PHOSPHORUS RETENTION IN NON-TIDAL PALUSTRINE FORESTED WETLANDS OF THE MID-ATLANTIC REGION

We: 1) quantified the areal extent of wetlands by type in Caroline County, VA to estimate the relative importance of non-tidal palustrine forested wetlands as a component of the wetland resources in Virginia's mid-Atlantic coastal plain, 2) used a comparative literature review to identify factors that might be important in controlling P retention in these wetlands, and 3) evaluated three indices that have been used to compare P adsorption potentials in wetland soils. Non-tidal palustrine forested wetlands comprised 66.0% of the 11,372 ha of wetlands in Caroline Co. The majority were either seasonally (4000 ha) or temporarily (1596 ha) flooded wetlands dominated by broad-leaved deciduous species. These wetlands are both small (averaging 5.4 and 2.4 ha in area, respectively) and numerous in this region. Adsorption by Al and Fe (hydr)oxides and precipitation of Al, Fe, and Ca phosphates probably controls dissolved phosphate retention and long-term P storage in these wetlands, although P storage in the woody biomass of aggrading forests may also be important. Phosphorus adsorption isotherms, a single-point phosphorus adsorption index, and concentrations of oxalate-extractrable (noncrystalline) Al and Fe have all been used as comparative indices of P sorption potentials in wetland soils, although the latter may represent the best single index of P sorption capacity because of its direct relationship to the mechanisms controlling P adsorption in soil. When using these indices to compare wetland P sorption potentials, it is important to consider differences in soil depth, profile heterogeneity, and bulk density between sites. Actual amounts of phosphate retained by a wetland will also be influenced by the degree of phosphate loading it receives, as determined by hydrology, phosphate sources in the watershed, and the P sorption potentials of surrounding upland soils. Because of the low P sorption potentials of sandy soils in coastal plain uplands, non-tidal palustrine forested wetlands in the mid-Atlantic coastal plain may perform particularly important functions with respect to dissolved phosphate removal and retention on the landscape.