Organic phosphorus mineralization characteristics in sediments from the coastal salt marshes of a Chinese delta under simulated tidal cycles

Purpose Sediment organic phosphorus (OP) mineralization plays an important role in phosphorus cycling in coastal salt marshes. However, information on OP mineralization characteristics in sediments under tidal cycles is limited. Materials and methods Sediment cores were collected from salt marshes which were exposed to tidal cycles. A laboratory manipulation experiment was conducted under simulated tidal flooding cycles. After the completion of each incubation experiment, the inorganic phosphorus (IP) and OP fractions were analyzed and net phosphorus mineralization rates were calculated to investigate the sediment OP mineralization process. Sediment microbial characteristic and chemical properties were determined to identify factors that affect the mineralization of OP in the Yellow River Delta. Results and discussion During the first 2 days of incubation, the tidal cycles significantly increased the microbial biomass in the sediments, which accelerated the assimilation of IP by microorganisms and the transformation of IP into OP in their bodies, thereby causing a significant increase in OP in the sediments. After 2 days of incubation, the mineralization and immobilization of OP entered the adaptation stage and gradually reached the dynamic balance of various phosphorus fractions in sediments. The path analysis showed that microbial biomass phosphorus and carbon (MBP and MBC) and the activity of alkaline phosphatase (AKP) are the direct factors influencing OP mineralization in sediments. The indirect effects of soil organic matter (SOM), water content (WC), exchangeable aluminum (Al-0), and exchangeable iron (Fe-0) on net phosphorus mineralization rate were relatively high. Conclusions In the laboratory experiments, we showed that the recovery of tidal cycles caused a fluctuation in OP mineralization and immobilization in sediments, which reached equilibrium over time. The tidal effect was conducive to maintaining the stability of phosphorus in sediments of coastal salt marshes. The findings of this study can contribute to protecting water quality and improving primary productivity in coastal salt marshes by regulating the tidal cycles and the key environmental factors (e.g., SOM, WC, Al-0, and Fe-0) influencing OP mineralization.