Characterizing the potential of vegetated filter strips to retain dissolved phosphorus using rapid measures of soil P saturation

Abstract

In the state of Delaware, approximately 50% of the rivers have total phosphorus (P) concentrations greater than 0.1 mg L⁻¹, the concentration above which freshwater bodies are considered impaired by P. The structural best management practice commonly recommended by advisory agencies to control nonpoint P loss is the vegetated filter strip (VFS). In the Coastal Plain region of Delaware, where P saturated soils are commonly found, VFS should trap particulate P, but their effectiveness at preventing dissolved P losses is less certain. The objectives of this research were (1) to develop a rapid soil test to determine if a VFS has the potential to adequately mitigate dissolved P in agricultural runoff at a designated site and (2) to determine, in laboratory tillage simulation studies, if deep tillage prior to VFS planting can improve dissolved P retention within VFS. Soil samples were collected from existing VFS and adjacent fields, as well as from field margins where new VFS were to be planted, and were characterized for organic matter, water pH, water soluble P, Mehlich 3 extractable elements (P, Al, and Fe), and Mehlich 3 P Saturation ratio (M3-PSR). A subset of soils was selected to characterize P sorption and desorption properties of the VFS, including total P sorption capacity (PSC_t), remaining P sorption capacity (PSC_r), and the equilibrium P concentration at zero sorption (EPC₀). Both the ratio of PSC_r to PSC_t (an estimate of the percentage of remaining P sorption capacity) and EPC₀ were well correlated with M3-PSR (r²= 0.92, 0.93). Simulated deep tillage decreased M3-P by 62% compared with surface soils and decreased PSR by as much as 70%. Study results suggest that the M3-PSR soil test can lead to a better understanding of whether a VFS installed at a particular site can be expected to retain dissolved P in agricultural runoff. Likewise, M3-PSR can also be used to determine if deep tillage prior to VFS installation will be beneficial for overall dissolved P management.