Higher magnitude of sediment phosphorus release stimulated by ammonium than nitrate.

Purpose: Human activities have increased the amount of reactive inorganic nitrogen (RIN) in the biosphere. Recently, the effect of RIN on lake ecosystems (e.g., stress to aquatic organisms) has received increasing attention, while only a few studies have investigated the effect of RIN on phosphorus (P) dynamics, which is considered a key element in eutrophication. This study aimed to test the effect of high RIN on sediment P release. Materials and methods: We conducted a two-month RIN-addition experiment in nine 150 L tanks with three treatments: controls (without N addition), nitrate addition treatments (+ NO₃⁻), and ammonium-addition treatments (+ NH₄⁺). NaNO₃ and NH₄Cl were added at a concentration of 10 mg L⁻¹. Results and discussion: We found that the total phosphorus concentrations (TP) of the water were significantly higher in the + NO₃⁻ treatments than in the controls but lower than the TP in + NH₄⁺ treatments. Meanwhile, labile P at the sediment‒water interface indicated minor and significant P release in the + NO₃⁻ and + NH₄⁺ treatments, but P uptake in the control. Stimulated alkaline phosphatase activity was likely the dominant mechanism underlying the N-induced sediment P release. The magnitude of P release was higher in the + NH₄⁺ treatment than in the + NO₃⁻ treatment, likely because nitrate enhances the oxidation of sediment, counteracting to some degree the P-releasing effect. Conclusions: The findings imply that high ammonium pollution may cause increased risks of eutrophication than the same amount of nitrate; this is important for decision-making to control internal P loading under the scenario of continuous N amplification. [ABSTRACT FROM AUTHOR]