Abstract: To clarify the biogeochemical coupling mechanism of iron-sulfur-phosphorus in the intertidal zone of estuaries, this study selected the typical intertidal zone of the Liaohe River Estuary as the research area. The phosphorus species and their distribution characteristics, as well as the coupling processes of iron-sulfur-phosphorus were investigated by measuring the content of phosphorus, iron and sulfur species and key environmental variables in the sediment profiles at different tidal levels. The distribution characteristics of total phosphorus (TP, 516.48±133.30 mg·kg^-1), organic phosphorus (OP, 162.58±56.91 mg·kg^-1), and calcium-bound phosphorus (HCl-P, 187.04±95.58 mg·kg^-1) in the sediments showed that the form of inorganic phosphorus was dominant in the sediments and that calcium-bound phosphorus was an important form of inorganic phosphorus. The distribution of phosphorus in the sediments was significantly affected by the frequency of water inundation and human activities. The vertical variations in contents of iron-bound phosphorus and organic phosphorus were minor in the natural sediment profiles, while their contents changed significantly in the sediment profiles disturbed by human activities. In addition, the iron-bound phosphorus (NaOH-P) was significantly positively correlated with the trivalent iron ions (r=0.64) in the sediment profiles of the middle and low tide flats, indicating the controlling role of iron oxides as the carrier of phosphorus in the intertidal zone sediments. Concurrently, the dissimilatory iron reduction process disrupted the mineral structure by converting Fe(Ⅲ) to Fe(Ⅱ) and released bound phosphorus, which was particularly significant under the dynamic redox conditions induced by periodic water inundation in the intertidal zones. There was significant positive correlation between sulfate and OP content in the surface sediments, indicating the indirect regulation of sulfur cycling on phosphorus bioavailability. In the micro-reducing environment formed by the rising tide, the enhanced activity of sulfate-reducing bacteria facilitated the release of soluble phosphorus through sulfide-mediated iron oxide desorption and organic matter mineralization. As the largest coastal wetland in northern China, the Liaohe River Estuary has significantly higher total phosphorus content in sediments than other bays in the Bohai Sea, with a high risk of eutrophication. These results offer critical theoretical foundations for understanding phosphorus cycling mechanisms and advancing eutrophication risk control strategies in coastal wetlands.

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