Interactions between iron and sulfur on the release and dynamics of phosphorus in eutrophic lake sediments

This study aimed to provide in situ evidence for the simultaneous coupling of iron (Fe), sulfide (S(-II)) and phosphorus (P) in the sediment‒water interface (SWI) under anaerobic conditions, thereby enhancing the understanding of the process and mechanism of phosphorus dynamics in eutrophic lake sediments.

A 300-day redox-cycle process of aerobic-anoxic-anaerobic was conducted in six periods. High-resolution methods, including diffusive gradient in thin films (DGT) and pore water sampling technology (HR-Peeper) were used to measure Fe, S(-II) and P in the vertical sediment. A 96-well microplate spectrophotometer was employed to detect the dissolved and labile concentrations of Fe(II) and P. The determination of S(-II) was performed using the computer imaging densitometry (CID) scanning method. The ratio of the labile to the soluble concentration (R_diff, C_DGT/C_SOL) was calculated to characterize the kinetics of Fe and P release from sediments into the overlying water.

The vertical distribution of labile and soluble P, Fe(II) and S(-II) in water and sediment was measured. Findings reveal concentration peaks of these elements at 10–40 mm depth in the SWI, with a significant positive correlation between them. The Fe–P coupling is the main driver of phosphorus release, with S(-II) participating in this process. Organic matter is utilized as an electron donor by sulfate-reducing bacteria, which reduces Fe(III) to Fe(II) and forms FeS precipitation, further promoting the release of Fe–P. The ratio of labile to dissolved concentration (C_DGT/C_SOL) peaks (20–40 mm in the SWI) during the transition from aerobic to anaerobic conditions, indicating a strong replenishment ability of sediment to pore water at this stage.

Under anaerobic conditions, there is a synchronous vertical distribution of P, Fe(II), and S(-II), with the reduction of Fe oxides being the main cause of P release from sediments. Under highly anaerobic conditions, S(-II) indirectly affects the dynamic of P by combining with Fe(II), potentially becoming the dominant factor. Changes in R_diff-P and R_diff-Fe infer that in the SWI of Lake Taihu sediments, Fe and P have a high capacity to replenish from the sediment particles to the pore water.