Water Quality Impacts of Wetland Restoration: What's the Source of All of That Phosphorus?.

Wetland restoration is done, in part, to improve water quality. However, in cases where wetlands are restored using agricultural land left high in P from years of fertilization, saturated and reduced soil conditions may cause P to be released from the wetland to nearby surface waters and foster eutrophication. In order to protect water quality during wetland restoration, a better understanding of changes to the soil P pool following wetland restoration is needed. The objective of this study, in conjunction with a study site phosphorus balance, was to determine the change in soil total P (TP_soil), and soil P fractions over the 8 years since the study site was restored to wetland. The study was conducted at a Carolina bay wetland that is representative of most wetlands restored from agricultural land in the southeastern U.S. The 256 ha research wetland, Juniper Bay, was restored from agricultural soils in 2004 after being cleared of timber, drained, fertilized, and placed under row crop production for up to 30 years. This caused an accumulation of soil P in the upper 1 m of soil.  Once restored, excess P exited the wetland, increasing drainage water P concentrations to .300 mg P L^-1, three times the concentration known to contribute to eutrophication. To determine the source of this P, a series of extractions were performed on archived, pre-restoration samples, and again at the same locations eight years after restoration. Soil P determinations included nitric-perchloric acid digestion (TP_soil) and Mehlich III P (plant-available P) extractions of soil P at depths of 0-15 cm, 15-30 cm, and 30-100 cm in depth. The results of this study will be used to better understand changes to the soil P pool, and determine potential sources of P contributions to surface waters.