Gradients of Anthropogenic Nutrient Enrichment Alter N Composition and DOM Stoichiometry in Freshwater Ecosystems

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 WYMORE ET AL.

Shifting stoichiometry: Long‐term trends in stream‐dissolved organic matter reveal altered C:N ratios due to history of atmospheric acid deposition

“…a stoichiometrically stable DOM pool, despite the large changes in DOC concentrations (Wymore et al, 2021c). Sites affected by atmospheric acid deposition generally show increases in DON evidenced by the significant one-sample t tests (Figure 6).…”

“…DOM varies in availability to biota along the hydrologic continuum (McArthur et al, 1985) and its composition and properties are closely linked to the surrounding landscape (Jaffé et al, 2008;Mattsson et al, 2005;Wymore et al, 2021c;Yates et al, 2019).…”

“…Other lines of evidence, however, suggest that concentrations of DOC and DON can respond differently to environmental change such as changes in the concentrations of inorganic nutrients (Lutz et al, 2012;Wymore et al, 2015Wymore et al, , 2021cYates et al, 2019) and seasonal variability in precipitation and stream runoff (Bernal et al, 2005). Recent evidence has pointed to the stream DOC:DON ratio varying according to the extent of nutrient enrichment in catchments, diverging from the soil DOC:DON ratio as systems become more nutrient-enriched through land-use change and increasing human population density (Yates et al, 2019).…”

Context dependence in a tropical forest: Repeated disturbance reduces soil nitrate response but increases phosphate

“…Nitrogen availability may also drive changes in the stoichiometry of dissolved organic matter (DOC:DON) found in soil solution posthurricane, as observed previously in soil solution following N fertilization (McDowell et al, 1998). A recent synthesis of DOC:DON in streams from multiple biomes also shows that elevated levels of N are associated with lower C:N of dissolved organic matter (Wymore et al, 2021), suggesting that the nitrogen content of dissolved organic matter in both soils and streams is surprisingly plastic in response to disturbance. Differences in the timing, magnitude, and duration of soil solution increases in K + and NO 3 À suggest that their concentrations are controlled by different mechanisms, even though their dynamics are linked at the plant physiological level (Raddatz et al, 2020) and both ions show increased concentrations in stream water in response to whole-watershed disturbances in temperate watersheds and our topical montane study site (Fakhraei et al, 2020;McDowell et al, 2013).…”

“…Nitrogen availability may also drive changes in the stoichiometry of dissolved organic matter (DOC:DON) found in soil solution posthurricane, as observed previously in soil solution following N fertilization (McDowell et al, 1998). A recent synthesis of DOC:DON in streams from multiple biomes also shows that elevated levels of N are associated with lower C:N of dissolved organic matter (Wymore et al, 2021), suggesting that the nitrogen content of dissolved organic matter in both soils and streams is surprisingly plastic in response to disturbance.…”