Gradients of anthropogenic nutrient enrichment alter N Composition and DOM stoichiometry in freshwater ecosystems

• Published in: Global Biogeochemical Cycles
• Main Authors: Wymore, Adam S., Johnes, Penny J., Bernal, Susana, Brookshire, E. N. Jack, Fazekas, Hannah M., Helton, Ashley M., Argerich, Alba, Barnes, Rebecca T., Coble, Ashley A., Dodds, Walter K., Haq, Shahan, Johnson, Sherri L., Jones, Jeremy B., Kaushal, Sujay S., Kortelainen, Pirkko, López-Lloreda, Carla, Rodríguez-Cardona, Bianca M., Spencer, Robert G. M., Sullivan, Pamela L., Yates, Christopher A., McDowell, William H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Geophysical Union 2021
• Subjects: nitrogen cycle; total dissolved nitrogen; dissolved organic matter; dissolved organic carbon; dissolved organic nitrogen; stoichiometry; freshwater ecosystems; nitrogen; carbon; organic chemistry; organic material; inorganic chemistry; chemistry; organic compounds; interaction; biogeochemical cycles; organic substances; freshwater; DOM; DON; TDN; deserts; boreal; boreal biome; arctic; arctic biome; temperate; temperate biome; tropical; tropical biome; desert biome; geosciences; kemia; stoikiometria; hiili; liuennut orgaaninen hiili; typpi; orgaaninen aines; aineiden kierto; makea vesi; virtavedet; vesiekosysteemit; aavikot; boreaalinen vyöhyke; kylmä vyöhyke; lauhkea vyöhyke; trooppinen vyöhyke; geotieteet; Arctic
• Online Access: http://hdl.handle.net/10138/337689; https://doi.org/10.1029/2021GB006953

Plain language summary Ammonium and nitrate in freshwaters have received considerable attention due to their clear ecological and health effects. A comprehensive assessment of N in freshwaters that includes DON is lacking. Including DON in studies of surface water chemistry is important because it can cause eutrophication and certain forms can be rapidly removed by microbial communities. Here, we document how elevated levels of TDN impact the concentrations and relative proportions of all three forms of dissolved N and the stoichiometry of DOM. Our results suggest that human activities fundamentally alter the composition of the dissolved nitrogen pool and the stoichiometry of DOM. Results also highlight feedbacks between the C and N cycles in freshwater ecosystems that are poorly studied. A 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 ...