Small‐Scale Topographic Variability Shapes Molecular Diversity of Organic Matter in Coastal Dune Soils

ABSTRACT Background Dissolved organic matter (DOM) plays a crucial role in carbon and nutrient cycling across terrestrial, aquatic, and marine ecosystems. Most terrestrial OM originates from litter and soils, where it is mobilized by percolating rainwater before entering the groundwater. Consequently, groundwater OM composition can be influenced by soil processes such as organic matter processing above the groundwater surface. Aims This study aimed to investigate how small‐scale topographic variability shapes the molecular composition of soil‐derived organic matter in a barrier island dune system and its potential implications for the island's freshwater lens. Methods A sequential extraction approach was applied to compare the molecular fingerprints of water‐ and acid‐extractable OM (WEOM and AEOM) from topsoils, representing readily leachable and mineral‐associated OM fractions. Utilizing ultrahigh‐resolution mass spectrometry, we analyzed the molecular composition of OM from north‐ and south‐facing brown dune slopes and gleyic interdunal valleys. Results WEOM from north‐facing slopes had a chemically reduced, plant‐derived composition, whereas WEOM of south‐facing slopes was more microbially processed and enriched in aromatics. These differences were driven by contrasting microclimates and litter qualities, resulting from meter‐scale variation in topography. Interdunal valleys exhibited high dissolved organic carbon (DOC) concentrations and a more oxidized, partly degraded WEOM signature, likely influenced by fluctuating groundwater levels. AEOM showed higher DOC concentrations and a strong microbial imprint, while still reflecting site‐specific environmental conditions. Conclusions Our findings provide molecular‐level insights into how small‐scale topographic heterogeneity shapes soil OM chemodiversity in coastal dunes. Interdunal valleys can act as potential hotspots of OM leaching.