Land use and ecosystem properties explain the composition and stability of dissolved organic matter across Northern Hemisphere headwater catchments

We analysed the chemical composition of dissolved organic matter (DOM) and the decomposition rates of cotton-strips in several headwater streams of the Northern Hemisphere. We conducted a coordinated distributed experiment to investigate concentration of dissolved organic carbon (DOC), DOM spectroscopic and fluorescence signatures including PARAFAC-resolved excitation emission matrices (EEMs), organic matter decomposition rates (OM stability), dissolved concentrations of CO 2 -C and CH 4 -C, and physio-chemical water quality parameters. We aimed to uncover relationships between land use classes (LULC), DOM composition and OM stability, in streams across different latitudes of Europe and the United States of America. Our data showed that physio-chemical water quality strongly reflects land use classes, and that in both agricultural and urban streams terrestrial DOM was lower compared to forested streams, indicating ecological disconnection with the riparian zone and higher photooxidation of chromophoric DOM (CDOM). However, on pan-continental scales, DOM characteristics instead can be pinned on abiotic and biotic controls, arising from a multitude of biogeographical factors within catchments, that dominate compositional differences. This leads to persistent chemodiversity in DOM composition under different LULC classes. Our results underscore the importance of coordinated distributed experiments to unravel local catchment dynamics to draw conclusions for global change assessments on DOM composition. We show that DOM is strongly affected by land use on the local scale, but geospatial and ecosystem properties superimpose these differences on larger scales.