Impact of Structural Heterogeneity of Pyrogenic Organic Material (PyOM) on Humification Processes in Fire-Affected Soils

摘要

Approximately 10 to 15×106 ha of boreal and temperate forest, 20 to 40×106 ha of tropical forests and up to 500×106 ha of tropical and subtropical savannas, woodlands and open forests are burnt every year (Goldammer, 1993). Beside the production of volatiles, vegetation fire transforms various amounts of labile organic components into recalcitrant dark colored and highly aromatic structures. They are incorporated into humic material of the soil and can substantially contribute to its darkness. Because in sediments and soils, this pyrogenic organic material (PyOM) can survive for millennia and it is assumed to represent an important sink within the global carbon cycle. However, if assumed that biomass burning occurred at the same rate than nowadays since the last glacial maximum, PyOM should account for 25 to 125% of the total soil organic matter pool (Masiello, 2004). In order to elucidate the real importance of PyOM as a C-sink, a good understanding of its chemistry and the processes involved in its humification is crucial. Although there are several models describing “Black Carbon” (BC) on a molecular level are reported, a commonly accepted view of the chemistry involved in its formation is still missing. Its biogeochemical recalcitrance is commonly associated with a highly condensed aromatic structure. However, recent studies indicated that this view may be oversimplified for PyOM derived from vegetation fires (Knicker et al., 2008). Alternatively, it was suggested that such combustion residues are a heterogeneous mixture of heat-altered biopolymers with domains of relatively small polyaromatic clusters, but considerable substitution with N, O and S groups.