biomass, C, N and P during pyrolysis (data from Enders et al (2012); typical losses followed by range in brackets)


contaminants (IBI, 2013), in keeping with related efforts to make composts and other soil amendments safe for soil. Despite these common criteria, it would be wrong to con-

clude that biochar is a narrowly defi ned material. In fact, biochars can have very different properties, which have to be recognized, as discussed throughout this book.