Glycine-driven abiotic humification: Differential mediation of montmorillonite and kaolinite on the structural evolution of humic-like substances.

Abiotic humification via the Maillard reaction represents an indispensable geochemical process driving humic-like substance (HLS) formation and potential soil organic carbon (C) stabilization. Clay minerals act as critical modulators of this process, yet the differential mediating mechanisms of montmorillonite (Mont) and kaolinite (Kao)-two dominant soil clay minerals with distinct crystal structures-on glycine (Gly)-driven structural evolution of HLS remain poorly understood. Specifically, how Gly dosage couples with clay mineral types to modulate condensation reactions and structural differentiation of HLS has not been systematically clarified. In this study, a sterile abiotic reaction system was established with glucose and catechol as co-precursors, and five Gly concentration gradients (0, 0.03, 0.06, 0.12, and 0.24 mol/L) were set to investigate the regulatory effects of Gly on Maillard-derived HLS formation and structural evolution under Mont and Kao mediation. 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 Amax relative increment reached 261.3% in Kao system versus 148.3% in Mont system, while Mont maintained more stable kinetic inflection x0 values with weaker fluctuation across Gly gradients. The optimal Gly concentration for humic-like acid (HLA) C (CHLA) accumulationwas 0.06 mol/L in both clay systems, where the maximum CHLA 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 CHLA/fulvic-like acid (FLA) C (CFLA) 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 CHLA/CFLA 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.