Wymore, Adam S.Johnes, Penny J.Bernal, SusanaBrookshire, E. N. JackFazekas, Hannah M.Helton, Ashley M.Argerich, AlbaBarnes, Rebecca T.Coble, Ashley A.Dodds, Walter K.Haq, ShahanJohnson, Sherri L.Jones, Jeremy B.Kaushal, Sujay S.Kortelainen, PirkkoLopez-Lloreda, CarlaRodriguez-Cardona, Bianca M.Spencer, Robert G. M.Sullivan, Pamela L.Yates, Christopher A.McDowell, William H.2021-11-172021-11-172021-080886-6236e2021GB006953http://hdl.handle.net/10919/106665A comprehensive cross-biome assessment of major nitrogen (N) species that includes dissolved organic N (DON) is central to understanding interactions between inorganic nutrients and organic matter in running waters. Here, we synthesize stream water N chemistry across biomes and find that the composition of the dissolved N pool shifts from highly heterogeneous to primarily comprised of inorganic N, in tandem with dissolved organic matter (DOM) becoming more N-rich, in response to nutrient enrichment from human disturbances. We identify two critical thresholds of total dissolved N (TDN) concentrations where the proportions of organic and inorganic N shift. With low TDN concentrations (0-1.3 mg/L N), the dominant form of N is highly variable, and DON ranges from 0% to 100% of TDN. At TDN concentrations above 2.8 mg/L, inorganic N dominates the N pool and DON rarely exceeds 25% of TDN. This transition to inorganic N dominance coincides with a shift in the stoichiometry of the DOM pool, where DOM becomes progressively enriched in N and DON concentrations are less tightly associated with concentrations of dissolved organic carbon (DOC). This shift in DOM stoichiometry (defined as DOC:DON ratios) suggests that fundamental changes in the biogeochemical cycles of C and N in freshwater ecosystems are occurring across the globe as human activity alters inorganic N and DOM sources and availability. Alterations to DOM stoichiometry are likely to have important implications for both the fate of DOM and its role as a source of N as it is transported downstream to the coastal ocean.application/pdfenPublic Domain (U.S.)itrogen cycletotal dissolved nitrogendissolved organic matterdissolved organic carbondissolved organic nitrogenstoichiometryfreshwater ecosystemsArticle - Refereedhttps://doi.org/10.1029/2021GB0069533581944-9224