Long-term whole-soil warming restructures the molecular composition of soil dissolved organic matter

Dissolved organic matter (DOM) represents one of the most dynamic organic carbon pools in soils, and its molecular composition ultimately governs carbon mobilization, transformation, and stabilization. However, the profound differentiation in both quantity and quality of DOM across the entire soil profile under long-term warming remains poorly resolved. Therefore, this study conducted an eight-year whole-soil warming experiment (+4 °C) in the alpine grasslands of the Tibet Plateau to investigate the response of DOM molecular composition to warming across four soil layers: 0–10 cm (surface), 10–30 cm (shallow), 30–60 cm (middle), and 60–100 cm (deep). Overall, warming significantly increased DOM concentrations in surface soil, whilst DOM in shallow, middle and deep soil layers showed only marginal increases that did not reach statistical significance. In contrast, warming drives the reconfiguration of DOM composition across the entire profile, primarily concentrated in molecules such as CHO, CHON, CHOS, and CHONP, exhibiting pronounced depth dependence. Concurrently, the relative intensity of aromatic and highly unsaturated compounds in the surface and shallow soils was lower under warming treatment compared to the control, whereas the aliphatic and peptide compounds exhibited the opposite trend. This finding indicates that warming induces a shift in DOM composition from relatively humic towards one dominated by aliphatic/nitrogen-rich components. Furthermore, the relative intensities of carboxyl-rich aliphatic molecules (more recalcitrant DOM fraction) in middle and deep soils significantly increased under warming conditions. Collectively, these results demonstrate that long-term whole-soil warming reshapes the DOM composition through depth-specific pathways, underscoring that deep-soil DOM can respond fundamentally differently from topsoil.