Microbial inoculation and temperature regulate mineral-associated organic carbon formation in technosols: molecular-level insights into distiller’s grains transformation

Whether mineral-associated organic carbon (MAOC) formation follows the same molecular pathways when the organic input is a pre-fermented industrial residue — already enriched in microbial metabolites — as when it is fresh plant litter remains unknown. Here, we investigated MAOC formation during transformation of distiller’s grains (DG; C/N = 3.78), a pre-fermented baijiu waste, in Technosols with a defined mineral matrix (quartz, montmorillonite, goethite, boehmite) under factorial manipulation of microbial inoculation and temperature (15, 25, 35 °C) over 120 days. Multi-analytical characterisation (density fractionation, Py-GC/MS, FTIR, selective dissolution, amino sugar biomarkers, amplicon sequencing) revealed three principal findings. First, microbial inoculation at 25 °C increased MAOC by 23.6 % (to 18.85 g C/kg), with microbial necromass carbon most strongly associated with MAOC in exploratory analyses. Notably, the MNC/MAOC ratio (0.50–0.60) substantially exceeded values reported for natural soils (0.30–0.45), consistent with a shortened microbial processing sequence in which DG’s pre-existing metabolites elevate the MNC/MAOC ratio. Second, MAOC formation showed a mineral-selective pattern not previously reported in model soil systems: organo-iron complexes (Fe p ) correlated strongly with MAOC (r = 0.84, p < 0.01), while aluminium phases showed no significant treatment response, indicating that iron oxide transformation — not aluminium — was the mineral phase most strongly associated with MAOC stabilisation under fermentation waste inputs. Third, elevated temperature (35 °C) not only reduced net MAOC through substrate depletion but shifted necromass composition (FNC/BNC declined from 1.32 to 1.64 to 0.92–0.95) and triggered necromass decline after Day 60. Partial-correlation analysis further indicated that MAOC was preferentially associated with bacterial necromass despite a fungal-dominated pool, and TG-DSC thermal-stability data indicated that the inoculated, N-rich MAOC was thermally the least stable, consistent with an early, transient accumulation state rather than long-term stabilisation. These results suggest that the MCP framework, while fundamentally applicable, requires a “substrate processing history” dimension to account for pre-fermented inputs, and that iron-bearing minerals should be prioritised in Technosol design for fermentation waste valorisation.