Biogeochemical indices of phosphorus retention and release by wetland soils and adjacent stream sediments

Eutrophication is still a water quality problem within many watersheds. The Lake Okeechobee Basin, Florida, USA, like many watersheds is impacted by eutrophication caused by excess phosphorus (P). To meet water quality criteria to reduce this impairment, several levels of information on P dynamics within the Basin are required. The use of biogeochemical indices to help determine P retention/release of different landscape units such as wetlands and streams provides useful information on P dynamics. The objective of our study was to determine P retention/release indices for a range of wetland soils and their adjacent stream sediments. We sampled several wetlands and adjacent streams within Okeechobee's Basin, which represented a range of P impacted systems. Regression analyses suggest that a single incubation of sediment/soil equilibrated at 1000 mg P kg(-1) was sufficient (> 96% of the time) to estimate maximum P sorption capacity (S-max). Using this single incubation, sampled wetlands had nearly twice the P sorption capacity (238 +/- 21 mg P kg(-1)) of stream sediments (146 +/- 14 mg P kg-1). Stream sediments also had a greater P saturation ratio (PSR) than wetland soils, indicating that sediment had a greater potential to release P. Phosphorus sorption under ambient P conditions (soil equilibrated with ambient site water) covaried best with P concentrations in site surface water and, as concentrations increased, P sorption also increased. Finally, we used soil P storage capacity (SPSC) to help estimate the ability of soils and sediments to retain additional P loadings and found that wetland soils had a greater ability to retain P. Phosphorus sorption was predicted equally well (> 73%) using either ammonium oxalate or 1 M HCl extractable Fe and Al. The use of indices to quantify P dynamics of different landscape units can inform watershed management and policies aimed at reducing P loads to receiving water bodies.