AbstractPhosphorus (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.

Phosphorus Transport in Agricultural Subsurface Drainage: A Review

Kevin W. King,* Mark R. Williams, Merrin L. Macrae, Norman R. Fausey, Jane Frankenberger, Douglas R. Smith, Peter J. A. Kleinman, and Larry C. Brown

Nutrient enrichment is a primary water quality concern facing many local, regional, and national enti-ties throughout the world. Hypoxic and anoxic areas

in coastal, marine, and freshwater bodies worldwide continue to increase in size and frequency and are prompting concern on the deleterious effects to tourism, fisheries, and ecosystem func-tion in Gulf of Mexico (Rabalais et al., 2001), Baltic Sea (Conley et al., 2002), Black Sea (Diaz, 2001), Chesapeake Bay (Boesch et al., 2001), China Sea (Chen et al., 2007), the Great Lakes (Rockwell et al., 2005; Hawley et al., 2006), Lake Winnipeg (Schindler et al., 2012), and throughout the United Kingdom and Europe (Haygarth et al., 2000; Heathwaite and Dils, 2000). In many of these areas, diffuse or nonpoint sources are rapidly replacing point sources as the major focus of nutrient enrichment (Kleinman et al., 2011). Phosphorus (P) is generally the limit-ing nutrient in freshwater ecosystems, and excess P loading from nonpoint sources has been identified as one of the primary causes of freshwater eutrophication (Rockwell et al., 2005; Schindler et al., 2008). Nitrate-nitrogen has historically been linked to hypoxic areas in coastal waters, but recent evidence suggests that P may also play a critical role (Dodds, 2006). Total P concentra-tions ≥0.02 mg L-1 are often considered problematic (Correll, 1998). In the United States, water quality standards for P are determined at the state level. However, only Illinois, Minnesota, and Wisconsin have set statewide numerical criteria for P levels or critical P concentrations for specific surface water bodies (USEPA, 2014). A critical total P concentration of 0.03 mg L-1 has been established in the Canadian provinces of Quebec and Ontario to protect against eutrophication (Environment Canada, 2004). Additionally, total P standards ranging from

Abbreviations: DRP, dissolved reactive phosphorus; DWM, drainage water management; FD, free drainage; STP, soil test phosphorus.

K.W. King, M.R. Williams, and N.R. Fausey, USDA–ARS, Soil Drainage Research Unit, 590 Woody Hayes Dr., Columbus, OH 43210; M.L. Macrae, Univ. of Waterloo, Dep. of Geography and Environmental Management, 200 University Ave. W., Waterloo, ON N2L 3G1, Canada; J. Frankenberger, Purdue Univ., Dep. of Agricultural and Biological Engineering, 225 S. University St., West Lafayette, IN 47907; D.R. Smith, USDA–ARS, Grassland, Soil and Water Research Lab., 808 E. Blackland Rd., Temple TX 76502; P.J.A. Kleinman, USDA–ARS, Pasture System and Watershed Management Research Unit, Bldg. 3702, Curtin Rd., University Park, PA 16802; L.C. Brown, Ohio State Univ., Dep. of Food, Agricultural, and Biological Engineering, 590 Woody Hayes Dr., Columbus, OH 43210. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA. USDA is an equal opportunity provider and employer. Assigned to Associate Editor Zach Easton.

Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. J. Environ. Qual. doi:10.2134/jeq2014.04.0163 Received 14 Apr. 2014. *Corresponding author (kevin.king@ars.usda.gov).

Journal of Environmental QualityPHOSPHORUS FATE, MANAGEMENT, AND MODELING IN ARTIFICIALLY DRAINED SYSTEMS

SPECIAL SECTION

Published January 9, 2015

Journal of Environmental Quality

0.01 to 0.087 mg L-1 have been proposed for many specific water bodies spanning several ecozones across Canada (Chambers et al., 2009).

