Evidence for the Formation of Recalcitrant Carboxyl-Rich Alicyclic Molecules (CRAM) in an Organic Soil Profile

Recalcitrant soil organic matter (rSOM) plays a pivotal role in global carbon (C) sequestration within soil ecosystems, yet its formation remains enigmatic due to limited insights into its molecular composition and synthesis processes. Specifically, carboxyl-rich alicyclic molecules (CRAM) are recognized for their thermodynamic stability owing to their distinctive molecular structure and hydrophobic properties. In this study, we investigate the presence and transformation of CRAM structures within an organic soil profile employing 1D and 2D 1H nuclear magnetic resonance (NMR) spectroscopy. We conducted organic matter extraction from distinct soil horizons from a soil profile at the Muck Research Station in Kettleby, Canada, utilizing acetonitrile (ACN) and n-hexane (HEX) as organic solvents. ACN effectively extracts condensed structures with O/C ratios exceeding 0.5, while hexane selectively targets lipid-like compounds with low O/C ratios (<0.1). The ACN and HEX solvents are known to represent rSOM components reliably. Notably, all samples exhibited a broad resonance assigned as CRAM with more resolved spectra from the Cg horizon, observed in the 1D 1H NMR. Transformation of CRAM in the Cg and Oh horizons towards increased aliphatic characteristics at the expense of functionalized protons was evident. Further analysis involved 1H-13C HSQC NMR spectroscopy, identifying five major spectral regions: Region I (δH: 0.5–1.5 ppm and δC: 10–30 ppm, associated with purely aliphatic groups), Region II (ca. δH: 1.5–3.5 ppm and δC: 30–45 ppm, linked to CRAM), Region III (ca. δH: 3.0–4.0 ppm and δC: 50–65 ppm, characterized by aromatic methoxyl groups indicating ethers and esters), Region IV (ca. δH: 3.25–4.5 ppm and δC: 65–80 ppm, associated with carbohydrates), and Region V (ca. δH: 6.25–7.5 ppm and δC: 115–135 ppm, representing aromatic structures). Significantly, CRAM components in the Cg horizon displayed distinct compositions from those derived in the upper horizons, suggesting microbial reworking and radical-assisted synthesis of biomolecules or precursors contributing to CRAM formation within the soil profile. Overall, the HSQC cross-peak positioning in all samples exhibited closer similarities to dissolved organic matter (DOM) of microbial origin rather than plant-derived DOM. In the lower Cg horizon, we observed a greater diversity of aliphatic and carbohydrate cross-peaks, highlighting both a higher degree of newly produced organic moieties and the potential for early diagenetic rSOM formation within the soil profile. These findings underscore the substantial role of CRAM in long-term carbon sequestration within soils.