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

Abstract

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.