In agricultural landscapes, there are two primary pathways for P to enter surface waters: surface runoff and subsurface flow. Early work on P transport from agricultural fields focused on surface runoff and developing management practices that reduced soil erosion. Phosphorus transport via subsurface flow pathways was often deemed “negligible” due to low P concentrations and the affinity of subsoil to bind P (Baker et al., 1975). As late as 1980, the vertical movement of soluble P was generally dismissed as “tile drainage rarely contains more than trace quantities of soluble P” (Logan et al., 1980). Perceptions of subsurface P transport, however, have evolved over the last several decades. Phosphorus concentrations and loads measured in subsurface flow pathways began to receive attention in the 1990s and resulted in a review by Sims et al. (1998) that evaluated current research on subsurface P movement. The authors concluded that subsurface transport of P can be significant in watersheds with high soil P levels (e.g., animal and specialty crop watersheds), low P sorption capacity (e.g., watersheds with sandy soil and soils with high organic matter content), and artificial drainage (e.g., tiles and ditches). Since the review by Sims et al. (1998), many studies have been completed to highlight the importance of subsurface P transport, to identify the important processes and mechanisms that influence subsurface P movement, and to evaluate practices and treatments that decrease subsurface P loads to surface waters. The objectives of this paper are (i) to provide a comprehensive review and critical evaluation of P transport in subsurface tile drainage, (ii) to determine factors that control P losses, and (iii) to identify gaps in the current scientific understanding of the role of tile drains in P transport. Measuring P concentrations and loads in tile drains is a commonly used approach to quantify vertical movement of P through soils. Subsurface drainage is also considered “the most extensive soil and water management activity in agriculture” (Pavelis, 1987). Approximately 43 million ha of land in the United States and 16 million ha in Canada either require or benefit from subsurface drainage (Skaggs et al., 1994). The benefits of subsurface drainage include (i) greater water infiltration; (ii) increased soil aeration, bearing strength, temperature, and nitrate-nitrogen production; (iii) promotion of germination, deeper root growth, and greater crop yields; (iv) reduced surface erosion; and (v) an effectively longer growing season (Fausey, 2005; Irwin, 1986, 1989; Smedema and Rycroft, 1983).

The initial section of this review examines the need, extent, and status of subsurface drainage in North America. This is followed by a review of the factors that are known to significantly affect subsurface P transport. Subsurface P transport as affected by 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 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) are discussed within the framework of intensively drained agricultural settings. A review of structural, treatment, and management approaches to mitigate subsurface

transport of P is also presented. These include practices that disconnect fast flow pathways between surface soils and tile drains, drainage water management, in-stream or end-of-tile treatments, and ditch design and management. The paper concludes by indicating future research directions that can contribute to an improved process-based understanding of subsurface P transport.

The Need, Extent, and Status of Subsurface Drainage in North America

A significant portion of the world’s most productive agricultural lands are located in regions where wetlands were once the primary landscape feature. For example, it is estimated that there were 18 million ha of wetlands in the Mississippi River Basin at the time of settlement (Dahl, 1990). The prevalence of wetlands in the midwestern United States and eastern Canada (i.e., Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, Newfoundland, and Labrador) was due to the dominance of fine, glacially derived soils with poor drainage (Skaggs et al., 1994; Madramootoo et al., 2007). During the past 150 yr, surface ditches and subsurface drains have been widely installed across these regions to facilitate settlement and to increase crop production on lands converted to agriculture by removing excess water from the soil profile. In many locations, subsurface drainage is required to make agricultural production possible. In other areas, adding subsurface drainage can increase crop yields by 5 to 25% annually (Eidman, 1997). Drainage reduces the risk of crop loss from excess water stress, decreases crop susceptibility to pests and diseases, and provides more uniform crop production amid climate variability (Blann et al., 2009). Farmers also have greater control of field operations, including earlier planting, less soil compaction, increased trafficability, and a wider choice of crops and crop varieties (Spaling and Smit, 1995; Kornecki and Fouss, 2001; Fausey, 2003, 2005).

