Dissolution of Phosphorus Into Pore-Water Flowing Through An Organic Soil From a Wetland Restoration Site.

Understanding the dissolution and transport of phosphorus (P) from organic soils in areas devoted for wetland restoration is critical for devising management strategies to manage off-site P exportation. This study was conducted to evaluate the dissolution of P in water flowing through the vadose zone-shallow ground water continuum of an organic soil from a wetland restoration site. Three 90 × 50 × 8 cm flow cells were packed with organic surface soil material from a Ponzer muck (Terric Haplosaprists). The flow cells were instrumented with solution samplers and redox electrodes at 5 cm below, and at 5 and 20 cm above a simulated water table (WT) to collect soil solution and monitor the redox potential (Eh), respectively.  Distilled water was supplied at rates of 1.2, 2.4, and 3.6 L d^-1 to one end of the flow cells while maintaining a WT at 12 cm above the bottom.   Phosphorus concentration in the outflow was consistently above the USEPA water quality limit. Changes in pore-water velocity did not alter amounts of P leached within the duration of the experiment. Phosphorus dissolution at 5 cm below and 5 cm above the WT was significantly higher than at 20 cm above the WT. This indicated that P that was leached out of the flow cell originated from the lower part of the capillary fringe and the upper part of the saturated zone.  Data on dissolved reactive P, total iron, and dissolved organic carbon in soil solution, combined with Eh data and the relatively high soil aluminum content, point to the following possible mechanisms as largely responsible for the increase in solution P: i) increased ligand exchange of dissolved organic matter (DOM) for mineral adsorbed phosphate (PO₄) and ii) DOM-enhanced dissolution of Fe and Al with concomitant release of PO_4. These results indicate that off-site exportation of P could be minimized if the WT in a wetland restoration site containing high organic matter is kept below the P-enriched surface layers.