<p>Chemical sequential extraction and high-resolution diffusive gradients in thin-film (DGT) techniques were employed to study the mobilization behavior and underlying mechanisms of phosphorus (P) in estuarine sediments. This study focused on two contrasting subtropical estuaries in the Maowei Sea, northern Beibu Gulf: the Maoling River Estuary (MLRE, dominated by natural processes) and the Qin River Estuary (QRE, influenced by coastal shrimp pond effluent). Sediment samples were collected in December 2020 (winter) and July 2021 (summer). Overall, the contents of inorganic P, organic P (OP), and DGT-labile P in core sediments of the QRE were significantly higher than those in the MLRE. Notably, the average concentration of DGT-labile P in the QRE sediments (7.82 ± 4.94&#xa0;µmol L⁻¹) was approximately 4.6 times greater than that in the MLRE sediments (1.69 ± 0.96&#xa0;µmol L⁻¹). During the investigation, the QRE sediments consistently acted as a net source of P to the overlying water. In contrast, the MLRE sediments functioned as a source of P in winter but transitioned to a weak sink in summer. Sedimentary P release was driven by dissimilatory sulfate reduction and dissimilatory Fe (III) reduction, regulated mainly by the content and speciation of sedimentary P and sedimentary organic matter (SOM). In the MLRE, where sedimentary P and SOM contents were relatively low, P release was governed by the migration and transformation of multiple P forms, including exchangeable P (Ex-P), Fe-bound P (Fe–P), OP and authigenic apatite P (Ca–P). In comparison, due to shrimp pond effluent inputs, the QRE exhibited higher sedimentary P and SOM contents, and P release was primarily controlled by Fe–P mobilization. These results highlight that coastal shrimp aquaculture can supply substantial amounts of high-quality SOM and P to estuarine sediments, significantly altering sedimentary P cycling.</p>