Vertical convergence of soil dissolved organic matter composition across Central European landscapes

Soil dissolved organic matter (DOM) plays a critical role in terrestrial carbon cycling and its transfer to aquatic systems, yet its vertical dynamics, particularly in deeper soil layers, remain poorly understood. We investigated water-soluble organic matter (WSOM) composition, used here as a proxy for soil DOM composition, in ∼1,500 soil samples from 330 cores across four Central European regions (Ore Mountains, Tyrolean Alps, Black Forest, Sauerland). Samples were collected from profiles extending to 4 m and analyzed using absorbance and fluorescence spectroscopy coupled with Parallel Factor Analysis (PARAFAC). Dissolved organic carbon (DOC) concentrations declined with depth, while DOM composition shifted toward greater protein-like contributions, consistent with intensified microbial reworking of organic matter. DOM exhibited vertical structuring: upper layers differed regionally, mid-depth layers (11–100 cm) showed the highest compositional heterogeneity, and deeper layers (101–400 cm) converged toward remarkably similar signatures across all regions. Depth was the dominant driver of DOM composition, outweighing regional context, land use, and topography. The observed convergence is consistent with microbial reworking of DOM at depth. These findings highlight deep soils as a reservoir of transformed DOM that likely influences groundwater chemistry and baseflow DOM export to streams. These depth-dependent patterns underscore the importance of subsoil processes in shaping the composition of terrestrial–aquatic carbon fluxes. By combining extensive spatial coverage with deep soil profiles, our study provides a comprehensive assessment of vertical DOM composition in temperate soils and highlights the need to incorporate deep soil dynamics into models of carbon cycling and hydrological connectivity.