Saltwater intrusion enhances phosphorus availability

Globally sea-level rise is expected to intensify saltwater intrusion into tidal freshwater marshes, ye the time-dependent effects of salinity on phosphorus (P) biogeochemistry remains inadequately understood. Herein, we conducted a field transplant experiment simulating saltwater intrusion to examine how salinity influences soil P availability, organic P mineralization, and phoD‐harbouring bacterial community in subtropical tidal marshes. Our results showed that simulated saltwater intrusion significantly elevated soil pH, electrical conductivity, porewater salinity, and concentrations of major ions (SO42- and Cl-), effectively altering the abiotic conditions governing P dynamics. Soil P availability increased significantly, with resin-P and NaHCO3-Pi concentrations rising, while less available P fractions (NaOH-Pi and NaOH-Po) declined. Saltwater intrusion inhibited soil organic P mineralization, especially during the later stage of transplant incubation. Notably, the abundance and diversity of phoD-harboring bacterial communities increased in transplanted soils during the early incubation period, indicating a functional compensatory response to salt stress. Over time, these communities shifted toward brackish-like compositions, driven by salinity acting as a deterministic environmental filter. Correlation analysis indicated that saltwater intrusion–induced ionic strength, the abundance and diversity of phoD-harboring bacteria, redistribution and key microbial taxa jointly regulated P fraction and availability in tidal marshes. Overall, this study reveals a strong “two-phase” conceptual framework: an initial shock of enhanced P availability driven by abiotic mobilization and microbial functional shifts, followed by a longer-term transition toward a brackish-like state in both microbial communities and P fractions. These findings deepen our understanding of P cycling in coastal marshes under future seawater intrusion scenarios, highlighting the critical roles of P fraction redistribution and time-dependent responses in regulating P availability.