Distribution of black carbon and black nitrogen in physical soil fractions from soils seven years after an intense forest fire and their role as C sink

After vegetation fires, incorporation of pyrogenic organic matter (PyOM) into soil organic matter (SOM) shifts its composition toward higher aromaticity and to an increase of N-heterocyclic constituents, formerly introduced as black nitrogen (BN). To investigate the medium-term impact of these shifts on the quality of SOM and its role as an important C sink, the A horizon from soils of the fire-prone Sierra de Aznalcóllar (Southern Spain) were sampled 4 weeks and 7 years after a severe fire. The solid-state ¹³C and ¹⁵N nuclear magnetic resonance (NMR) spectra of the samples obtained 4 weeks after the fire indicated quick incorporation of PyOM into SOM. Correspondingly, pyrrole-type N dominated the organic N fraction. Seven years after the fire, the aryl C contribution decreased from 46% to 23% of organic C, although it was still higher at the burnt sites than in the unburnt reference soil (16%). This fast loss of PyOM may be due to erosion, transport into deeper soils or microbial decomposition. The contribution of the latter is in line with former incubation experiments with burnt soils from the same area. Comparably, in the recovered soil, BN was almost completely substituted by amide N. Studying the partitioning of PyOM among the density and particle size fractions of the soils, indicated that after medium-term recuperation, most PyOM occurred in the free and occluded particulate OM fractions (fPOM and oPOM). The low protection against microbial degradation and the low density of these fractions may explain the high PyOM losses from the studied soils either by decomposition or by transport. We suggested that formation of PyOM-soil mineral associates represents an important step for reducing losses of fire-derived charcoal due to biochemical mineralization and thus to its sequestration in soils.