Sulfide Induced Mobilization of Phosphorus in Constructed Wetland Soils: Iron, Sulfide, Phosphate Interactions.

The biogeochemical cycling of iron and sulfide affect both the availability and mobility of phosphorus in wetland sediments. The concentration of sulfate is typically low in freshwater aquatic environments, however, sulfur cycling in these systems can be both spatially and temporally dynamic. This study evaluated the role of sulfide induced mobilization of phosphorus in constructed wetland sediments. The temporal (biweekly sampling) and spatial (3 wetland environments) variability of pore water chemistry (i.e., TDP, DRP, Fe²⁺/Fe³⁺, SO₄, Sulfide, pH, EC, and anions/cations) was measured within the wetland as a function of soil depth (i.e., 2.5, 10, 30 cm), redox status, and soil biogeochemical characteristics. Additionally, the temporal (early and late season sampling) and spatial variability of soil chemistry (i.e. sulfide chemistry [acid volatile sulfide, AVS; chromium reducible sulfur, CRS], iron chemistry [0.5M HCl, 6.0M HCl], P chemistry [TP, anaerobic P sorption isotherms]) was evaluated within the wetland as a function of depth. Results from this study show that the wetland sediment was highly reduced (Eh range: 200 to -200), with high concentrations of acid volatile sulfide in the surface (2.5 cm) layer (AVS: 2-9 μmol cm^-3). In general, the concentration of AVS decreased with depth at all sites, with 0-3 μmol cm^-3 sulfide and 0-1 μmol cm^-3 sulfide at 10 and 30cm respectively. Ferric iron increased with depth, while ferrous iron decreased with depth, indicating higher reducing conditions in the surface layers. High sediment deposition environments had a higher iron buffering capacity due to the continued influx and deposition of fine textured sediment enriched in iron oxides. However, these areas were also the most enriched in P due to high P concentrations in inflowing water. This study showed that sulfide induced P flux can play an important role in the retention and cycling of P in wetland sediments.