Effects of wetland types on dynamics and couplings of labile phosphorus, iron and sulfur in coastal wetlands during growing season

Wetland type plays an important role in controlling the phosphorus (P) biogeochemical cycle, while its effect on labile P dynamics and coupling with iron (Fe) and sulfur (S) in coastal wetlands remains unclear. In this study, chemical sequential extraction and high-resolution diffusive gradients in thin-film (DGT) techniques were employed to investigate P forms, mobilization, and labile Fe-S-P coupling in several coastal wetland types [i.e., natural wetland (NW), aquaculture pond (AP), artificial (ARW) and natural restored wetlands (NRW)]. Compared with NW, AP decreased the total P by 40.6%. The concentrations of soil organic P and inorganic P (including NaOH-extractable P and HCl-extractable P) were significantly increased in ARW, but decreased in AP and NRW. DGT-labile P, Fe, and S concentrations changed significantly in different wetland types, and the labile P concentrations in AP were significantly higher than those in the others. Similar spatial distribution dynamics and significant positive relationships between labile P, Fe, and S concentrations in NW and AP confirmed that intense reduction in iron and sulfate are the key mechanisms regulating P mobilization. However, these relationships were decoupled in restored wetlands, suggesting that the Fe redox-coupled P mobilization and sulfate reduction were sensitive to wetland changes. The diffusion fluxes of P across the soil-water interface were positive in AP (0.619 pg·cm-2·s-1), indicating that P was released from soil to the overlying-water. We concluded that coastal wetland types altered soil P forms, availability, and labile Fe-S-P coupling, and the natural restored wetland could help stabilize the soil P pool and eventually controlled the mobilization and release of P.