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.

中文翻译：

---

检查沟渠田间磷转移的来源和途径

了解风暴期间溶解磷 (P) 的动员和运输过程对于管理具有明渠排水和遗留土壤磷的平坦景观中的磷至关重要。 在这项研究中，我们在沟渠排水场地使用了常规基流监测和密集风暴采样在马里兰州的下东海岸（2017 年 7 月至 2018 年 9 月）评估浓度-排放 (CQ) 关系以及化学和同位素水文过程线分离是否可以深入了解沟渠排水中溶解磷的动员和运输过程。使用分段回归模型，我们确定溶解 P 的长期 CQ 关系在 6.4 L s ^–1的排放阈值上下不同。密集风暴采样显示，小风暴 ( n = 3) 发生在或低于排放阈值时通常表现出复杂的滞后和溶解的 P 稀释模式，这与更深（> 122 cm）的地下水输入一致，溶解的 P 浓度低（0.04 mg L ^–1）。相比之下，远高于排放阈值 ( n  = 4) 的大风暴导致地下水位上升和优先流动路径，最有可能利用溶解的富含磷的浅层 (<20 cm) 土壤水 (0.89 mg L ^–1)，产生一致的顺时针滞后和溶解的 P 冲洗模式。值得注意的是，在两次最大的暴风雨中，化学和同位素水位图的分离显示出大量的事件水份（59-68％），强烈暗示了通过优先流动途径快速输送溶解的P的作用。研究结果强调需要减轻垂直 P 分层，作为减少来自沟渠排水景观与遗留 P 的溶解 P 冲刷的一种手段。