<p>Original data.</p>

A combination of ultraviolet-visible spectroscopy, total organic carbon (TOC) analysis, elemental analysis, Fourier transform infrared (FTIR) spectroscopy, and partial least squares structural equation modeling (PLS-SEM) was applied to characterize HLS aromatic condensation, mineral–organic interactions, and abiotic humification pathways. Results showed that elevated Gly concentrations continuously increased the supernatant E₄/E₆ ratio and TOC content; this trend reflected enhanced adsorption of highly aromatic HLS fractions onto mineral surfaces, leaving simpler-structured components in the aqueous phase.

The E₄/E₆ <i>A</i>_<i>max</i> relative increment reached 261.3% in Kao system versus 148.3% in Mont system, while Mont maintained more stable kinetic inflection <i>x</i>_<i>0</i> values with weaker fluctuation across Gly gradients. The optimal Gly concentration for humic-like acid (HLA) C (C_HLA) accumulationwas 0.06 mol/L in both clay systems, where the maximum C_HLA value of Kao group reached 3.00 g/kg, 69.5% higher than the 1.77 g/kg observed in Mont treatments.

Over the full incubation period, the average C_HLA/fulvic-like acid (FLA) C (C_FLA) ratio of Kao system increased by 492.8%, merely 160.5% for Mont; Mont-derived HLA exhibited consistently higher H/C atomic ratios (1.28–1.94) than Kao HLA (0.94–1.83), verifying weaker aromatic condensation of Mont-produced humic fractions. Kao likely facilitated higher aromatic condensation, molecular maturity, and C_HLA/C_FLA ratio of HLS, whereas Mont restricted macromolecular polymerization through interlayer spatial confinement, producing HLA with higher H/C ratios and simpler molecular structures.

FTIR spectra confirmed divergent responses of the two clay minerals to Gly gradients and strengthened mineral–organic coupling at the optimal 0.06 mol/L Gly dosage, with Kao displaying a wider fluctuation range (0.50–5.99) of polysaccharide C–O band intensity than Mont (0.91–1.01). PLS-SEM further verified that Gly addition directly drove early HLS formation, and mineral–organic coupling may serve as a key intermediate pathway related to HLS structural evolution and mineral-associated C retention.

These findings likely reveal the structure-dependent regulatory patterns of typical soil clay minerals in abiotic humification under laboratory simulation, providing a preliminary mechanistic reference for optimizing precursor conditions to promote soil C stabilization.</p></div>