Long-term degradation effect on the molecular composition of black carbon in Brazilian Cerrado soils

The effect of long term degradation on soil black carbon (BC) is important for correctly interpreting the role of BC in the global carbon cycle and in biochar studies. To address this, we studied three soil profiles (0-2 m depth, > 9000 yr) in undisturbed Brazilian Cerrado vegetation naturally affected by wildfires. The molecular composition of several soil organic matter (OM) pools was studied using analytical pyrolysis. Other analyses included general chemical and physical characteristics and micromorphology. The soil OM fractions included the free light fraction (FLF; particulate OM), the occluded light fraction (OLF; particulate OM within aggregates), the 0.1 M NaOH extractable OM (EXT; comparable with the combined humic acid and fulvic acid fractions) and the remaining residue (RES; treated with HF/HCl; comparable with the humin fraction). Although each fraction represents a continuum of material, they were assigned a different degradation level. The light fractions represent relatively intact BC (i.e. charcoal) of which the OLF is more degraded than the FLF, the EXT contains more decomposed material and the RES represents residual OM that is difficult to decompose. The largest contribution to the total soil OM was from RES and EXT, together accounting for > 80% of the total soil C. The relative contribution from BC-associated pyrolysis products was generally large for all fractions (between 5% and 57%) and showed an increase with depth in the OLF and RES. Based on factor analysis exclusively applied to BC-associated pyrolysis products from all four OM fractions, two BC degradation indices were extracted: Index 1, reflecting depolymerization and relative enrichment of N, and Index 2, reflecting a relative increase in condensed structures. Both indices gradually increased with depth for all fractions, except FLF. Based on these indices, we selected some simple ratios that showed good correlation with them (r2 > 0.7). For Index 1, reflecting depolymerization, adequate ratios for all fractions included benzene/indene and benzene/C1 naphthalenes. For Index 2, reflecting the relative accumulation of condensed BC, adequate ratios for all fractions included biphenyl/C2-benzene, fluorene/C2-benzene and 4-ring PAHs/C2-benzene.