Erosion-Driven Vertical Decoupling: Molecular Signatures of Dissolved Organic Matter and Microbial Coevolution in Subsoil Hotspots

Dissolved organic matter (DOM) is crucial to agricultural biogeochemical cycles, yet its molecule characteristics and interactions with microbial communities in eroding landscapes─particularly subsoil─are poorly understood. Using ultrahigh-resolution mass spectrometry and high-throughput sequencing, we investigated DOM and microbiota across weakly acidic silty loam topsoil (0-20 cm) and subsoil (80-100 cm) along eroding-depositional gradients in a maize-cropped Mollisol agroecosystem. The overall chemodiversity, composition, and complexity of the DOM molecule pool were higher in topsoil than subsoil, but their topographical differences were predominantly observed in the subsoil, indicating erosion-induced homogenization of surface molecular fingerprints. Depositional subsoil increased DOM molecular diversity and accumulated recalcitrant lignin/tannin compounds, contrasting lipid enrichment at the eroding subsoil. Thermodynamic analysis via substrate-explicit models confirmed reduced DOM bioavailability in depositional subsoil compared to its erosional counterpart. Across both soil layers, microbial α-diversity metrics declined at eroding sites but increased at depositional sites. Crucially, these metrics negatively correlated with the diversity and complexity of DOM molecules in the topsoil yet positively correlated in the subsoil. These divergent patterns reflect depth-dependent microbial strategies─surface microbes favored labile substrates, while subsurface specialists utilized complex compounds. Our findings highlight subsoil as a critical zone where erosion-deposition processes establish distinct DOM-microbe interfaces governing soil organic matter stabilization.