Gradients of Anthropogenic Nutrient Enrichment Alter N Composition and DOM Stoichiometry in Freshwater Ecosystems. Issue 8 (5th August 2021)

• Record Type: Journal Article
• 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.
• Is Part Of: Global biogeochemical cycles. Volume 35:Issue 8(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-08-05
• Subjects: itrogen cycle -- total dissolved nitrogen -- dissolved organic matter -- dissolved organic carbon -- dissolved organic nitrogen -- stoichiometry -- freshwater ecosystems; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2021GB006953
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: 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 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. 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. Key Points: We synthesize a global data set of stream chemistry to examine how the composition of dissolved N and DOC:DON ratios respond to N enrichment Under low total dissolved N concentrations, the dominate form of N is highly variable but switches to primarily inorganic forms at high TDN With N enrichment, DOM becomes more N‐rich (lower DOC:DON ratios) while concentrations of DON are less associated with concentrations of DOC