Pyrogenic Organic Matter in Japanese Andosols: Occurrence, Transformation, and Function

The aim of this chapter was to provide invaluable information for a better understanding of the potential role of biochar by integrating the existing publications on pyrogenic organic matter with special reference to Japanese Andosols. Charred plant residues (CPRs) were produced by carbonization during the burning of vegetation, and in soils, they were progressively degraded into smaller particles (charred plant fragments [CPFs]). During this process, part of the CPRs were subjected to oxidation and hydration and were transformed into fulvic acids (FAs) and, especially, black humic acids (HAs) with graphite-like (turbostratic) structures consisting of highly polycondensed C-layer planes. The CPFs containing these acids seemed to be more stabilized by forming complexes with inorganic matrices in the silt-sized aggregate fraction. It was assumed that the formation not only alleviated Al toxicity for plant growth but also hindered the synthesis of short-range-order minerals. In the Japanese Andosols (68 samples) studied, quantitative contribution of CPF C to soil organic C (SOC) was 5% or more in 38 samples, with a maximum value of 33%. The CPF C content was significantly correlated with the SOC, HA C, or FA C content. Similar relationship was found between the CPF C/SOC level and the SOC content. The proportion of NaOH-extractable black HAs and FAs in the CPFs to those in the whole soils reached 44% for the HAs and 10% for the FAs. It was concluded that pyrogenic organic matter such as CPR-derived CPFs, black HAs, and FAs is the crucial constituent responsible for the high humus accumulation in Japanese Andosols.