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

Authors: Osterholz, William - Will; Hanrahan, Brittany; King, Kevin

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2020
Publication Date: 3/31/2020
Citation: Osterholz, W.R., Hanrahan, B.R., King, K.W. 2020. Legacy phosphorus concentration-discharge relationships in surface runoff and tile drainage from Ohio crop fields. Journal of Environmental Quality. 49(3):675-687. https://doi.org/10.1002/jeq2.20070.
DOI: https://doi.org/10.1002/jeq2.20070

Interpretive Summary: Agricultural soils contain phosphorus from past fertilizer applications, also called legacy phosphorus. Surface runoff and tile drainage from agricultural fields can transport this legacy phosphorus to waterbodies where it causes harmful algae blooms. Additional understanding of the dynamics of this legacy phosphorus loss may improve the development and implementation of mitigation strategies. This study established relationships between legacy phosphorus concentrations and the daily amount of flow via surface runoff and subsurface tile drainage across 41 fields in northwest Ohio. For both surface runoff and tile discharge the concentration of legacy phosphorus increased as the flow rate increased. Second, phosphorus concentrations in tile drainage highly variable over the course of the monitoring, but were less variable in surface runoff. Overall this work shows that high daily flows caused by large precipitation events increased the concentration of legacy phosphorus leaving agricultural fields, and that transportation of legacy phosphorus through tile drains changes dramatically over time. This work provides insight into the drivers of harmful algae blooms in Lake Erie, and points to directions for future research efforts, particularly investigating the causes of changes in phosphorus concentrations in tile drainage.

Technical Abstract: Legacy 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 (HNABs) as well as 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 northwest (NW) Ohio, USA, were utilized to develop regressions between legacy P concentrations (C) and discharge (Q) for total P (TP) as well as dissolved reactive P (DRP). Tile drainage CTP and CDRP both increased as Q increased, and CTP tended to increase at a greater rate than CDRP. Surface runoff showed greater variation in C-Q regressions, indicating the response of TP and DRP to elevated Q was field-specific. The CTP and CDRP in both tile drainage and surface runoff demonstrated a positive relationship with soil test P. Additionally, the relative variability of C and Q was explored using a ratio of coefficients of variation (CVC/CVQ), which indicated that tile drain TP and DRP losses were chemodynamic while losses via surface runoff demonstrated both chemodynamic and chemostatic behavior. The chemostatic behavior indicates that legacy P losses were strongly influenced by variation in P source availability and transport pathways. These results highlight the need to focus mitigation efforts on hotspots of legacy P loss, particularly during high Q events. Progress towards legacy P mitigation will require further characterization of the drivers of variability in CTP and CDRP, including weather, soil, and management related factors.