Land use and metal identity govern molecular pathways of soil DOM complexation and transformation

Soil dissolved organic matter (DOM) plays a critical role in controlling the mobility, bioavailability, and transformation of heavy metals in terrestrial ecosystems. However, the molecular-level mechanisms underlying its interactions with metals under different land use regimes remain poorly understood. In this study, we combined advanced molecular characterization techniques—Fourier transform ion cyclotron resonance mass spectrometry (FT–ICR MS), excitation–emission matrix-parallel factor analysis (EEM–PARAFAC)—with a molecular reactionomics framework to explore the selective complexation of DOM with Cd(II), Pb(II), and Cu(II) in soils from farmland, fallow land, forest, and orchard. EEM–PARAFAC analysis revealed that humic-like components were the primary binding units, showing the strongest affinity for Cu(II) (log K up to 6.013), followed by Pb(II) and Cd(II). FT–ICR MS results indicated that variations in DOM molecular composition were closely associated with metal-chelation capacity, with lignin-like compounds being the dominant ligands in all systems. Pairwise mass difference (PMD) network analysis showed that heavy metal exposure universally triggered oxygenation, decarboxylation, and dealkylation reactions, though these transformations followed metal-specific pathways: Cu(II) induced extensive oxidation, including carboxyl group cleavage and aromatic ring opening, due to its high redox potential and strong Lewis acidity; Pb(II) facilitated aromatic-carboxyl dissociation and multidentate coordination; and Cd(II) primarily promoted alkyl-chain cleavage in lipid-like molecules, simplifying their molecular structure. These findings offer valuable insights into the dynamic transformation processes and structural adaptations of DOM under metal stress, which could inform the development of predictive models and strategies for DOM-mediated heavy metal remediation.