Excessive nutrient inputs from agricultural and urban fertilizer use and inadequate wastewater treatment have led to nutrient accumulation in freshwater systems. Reservoirs slow downstream nutrient transport, retaining nutrients in sediments and biomass and promoting internal cycling of legacy nutrients. Consequently, reservoirs are often hotspots of algal and cyanobacterial growth associated with degraded water quality and harmful algal blooms. While intact sediment core experiments are commonly used to quantify internal phosphorus (P) fluxes, internal fluxes of dissolved nitrogen (N) species are frequently overlooked, despite their potential importance, often due to assumptions that external N inputs dominate eutrophication or the availability of alternative methods to assess N dynamics, such as stable isotope tracer studies.
In this study, we quantified nitrate+nitrite‑N (NOx‑N) and ammonium (NH₄‑N) fluxes using data from historical sediment P‑flux experiments conducted across reservoirs in Northwest Arkansas and Eastern Oklahoma. Although these prior studies focused on dissolved P dynamics, concentrations of NOx‑N and NH₄‑N were also measured. Intact sediment cores were collected from reservoirs between 2015 and 2025 and incubated in the dark, at ~20 °C under oxic and anoxic conditions. Changes in dissolved NOx‑N and NH₄‑N concentrations over time were used to estimate sediment–water N fluxes. NOx‑N fluxes in oxic cores ranged from −2.2 to 4.1 mg m⁻² h⁻¹ and from −4.5 to 0.8 mg m⁻² h⁻¹ under anoxic conditions. Ammonium fluxes showed distinct redox responses: oxic cores exhibited both positive (1.3 to 2.6 mg m⁻² h⁻¹) and negative (−4.6 to −0.1 mg m⁻² h⁻¹) fluxes, whereas anoxic cores consistently released NH₄‑N (0.34 to 3.0 mg m⁻² h⁻¹). Together, these results highlight a critical but underutilized opportunity to quantify internal nitrogen loading from existing sediment core experiments traditionally focused on phosphorus.