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.

Incorporating Rainfall-Runoff Events into Nitrate-Nitrogen and Phosphorus Load Assessments for Small Tile-Drained Catchments

“…For both P forms monitored in drainage waters, no such differences were found. During the REs, a dilution of N-NO 3 in drainage waters usually prevailed, whilst for both P-PO 4 and P tot , the concentrations usually rose with elevated discharge, which is in accordance with many monitoring campaigns in tile-drained catchments [5,12,18,25,31]. The only exceptions were sites PD1 and PD2, with Cfw N-NO 3 higher in ESs than in RSs.…”

“…This means that, for both the P-PO 4 and P tot loads, the runoff pattern seems to play in general a more dominant role than land use within the monitored catchments, as discovered also by other studies in similar and different climate and cropping systems [5,9,12,19]. The differences in total runoff in sites with similar geographical conditions were caused most probably by the different areas of their recharge zones [43].…”

“…Given that the average duration of an RE within a hydrological year was 30 (5.3 to 81.3) days across all the sites and seasons, the loss of phosphorus by subsurface runoff should be given considerable attention in central European tile-drained catchments, as recommended by many other studies in tile-drained landscapes of diverse natural and agricultural characteristics [5,6,8,10,34,35]. Since the algorithms that do not use a continuous record of flow rates (M1 to M3) has considerable poorer performance, yielding biased and imprecise results, the use of continuous flow measurement is necessary to capture discharge dynamics and to calculate the real nutrient fluxes from small tile-drained catchments with reasonable certainty [25,27,33].…”

“…Intensively managed tile-drained landscapes have been found to have a considerable potential for N and P losses, with regard to different soil conditions and cropping systems, especially during periods with elevated flows [3][4][5][6][7][8][9]. Nutrient concentrations in drainage waters may change rapidly within a rainfall-runoff event due to variable pre-event and event soil/catchment moisture conditions, related soil biogeochemical processes [10][11][12][13], precipitation characteristics, and water flow paths into drainage and with different origin and residence times [14][15][16][17].…”

“…These studies often conclude that the most dominant factors influencing nutrient fluxes are precipitation characteristics and catchment hydrological connectivity. This pertains, compared to nitrogen, to a greater extent to phosphorus, as high P concentrations in water from land drainage are associated predominantly with elevated flows, both from ploughland and grassland [4,5,[32][33][34]. Load assessment approaches without continuous discharge measurements and event sampling tend to underestimate P loads especially [33,35].…”