Phosphorus (P) leaching from agriculture is a major driver of water eutrophication in downstream rivers and lakes. In drained lowland areas with intensive agriculture, a reduction in the fertilizer applications may be insufficient to improve the water quality in the short term as the P accumulated in the soil during decades of high fertilization may continue leaching for many years. A complementary approach to reduce P exports from agriculture is to implement edge-of-field mitigation measures at the farm scale. The selection of effective measures requires a detailed insight into the chemical and hydrological transport mechanisms. Here, we determined the main P sources, processes, and transport routes at the farm scale to support the selection of appropriate mitigation measures. We quantified the legacy P, the different P pools stored in the upper soil, and related it to the yearly P export downstream. To do this, we combined high-resolution monitoring data from the soil, groundwater, surface water, and ditch sediments. The legacy P in the topsoil was high, about 2500 kg ha⁻¹. The predominant subsurface flow and the subsoils’ P sorption capacity retained the P mobilized from the topsoil and explained the relative moderate flux of P to surface waters (0.04 kg ha⁻¹ during the 2018–2019 drainage season). The dissolved P entering the drainage ditch via groundwater discharge was bound to iron-containing particles formed due to the oxidation of dissolved ferrous iron. Once leached from the soil to the drainage ditch, resuspension of P-rich sediment particles during flow peaks were the most important P transport mechanism (78%). Therefore, we expect that hydraulic constructions that reduce flow velocities and promote sedimentation of P-containing particles could reduce the export of P further downstream.

从农业中浸出的磷是下游河流和湖泊水富营养化的主要驱动力。在集约化农业的干旱低地地区，减少肥料的使用可能不足以在短期内改善水质，因为在数十年的高施肥过程中，土壤中积累的磷可能会持续多年淋失。减少农业磷出口的一种补充方法是在农场范围内实施田间减缓措施。选择有效措施需要深入了解化学和水文传输机制。在这里，我们确定了农场规模的主要P来源，过程和运输路线，以支持选择适当的缓解措施。我们量化了遗留的P 上部土壤中储存的不同磷库，并将其与下游的年磷出口相关。为此，我们结合了来自土壤，地下水，地表水和沟渠沉积物的高分辨率监测数据。表层土壤中的遗留磷很高，约2500千克公顷^-1。主要的地下流量和地下土壤对P的吸附能力保留了从表层土壤中迁移出来的P，并解释了P向地表水的相对中等通量（2018-2019排水季节为0.04 kg ha ^-1）。经由地下水排放进入排水沟的溶解磷与由于溶解的二价铁的氧化而形成的含铁颗粒结合。一旦从土壤浸出到排水沟中，富P沉积物颗粒在流量峰值期间的重新悬浮是最重要的P运移机制（78％）。因此，我们预计降低水流速度并促进含P颗粒沉降的水力建筑将减少P向下游的出口。