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Understanding the processes that mobilize and transport dissolved phosphorus (P) during storms is critical to managing P in flat landscapes with open ditch drainage and legacy soil P. In this study, we used routine baseflow monitoring and intensive storm sampling at a ditch-drained site on Maryland's Lower Eastern Shore (July 2017-September 2018) to assess whether concentration-discharge (C-Q) relationships and chemical and isotopic hydrograph separation could provide insight into the processes that mobilize and transport dissolved P in ditch drainage. Using a segmented regression model, we determined that long-term C-Q relationships for dissolved P differed above and below a discharge threshold of 6.4 L s -1 . Intensive storm sampling revealed that small storms (n = 3) occurring at or below the discharge threshold generally exhibited complex hysteresis and dissolved P dilution patterns that were consistent with deeper (>122 cm) groundwater inputs with low dissolved P concentrations (0.04 mg L -1 ). In contrast, large storms occurring well above the discharge threshold (n = 4) induced rising water tables and preferential flow pathways that most likely tapped dissolved P-enriched shallow (<20 cm) soil waters (0.89 mg L -1 ), producing consistent clockwise hysteresis and dissolved P flushing patterns.Notably, chemical and isotope hydrograph separation during two of the largest storms revealed significant event water fractions (59-68%) that strongly suggested a role for the rapid delivery of dissolved P via preferential flow pathways. Findings highlight the need to mitigate vertical P stratification as a means for reducing dissolved P flushing from ditch-drained landscapes with legacy P. INTRODUCTIONThe eutrophication of water bodies from excess phosphorus (P) inputs is a global water quality concern (Oelsner & Stets, 2019). In the eastern United States, agricultural lands Abbreviations: C-Q, concentration-discharge; FI, flushing index; HI, hysteresis index; TDP, total dissolved phosphorus; UMES, University of Maryland Eastern Shore.

Soil health practices can improve soil conditions and provide ecosystem services, but increased risk of phosphorus (P) loss can be an unintended consequence. We investigated conservation tillage and cover crops effects on soil P stratification, P accumulation at depth, and soil aggregation for sandy Coastal Plain soils from the Mid-Atlantic United States soil cores from 10 agricultural fields with 0-15 years of conservation tillage or cover cropping were analyzed for Mehlich-3 P and dry aggregate stability. We found no evidence that conservation tillage or cover cropping caused P stratification or accumulation in study fields that were already enriched with P prior to soil health implementation. Annual particulate, dissolved runoff, and leachate P loads decreased when estimated using the North Carolina Phosphorus Loss Assessment Tool assuming no-till and cover crops (soil health) compared to conventional till and winter fallow (conventional). We suggest that soil health practices are unlikely to exacerbate P losses from high P Coastal Plain soils beyond their initial risk profile.

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