Phosphate removal capacity of palustrine forested wetlands and adjacent uplands in Virginia

We examined the ability of soils in six nontidal palustrine forested wetlands (PFOs) in Virginia's Piedmont (PD) and Coastal Plain (CP) (n = 3 per physiographic province) to remove dissolved inorganic P from solution, and we compared the P sorption capacities of wetlands with those of streambanks (within wetlands) and adjacent uplands. We hypothesized that wetland soils would have higher P sorption capacities than streambank and upland soils due to the higher concentration of noncrystalline (oxalate-extractable) Al and Fe (Al-o and Fe-o) favored by periodic flooding. We found that P sorption capacities varied both as a function of landscape position and soil depth. Wetlands had higher P sorption capacities than uplands in surface soils (0-15 cm), while below 50 cm the relationship was reversed. Streambank areas within wetlands generally had the lowest P sorption capacities. As hypothesized, Al-o was correlated with P sorption capacity in wetland soils (r(2) = 0.80) but so was soil organic matter (as estimated by mass loss on ignition [LOI]) (r(2) = 0.78); in fact, Al-o and organic matter were positively correlated in wetland soils (r(2) = 0.83). In contrast, clay and silt content were the two soil parameters most highly positively correlated with P sorption capacity in upland soils (r(2) = 0.87). Overall these results suggest that differences in soil chemistry exist among landscape positions (wetland, streambank, upland) that have important implications with regard to P sorption capacity. Since wetlands and uplands may remove P from different hydrologic sources (i.e., surface runoff in wetlands and groundwater in uplands), hydrology may be a key factor in determining water quality functioning.