Evidence for the formation of fused aromatic ring structures in an organic soil profile in the early diagenesis

The presence of fused aromatic ring (FAR) structures in soil define the stability of the recalcitrant soil organic matter (RSOM). FAR are important skeletal features in RSOM that contribute to its extended residence time. During the early diagenesis, FAR structures are formed through condensation and polymerization of biomolecules produced during plant residue and microbial product decay. Molecular level characterization of the RSOM extracted from an organic soil profile gives important insights into the formation of FAR. Advanced solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy, including recoupled long-range C-H dipolar dephasing experiments on extracted humic acids (HA) showed that they contain diagenetically formed FAR different from charcoal and lignin. Peaks characteristic of FAR are observed at all depths in the soil profile, with a greater prevalence observed in the HA extracts from the clay soil layer at the bottom. In the clay soil layer, 78% of the aromatic carbon was non-protonated, and this was 2.2-fold higher than the topsoil. These data further strengthen our understanding of the humification process that could occur in early diagenesis and help explain the importance of incorporating diagenesis as an important phenomenon for long-term carbon sequestration in soil.

Automated summary

The presence of fused aromatic ring (FAR) structures in soil determines the stability of recalcitrant soil organic matter (RSOM). FAR, important features in RSOM, contribute to its prolonged residence time. Molecular level characterization of RSOM extracted from an organic soil profile provides insights into the formation of FAR. Advanced solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy on extracted humic acids (HA) reveals diagenetically formed FAR that differ from charcoal and lignin. The prevalence of FAR is observed in all soil depths, with a higher occurrence in the clay soil layer at the bottom. These findings enhance our understanding of the humification process in early diagenesis and elucidate the significance of diagenesis in long-term carbon sequestration in soil.