Overlooked sulfur moieties in leaf litter drive the divergent fates of Cu and Cd during DOM humification.

Early humification of leaf litter generates dynamic dissolved organic matter (DOM) that regulates heavy-metal mobility at soil-water interfaces, yet the role of sulfur-containing moieties under oxygen-limited conditions remains unclear. Here, FT-ICR MS and fluorescence spectroscopy were used to track DOM evolution and its interactions with Cu(II) and Cd(II). Fresh DOM was dominated by protein-like component, associated with aliphatic N/S species, whereas humified DOM was enriched in humic-like components correlated with aromatic and lignin-like sulfur molecules. The progressive humification increased the Cu(II) complexation constant (logKM) from 3.46 to 5.38, which was attributed to the accumulation of reduced sulfur species (O/S < 5) in the aromatic humic matrix. These groups acted as redox-active centers, promoting Cu(II) reduction to Cu(I) (up to 82.4%) and subsequent desulfurization, with a tendency toward Cu2S sedimentation and irreversible immobilization in quartz-column simulations. By contrast, Cd(II) showed moderate affinity (logKM≈3.0-3.4) and mainly targeted protein-like sites. Molecular and transport analyses showed that Cd(II) mainly interacted with oxidized S/O sites and promoted condensation, whereas fresh DOM favored Cd retention and humified DOM enhanced Cu immobilization. These results reveal humification-driven differences in metal fate and inform predictions of heavy-metal stability and ecological risk at soil-litter interfaces.