Legacy phosphorus concentration-discharge relationships in surface runoff and tile drainage from Ohio crop fields

Legacy phosphorus (P) in agricultural soils can be transported to surface waters via runoff and tile drainage, where it contributes to the development of harmful and nuisance algal blooms and hypoxia. However, a limited understanding of legacy P loss dynamics impedes the identification of mitigation strategies. Edge-of-field data from 41 agricultural fields in northwestern Ohio, USA, were used to develop regressions between legacy P concentrations (C) and discharge (Q) for two P fractions: total P (TP) and dissolved reactive P (DRP). Tile drainage TP concentration (C-TP) and DRP concentration (C-DRP) both increased as Q increased, and C-TP tended to increase at a greater rate than C-DRP. Surface runoff showed greater variation in C-Q regressions, indicating that the response of TP and DRP to elevated Q was field specific. The relative variability of C and Q was explored using a ratio of CVs (CVC/CVQ), which indicated that tile drainage TP and DRP losses were chemodynamic, whereas losses via surface runoff demonstrated both chemodynamic and chemostatic behavior. The chemodynamic behavior indicated that legacy P losses were strongly influenced by variation in P source availability and transport pathways. In addition, legacy P source size influenced C, as demonstrated by a positive relationship between soil-test P and the C-TP and C-DRP in both tile drainage and surface runoff. Progress towards legacy P mitigation will require further characterization of the drivers of variability in C-TP and C-DRP, including weather-, soil-, and management-related factors.