The insignificant effect of increased sulfate concentration on nitrogen dynamics in eutrophic lakes: the neglected role of iron ions.

Context: Sulfate (SO₄²⁻) concentrations in eutrophic lakes are continuously increasing. Aims: This study aimed to explore the effects on nitrogen and their limiting factors in eutrophic lakes. Methods: We constructed a series of microcosms with different initial SO₄²⁻ concentrations (0–180 mg L⁻¹) to simulate the SO₄²⁻ concentration in eutrophic lakes. Key results: The NO₃⁻-N concentration rapidly decreased in all treatments. The respective 2.21 and 2.77 mg L⁻¹ concentrations of NO₃⁻-N recorded on Day 5 in the initial 150 and 180 mg L⁻¹ SO₄²⁻ concentration treatments were higher than in other treatments. The highest N₂O concentrations in all treatments showed insignificant differences. The existence of free Fe²⁺ that was released from iron reduction resulted in an insignificant effect of increased SO₄²⁻ concentration on nitrogen dynamics. With a decrease in Fe²⁺, the acid volatile sulfate (AVS) increased, and the highest AVS concentrations were positively correlated with the initial SO₄²⁻ concentration (7.20–12.88 mg kg⁻¹). Conclusion: This effect could be ascribed to the fact that the rapid combination of sulfide (∑S²⁻) and Fe²⁺ prevents ∑S²⁻ from producing a toxic effect on the denitrification. Implications: Our study highlighted the role of iron in eutrophic lakes, especially in the coupling of sulfate reduction and nitrogen dynamics. Increasing concentrations of sulfate in eutrophic lakes has gained a lot of attention. Cyanobacteria decomposition enhances sulfate reduction leading to the release of sulfide, which inhibits the activities and abundance of microorganisms because of its toxicity. However, in a system with high iron ion concentrations, reducing sulfide is rapidly bound, and its toxic effect on microorganisms is significantly inhibited, thereby eliminating its significant influence on the nitrogen dynamics and nitrogen oxide production. [ABSTRACT FROM AUTHOR]