Christopher Brownfield, D. Hesterberg, and M. Vepraskas. Department of Soil Science, NC State University, PO Box 7619, Raleigh, NC 27695-7619
Excessive phosphorus mobility may result when agricultural land is converted to a wetland. Strategies for predicting and ultimately preventing reductive dissolution and discharge of P from such soils under reducing hydrological conditions would benefit from understanding the dissolution mechanisms involved. Our research objective was to measure reductive dissolution of P from soil samples from a drained wetland (Carolina bay) in southeastern North Carolina, and relate P dissolution to P speciation and other soil properties. The <53 µm fraction of field moist surface-soil samples from organic and mineral soils at Juniper Bay were subjected to microbial reduction in a continuously-stirred reactor. The soil samples contained total P concentrations between 8 and 15 mmol kg-1 (248 to 465 mg P kg-1) in the < 2mm fractions, and 19 and 72 mmol kg-1 (589 to 2232 mg P kg-1) in the < 53µm fractions. Citrate-bicarbonate-dithionite-extractable iron (Fecbd) ranged from 7 to 60 mmol kg-1 in the < 53µm fraction, and the ratio of acid ammonium oxalate-extractable iron to Fecbd was > 0.76 for the < 53µm fraction in all samples. Despite the presence of poorly-crystalline Fe-oxides or organic-matter associated Fe, initial phosphorus K-XANES results indicated that phosphorus was not predominantly associated with iron. Trends in reductive dissolution of P in relation to Fe, Al, and organic matter dissolution will be discussed. Knowledge of soil P speciation in relation to reductive dissolution will be useful for gauging and mitigating P impacts on water quality as a result of wetland restoration at Juniper Bay.
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