Constructed wetlands can be important sinks of P in agricultural landscapes; however, the long-term ability of these systems to retain P often diminishes with time. This study used a spatially explicit statistical approach to characterize spatial patterns of soil properties and their correlation with P sorption in an 11-yr-old, seasonally saturated, constructed wetland receiving runoff from irrigated agriculture. The results were used to link the spatial pattern of P sorption with hydrologic, biogeochemical, and pedologic processes. A spatial sampling design was used and soil samples were measured for total P, bioavailable P, P sorption index (PSI), total C and N, texture, oxalate- and dithionite- extractable Fe and Mn, pH, electrical conductivity, water-extractable Mg, Ca, and Na, and sedimentation rate. Wetland hydrodynamics was a primary factor shaping the spatial distribution of wetland soil properties, creating three distinct hydrologic and biogeochemical zones (e.g., sediment deposition, transition, and Fe oxide transformation). Soil properties with the strongest correlation to the PSI included oxalate-extractable Fe, clay, total C, and silt. In the sediment-deposition zone, sedimentation was the dominant process influencing P sorption, contributing fresh sediments with reactive surfaces for P sorption. In contrast, the Fe oxide transformation zone received little sedimentation, resulting in greater exposure time for surface sediment alteration. Increased exposure resulted in dissolution of crystalline Fe oxides and reformation of poorly crystalline Fe oxides in a thin oxidative lens at the sediment-water column interface, thus increasing P sorption capacity. This spatially explicit investigation of P sorption in seasonally saturated wetland soils provided a robust framework within which to evaluate the wetland soil processes controlling P sorption capacity and the efficacy for long-term sorption potential.
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