Early settlers to North America came from Europe where wet soils had been drained for centuries. Tile drainage was introduced in the United States by John Johnston in the 1820s (Weaver, 1964). The first tile drains in the midwestern United States were installed in the late 1800s and consisted of pipes or sections of clay, concrete, or wood. Traditionally, subsurface drains were used strategically to drain wet areas in fields where surface water tended to pond. Extensive tile drainage in the north-central region of the United States was installed between 1870 and 1920 and between 1945 and 1960 (Zucker and Brown, 1998) and continues at a fast rate into the present. Since the 1970s, drainage pipes made from plastic tubing began to replace concrete and clay tile, and the random drainage of wet spots has increasingly been supplanted by systematic draining of entire fields. Subsurface drain spacing has also continued to decrease because farmers are placing additional drains between existing drains (Hubbard, 2005). Modern farming practices involving fewer farmers, larger fields, and larger equipment producing annual cash grain crops of corn (Zea mays L.) and soybeans (Glycine max L.) has created demand for more intensive drainage to ensure economical agricultural production (Madramootoo et al., 2007).

It is estimated that there are currently 8 million ha of drained land in Canada (Shady, 1989). Zucker and Brown (1998)

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indicate that between 18 and 28 million ha of cropland in the midwestern United States has subsurface tile drainage. The 2012 Agricultural Census included questions on farm drainage for the first time in 40 yr, which will provide updated information. Although data on the extent of subsurface drainage have not been collected in the United States for several decades, the total number of hectares reportedly drained with subsurface tiles has likely greatly increased (Sugg, 2007). One of the largest changes over this time has likely been in the intensity of drainage in these regions, which continues to increase (Blann et al., 2009).

Contribution of Subsurface Drainage to Phosphorus Transport

The addition of tile drains to previously undrained land can substantially alter the total water yield from a field or small watershed as well as the timing and shape of the hydrograph (Schilling and Helmers, 2008). Tile drainage tends to increase total water yield between 10 and 25% because it often increases the proportion of annual precipitation that is discharged to surface waters via subsurface flow relative to the amount that is stored, evaporated, or transpired (Serrano et al., 1985; Magner et al., 2004; Tomer et al., 2005). Thus, several studies suggest that discharge from tile drains constitutes the majority of stream flow in many agricultural watersheds across the midwestern United States and Canada. For example, King et al. (2014a) found that tile drainage contributed 51% of annual stream flow in a headwater watershed in Ohio. An estimated 42% of annual watershed discharge originated from tile flow in a watershed in Ontario, Canada (Macrae et al., 2007). Culley and Bolton (1983) and Xue et al. (1998) also estimated that 60 and 86%, respectively, of stream flow was derived from tile drains.

Although total water yield from a field tends to increase with the installation of subsurface drainage, surface runoff and sediment yield is often significantly decreased (Robinson and Rycroft, 1999; Skaggs et al., 1994; Dolezal et al., 2001). Several studies have noted that surface runoff is rarely observed in tile drained fields and only occurs during very wet conditions when the quantity of water on the fields exceeds the capacity of the subsurface drainage system (Dolezal et al., 2001; Macrae et al., 2007). However, surface runoff can also occur if precipitation falls at a rate that exceeds the infiltration capacity of the soil (e.g., heavy rains or snowmelt on frozen soil). Elevated nutrient losses have long been associated with surface runoff largely because of the abundance of nutrients in surface soil horizons and increased soil erodibility. This is particularly the case with P because most P in surface runoff from agricultural fields is associated with and adsorbed to fine sediments (Sommers et al., 1979; Cooper and Gilliam, 1987; Stone and Mudroch, 1989). It is therefore generally agreed that, through the installation of subsurface drainage, the amount of P and soil lost in surface runoff is decreased because the volume of surface runoff is reduced (Bengtson et al., 1995).

