Spatiotemporal changes in soil phosphorus characteristics in a submerged aquatic vegetation-dominated treatment wetland

In South Florida, stormwater treatment areas (STAs) are used to reduce phosphorus (P) in runoff from agricultural areas before water is discharged into the Everglades Protection Area. The Everglades STAs retain a significant amount of P and play an important role in Everglades restoration. Wetland soils generally are long-term sinks for P; therefore, the sustainability of STA treatment performance can be assessed by tracking changes in soil characteristics. This study evaluated the spatiotemporal changes in soil P and related physicochemical characteristics in the unconsolidated floc and underlying surface soil layer (0-10 cm) of a 920-ha submerged aquatic vegetation (SAV)-dominated wetland (STA-2 Cell 3). Physicochemical properties in soil cores collected in 2003, 2007, 2009, and 2015 were evaluated and compared using geostatistical methods. Results indicated a gradual increase in floc depth over time. Total P (TP) concentrations in the floc were significantly higher than in the surface soil. Slight but statistically nonsignificant increases of mean TP in floc were observed. There was a significant increase in P storage in the floc layer between 2003 and 2007, with more P stored in the surface soil layer. Interpolated maps showed consistently higher TP and P storage values in the floc and surface soil near inflow areas of the cell during all sampling events. Furthermore, the 2003 to 2015 change maps showed TP and P storage decreasing from inflow to outflow. Bulk density (BD) in floc was approximately half of surface soil BD. Significant decline in the percentage of ash-free dry weight (AFDW) in floc from 2003 to 2007 indicated an increase in mineral content. This is consistent with increases in total calcium (TCa) in the floc, which was up to four times higher than in the surface soil layer. This indicates that TCa plays a central role in defining the characteristics of SAV cells. Overall, despite the heterogeneity of sediment attributes in the system, temporal trends and spatial patterns were observed in the physicochemical characteristics of soils. These trends and patterns can be used to understand long-term changes in large-scale treatment wetlands. Such insights are useful for optimizing and sustaining the treatment performance of STAs.