Test procedures for biochar analysis in soils

Biochar is a product of the heating of organic matter under conditions of restricted oxygen. Biochar contains pyrogenic carbonaceous material (PCM) as well as inorganic ash constituents and, in some cases, partially or completely unpyrolysed organic matter. As a result, biochars can vary widely in their chemical composition and persistence under natural environmental conditions (Chapters 5–10). The chemical characteristics of biochars therefore overlap with all other materials across the PCM continuum that are variously known as char, charcoal, black carbon, soot, microcrystalline graphite and elemental carbon (Schmidt and Noack, 2000; see Chapters 1 and 6 for discussion).\ud \ud PCM occurs naturally in soils where biomass burning is a natural feature of the ecosystem (savannas, grasslands and some forests, and indeed can make up more than 50 per cent of total soil organic carbon (SOC) in some soils (e.g. Lehmann et al, 2008; Figure 24.1). PCM can also be abundant in soils where there have been significant historic (Schmidt et al, 2000), or prehistoric (Glaser and Birk, 2012) anthropogenic inputs of PCM to the soil. Natural PCM in a modern soil may therefore have been present in that soil for centuries to millennia. Over time this PCM becomes comminuted into the finer particle sizes (Skjemstad et al, 1999; Nocentini et al, 2010) and can undergo significant chemical alteration, particularly on particle surfaces (Kaal et al, 2008a; Ascough et al, 2011). Therefore the presence of indigenous PCM in many soils complicates the measurement and isolation of biochar added to soil, because indigenous PCM may or may not be similar, both physically and chemically, to biochar purposefully added to the soil