Acknowledgements: We are thankful to K Waxenberg, W Pallier, and L Mahony (Cambridge) for help with fieldwork. Additional support was provided by K. Klapproth and I Ulber with FT-ICR-MS measurements and C Schmalsch with LC-OCD samples. We also thank staff at the Natural Resources Canada – Canadian Forest Service (NRCan-CFS) for considerable support, including with logistics (K Webster, P Hazlett), development of extraction and assay protocols (V Rouleau, M-J Morency), conducting laboratory extractions and assays (E Smenderovac, D Chartrand, M-J Morency), and general field and laboratory assistance (J Schadenberg and many others). This work was funded by a Gates Cambridge Scholarship (OPP1144) awarded to E.C.F., NRCan-CFS Cumulative Effects programme awarded to E.J.S.E., H2020 ERC Grant FLUFLUX (716196) to G.S., H2020 ERC Grant sEEIngDOM (804673) to A.J.T., and grant from the Genomic Research and Development Initiative of the Government of Canada to C.M. and E.J.S.E.Funder: Genomic Research and Development Initiative of the Government of Canada, NRCan-CFS Cumulative Effects program awardsAbstractSoils are losing increasing amounts of carbon annually to freshwaters as dissolved organic matter (DOM), which, if degraded, can offset their carbon sink capacity. However, the processes underlying DOM degradation across environments are poorly understood. Here we show DOM changes similarly along soil-aquatic gradients irrespective of environmental differences. Using ultrahigh-resolution mass spectrometry, we track DOM along soil depths and hillslope positions in forest catchments and relate its composition to soil microbiomes and physico-chemical conditions. Along depths and hillslopes, we find carbohydrate-like and unsaturated hydrocarbon-like compounds increase in abundance-weighted mass, and the expression of genes essential for degrading plant-derived carbohydrates explains >50% of the variation in abundance of these compounds. These results suggest that microbes transform plant-derived compounds, leaving DOM to become increasingly dominated by the same (i.e., universal), difficult-to-degrade compounds as degradation proceeds. By synthesising data from the land-to-ocean continuum, we suggest these processes generalise across ecosystems and spatiotemporal scales. Such general degradation patterns can help predict DOM composition and reactivity along environmental gradients to inform management of soil-to-stream carbon losses.</jats:p
