The Influence of Seawater and Sulfate Reduction on Phosphate Release from Tidal Wetland Soils in the St. John’s River, Florida

Climate change and increasing sea level elevation are predicted to increase salinity in estuarine tidal wetlands in the Southeastern United States. Since much of the ecosystem function in these areas is predicated upon salinity regimes, many fundamental changes are likely to occur as a result. The influence of salinity and SO4 2- reduction on PO4 3- release from tidal wetland soils was evaluated along a salinity gradient at three sites in The St. John’s River, Florida using both field and laboratorybased methods. Porewater was sampled over the course of 10 months to determine ambient levels of SO4 2- and PO4 3-. Lab-based experiments, soils samples were subjected to seawater and SO4 2- treatments in an attempt to induce PO4 3- release. Salinity was lowest at Sixmile Creek (0.45 ± 0.1 g kg-1) and Goodby’s Creek (2.05 ± 2.3 g kg-1) and much higher at Sister’s Creek (27.81 ± 3.1 g kg-1). The organic content of soils was highest (82.35% ± 5.11) at Sixmile Creek, intermediate at Goodby’s Creek (64.45% ± 7.02) and lowest at Sister’s Creek (32.11% ± 9.61). Total soil P was highest at the freshwater Sixmile Creek (1101.64 ± 220.2 μg g-1), intermediate at the brackish Goodby’s Creek (719.61 ± 114.3 μg g-1) and lowest at the Sister’s Creek saltmarsh (475.85 ± 110.9 μg g-1). Porewater PO4 3- was higher at Sixmile and Goodby’s Creek sites (9.44 ± 15.6, 8.99 ± 14.7 !g L-1, respectively) compared to Sister’s Creek (0.6 ± 3.1 !g L-1). Porewater SO4 2- was lower at Sixmile (70.73± 57.58 !g L-1) and Goodby’s Creeks (124.35 ± 152.5 !g L-1) compared to Sister’s Creek (1931.41 ± 557.82 !g L-1). Temporal and spatial trends indicated that SO4 2- and PO4 3- in porewater was likely due to floodwater content and that direct reaction between analytes in soils was unlikely. The addition of aerated seawater failed to cause PO4 3- release from any sites. The incubation of soils under anaerobic conditions, in the presence of Na2SO4 induced SO4 2- reduction, but inhibited PO4 3- flux from both Sixmile and Goodby’s Creek, which is attributed here to likely S- toxicity (Roychoudhury et al., 1999). PO4 3- flux from Sister’s Creek increased in association with Na2SO4 concentration, likely due to more Fe availability to mitigate Stoxicity. Ambient seawater additions to soils under anaerobic conditions followed a similar trend, but the results were not statistically conclusive. Overall, both field and labbased data indicated that Tidal wetland porewater PO4 3- likely originates from floodwaters and that increased salinity and SO4 2- reduction did not directly enhance soil PO4 3- fluxes