Over the past two decades, tile drains have been identified as a potentially significant source of P in agricultural watersheds. It has been found that tile drains in some settings may export the same amount or more P as surface runoff ( Jamieson et al., 2003; Zhang et al., 2009; Reid et al., 2012). Indeed, Ruark et al. (2012) estimated that tile drainage contributed 17 to 41% of

cumulative total P loads in Wisconsin, whereas tiles supplied 16 to 58% of dissolved P loads. Elevated dissolved and particulate P concentrations from tile drains have been reported across the midwestern United States (e.g., Gentry et al., 2007), Canada (e.g., Kinley et al., 2007; Macrae et al., 2007), and Europe (e.g., Dils and Heathwaite, 1999; Gelbrecht et al., 2005). Total P concentrations in tile drains across studies are often highly variable in space and time (<0.01 to >8.0 mg L-1) but generally exceed critical levels for accelerated eutrophication (0.02–0.03  mg L-1). Phosphorus loads from tile drains can also be significant. Annual total P loads measured in tile drains under prevailing practices are often in the range of 0.4 to 1.6 kg ha-1 (Gaynor and Findlay, 1995; Gentry et al., 2007; Eastman et al., 2010; King et al., 2014b). The Ohio P Task Force (2013) estimated that the amount of P responsible for the algal blooms in the western Lake Erie Basin, when averaged over the total cropland area, is 0.6 to 1.1 kg ha-1.

Although elevated dissolved and particulate P concentrations and loads have been measured in tile drain effluent across North America and Europe, it is not immediately clear which spatial and temporal factors strongly influence P transport to tile drains and the partitioning between P losses in surface runoff and tile drainage. The following section elaborates on the factors and conditions that affect P losses in subsurface drainage, which include soil characteristics, drainage design, prevailing conditions and management, and hydrologic and climatic variables.

Factors Influencing Subsurface Phosphorus Transport to Tile Drains

This section describes the effects of soil characteristics, drainage design, management, and hydrologic and climatic factors on subsurface P transport (Fig. 1). The individual factors are discussed separately, but as McDowell and Sharpley (2001) note, “the loss of P is a function of, but not exclusively of, any one factor.”

Soil CharacteristicsPreferential Flow

Preferential flow paths or macropores provide a direct connection between the soil surface and tile drains. Preferential flow paths are broadly divided into two categories: cracks/fissures and biopores. Cracks and fissures form during the natural desiccation process when the matric soil suction exceeds the tensile strength of the soil (Kodikara et al., 2000; Peron et al., 2009) and are generally associated with fine-textured clayey soils. Studies have shown that medium- and coarse-textured soils generally have less P loss compared with soils with greater fractions of clay (Beauchemin et al., 1998). Biopores form as a result of biological activity such as old root channels or earthworm burrowing (Nielsen et al., 2010). Faster breakthrough and greater loss of soluble nutrients have been observed in soils with an abundance of root channels and worm burrows compared with soils where the macropore channels were disrupted (Singh and Kanwar, 1991; Dominguez et al., 2004). Preferential flow, whether through cracks and fissures or biopores, is a critical process in the movement of P to subsurface tile drainage (e.g., Stamm et al., 1998; Simard et al., 2000).

Journal of Environmental Quality

Preferential flow has been cited in several studies as an explanation for the relatively large amounts of P measured in tile drainage, a concept supported by observed responses in tile discharge at the onset of precipitation (Kung et al., 2000a; Paasonen-Kivekas and Koivusalo, 2006). For example, an investigation of chemographs illustrated that tracer concentrations measured in the tile drainage discharge peaked within 1 h after onset of precipitation, suggesting preferential flow to the subsurface tile (Laubel et al., 1999). The dissolved and particulate P concentrations that were observed in the initial tile discharge event after P application were substantially greater than concentrations from subsequent events (Laubel et al., 1999). Chapman et al. (2001) also measured particulate P in tile drains and indicated that the largest concentrations were measured in the first sample. Likewise, Scott et al. (1998) noted peak dissolved and total P concentrations before the discharge peak. After application of liquid manure, Dean and Foran (1992) noted marked increases in bacteria concentrations at 12 different tile drainage sites in as little as 20 min. Simultaneous response

to precipitation and elevated P concentrations in initial leachate samples are supporting evidence that preferential flow paths are a significant hydrologic and P transport pathway.

