title: Phosphorus transport in agricultural subsurface drainage: a review.
abstract: Phosphorus (P) loss from agricultural fields and watersheds has been an important water quality issue for decades because of the critical role P plays in eutrophication. Historically, most research has focused on P losses by surface runoff and erosion because subsurface P losses were often deemed to be negligible. Perceptions of subsurface P transport, however, have evolved, and considerable work has been conducted to better understand the magnitude and importance of subsurface P transport and to identify practices and treatments that decrease subsurface P loads to surface waters. The objectives of this paper were (i) to critically review research on P transport in subsurface drainage, (ii) to determine factors that control P losses, and (iii) to identify gaps in the current scientific understanding of the role of subsurface drainage in P transport. Factors that affect subsurface P transport are discussed within the framework of intensively drained agricultural settings. These factors include soil characteristics (e.g., preferential flow, P sorption capacity, and redox conditions), drainage design (e.g., tile spacing, tile depth, and the installation of surface inlets), prevailing conditions and management (e.g., soil-test P levels, tillage, cropping system, and the source, rate, placement, and timing of P application), and hydrologic and climatic variables (e.g., baseflow, event flow, and seasonal differences). Structural, treatment, and management approaches to mitigate subsurface P transport-such as practices that disconnect flow pathways between surface soils and tile drains, drainage water management, in-stream or end-of-tile treatments, and ditch design and management-are also discussed. The review concludes by identifying gaps in the current understanding of P transport in subsurface drains and suggesting areas where future research is needed.
authors.forename: Kevin W
authors.initials: KW
authors.lastname: King
authors.forename: Mark R
authors.initials: MR
authors.lastname: Williams
authors.forename: Merrin L
authors.initials: ML
authors.lastname: Macrae
authors.forename: Norman R
authors.initials: NR
authors.lastname: Fausey
authors.forename: Jane
authors.initials: J
authors.lastname: Frankenberger
authors.forename: Douglas R
authors.initials: DR
authors.lastname: Smith
authors.forename: Peter J A
authors.initials: PJ
authors.lastname: Kleinman
authors.forename: Larry C
authors.initials: LC
authors.lastname: Brown