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 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 13C 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.

<p>Understanding the molecular make-up of recalcitrant organic matter (rSOM) is important to postulate the capability of soil organic matter (SOM) to sequester carbon and mitigate climate change. Humic acid (HA) extracted from the river bed sediment (RS) from the West Holland river was analyzed, aiming to characterize and quantify the fused ring aromatic structures (FRA) portion. FARs can be formed through condensation and polymerization reactions and act as an important skeletal structure of the rSOM which has a mean residence time >1000 years. We conducted a series of nuclear magnetic resonance (NMR) experiments, 13C Direct Polarization Magic Angle Spinning (DP-MAS) NMR spectroscopy, and Dipolar dephased (dd) DPMAS NMR, chemical shift anisotropy (CSA) cross-polarization (CP) total sideband suppression (TOSS) NMR experiment and a dd-CSA filtered CPTOSS to accurately quantify the proportion of FRAs in the sediment HA sample. We compared the proportions of the functional groups of the RS with the surface (0-20 cm, TS) and deep (>90 cm, CS) soil HAs of the nearby Holland Marsh, Muck Crops Research Station to understand the linkages and the transformations of SOM happened while transportation (wind erosion and horizontal seepage) to the muck river sediment. We found that 90% of the aromatic C in the RS is non-protonated, and 32% of the aliphatic region was non-protonated. The DPMAS spectral comparison between RS, TS and CS clearly showed that RS contains characteristic peaks of both TS and CS. Moreover, the proportion of non-protonated aliphatics in RS (32%) is high compared to TS (18%) and CS (29%). Our results indicate that in muck river sediment soil HA, non-protonated aliphatics (CRAM-like structures) contribute to the rSOM more than FRAs, while in TS and CS, FRAs' contribution is higher than the non-protonated aliphatics. Collectively our results show the link between terrestrial organic matter transportation to the river sediment and the transformation that occur in the rSOM fraction in the river sediment SOM. This new knowledge allows us to understand the structural changes that happen in the sequestered carbon in different soil environments.</p>