Non-Growing Season Phosphorus Losses on the Eastern Shore - New Technologies to Identify the Scope of the Problem.

Phosphorus (P) management in ditch-drained agroecosystems on the Delmarva Peninsula presents unique challenges, especially during the non-growing season when high water tables and wet conditions can exacerbate subsurface P transfers. To address these challenges, we combined time-lapse electrical resistivity imaging (ERI) with conservative tracers to characterize subsurface hydrologic flow pathways that link legacy soil P sources with surface waters. The research site is located on Maryland’s Lower Eastern Shore, an intensely ditch-drained region of the Delmarva Peninsula that exemplifies the challenges of minimizing runoff P losses in areas where subsurface flows drive P transport. In this presentation, we summarize the results of several ERI/tracer studies that allowed us to examine the spatial extent, structure, and connectivity of subsurface hydrologic flow paths tied to P loss. We also highlight the potential of using stable water isotopes and conservative tracers in simple mixing models to determine the proportional contribution of subsurface flow to ditch discharge. Collectively, these studies will enable an improved understanding of hydrologic, soil, and management factors that determine the timing, extent, and duration of shallow subsurface hydrologic connectivity in ditch-drained systems with P-enriched soils. We close the presentation with some thoughts on potential management strategies that can be used in flat, drained landscapes to mitigate non-growing season P losses.