Addressing phosphorus hotspots using controlled drainage

Agricultural phosphorus (P) hotspots can be defined as localized areas of elevated soil test P (STP) concentration resulting from P application rates exceeding crop removal. They typically form over prolonged periods or following intensive P application and may enhance edge-of-field P loss. This study evaluated the individual and combined impacts of P hotspots and controlled drainage (CD) on edge-of-field P losses through subsurface drainage using a paired-field before–after–control–impact (BACI) design. Two adjacent farm fields in the Western Lake Erie Basin (Ohio) with identical agronomic management practices were monitored over four years (2020–2023). A P hotspot was induced in the “impact field” by applying P at 102 kg P/ha per event over two application events to 8% of the drainage area. Consequently, the surface layer (0–5 cm) STP concentration within the hotspot doubled (55 to 119 mg P/kg). Moreover, the average weekly total P (TP) concentration in subsurface drainage increased from 0.33 to 0.42 mg/L but increases in dissolved reactive P (DRP) were not statistically significant. When CD management was applied to the hotspot field, average weekly drainage discharge decreased by 79% (0.27 to 0.05 mm). During this period, average weekly DRP and TP concentrations in drainage discharge significantly increased, yet the reduced drainage volume led to an 89% decrease in weekly DRP and a 97% decrease in TP loads. These results indicate that reduced drainage discharge under CD resulted in substantially lower subsurface P loads, despite elevated P concentrations.