Catchment land use is associated with downstream transformation of extractable dissolved organic matter along contrasting Iberian rivers

Dissolved organic matter (DOM) exported from catchments regulates biogeochemical processing along river networks, but links between land use/land cover (LULC) and the composition of the operationally extractable DOM pool remain poorly constrained. We assessed how contrasting LULC patterns shape solid-phase extractable DOM along two Iberian rivers (Ebro and Mero) spanning forested headwaters, agricultural reaches, and urban zones. DOM isolates were characterized optically using synchronous fluorescence spectroscopy with Gaussian band fitting to quantify amino acid-, fulvic-, and humic-like fluorophores. Relative contributions shifted systematically downstream, and hierarchical clustering resolved three compositional groups consistent with the dominant LULC patterns. Forested headwaters were enriched in humic, aromatic material, consistent with preferential retention of hydrophobic, high-molecular-weight compounds by the styrene–divinylbenzene sorbent. Agricultural reaches, with higher nitrate and phosphate, showed stronger low-molecular-weight, microbially derived signatures that are less efficiently captured due to higher polarity and lower aromaticity. Urban sites displayed mixed optical fingerprints reflecting upstream inputs and local anthropogenic sources. Together, these results indicate that landscape setting and downstream hydrological processing jointly regulate extractable riverine DOM, emphasizing the need to account for LULC when interpreting basin-scale DOM dynamics.