Further evidence of the significance of preferential flow to tile drainage exists in the amount and source of particulate P measured in tile flow. The amount of particulate P reported in tile drainage is often quite large and greater than would be expected if matrix flow was the dominant flow path. Over a 3-yr period, Bottcher et al. (1981) reported that 70% of total P in tile drainage was particulate P. Also in that study, large amounts of particulate P were measured in the tile discharge resulting from the first precipitation event after fertilizer application. Soil P concentrations near the surface are generally more enriched compared with lower soil layers due to surface P applications. Thus, greater particulate P measured in the subsurface drainage implies that sediments and associated P in the tile drainage most likely originated from the surface layer (Bottcher et al., 1981). Increases in particulate P have also been measured after plowing (Schelde et al., 2006). In contrast, Geohring et al. (2001)

Fig. 1. Conceptual diagram of the factors that influence P movement to subsurface tile drainage.

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noted significant reductions in total P (dissolved + particulate) concentrations after disruption of macropores through plowing. Uusitalo et al. (2001) analyzed cesium-137 on bulk soil samples and samples collected in drain tile. They determined that particulate P concentrations carried in the tile drains were equivalent to that carried in surface runoff. Thus, if the P can be desorbed, particulate P transported in tile drainage discharge may be just as important of a source of bioavailable P as dissolved P (Uusitalo et al., 2001).

Some research has specifically been designed to quantify and partition the hydrology and P movement between matrix and macropore flow. For example, Gachter et al. (1998) used dye and bromide tracers in a plot-scale study in the Central Swiss Plateau and determined that approximately 35% of the leachate in a rainfall simulation was transported through preferential flow paths. Additionally, they estimated that >50% of the annual dissolved P losses was leached through the soil profile. Using chloride tracers, Stone and Wilson (2006) attributed up to 51% of the total storm discharge and 81% of total peak flow measured in tile drainage to preferential flow. Heathwaite and Dils (2000) designed a lysimeter and piezometer study in the Pistern Hill catchment of Leicestershire, United Kingdom, and determined that total P losses due to preferential flow were more than double the total P losses resulting from matrix flow. Vidon et al. (2012) used end-member mixing analysis and determined that >50% of the discharge from seven different rainfall events from two individual tile drains in Indiana was a result of preferential flow. During large rainfall events, dissolved P transport to tile was regulated by preferential flow (Vidon and Cuadra, 2011). These studies provide evidence that discharge and associated P moving into and through the subsurface drainage network originating from preferential flow is significantly greater than previously thought.

Phosphorus Sorption CapacityThe P sorption capacity of a soil is critical for determining the

fate of P once it is applied to the soil (Kleinman and Sharpley, 2002). Phosphorus leaching has been strongly correlated to soil texture (Leinweber et al., 1999). In general, there exists a negative correlation between P sorption capacity and larger fractions of sand and organic matter (Fox and Kamprath, 1970; Daly et al., 2001). Indeed, dissolved P concentrations measured in tile drainage waters from six mineral soils were approximately two orders of magnitude less when compared with concentrations from three organic soils in Ontario, Canada (Miller, 1979). Fine-grained particles have the greatest capacity to sorb P (Schuman et al., 1973; Stone and Mudroch, 1989; McDowell et al., 2001; Poirier et al., 2012). It follows that finer-textured soils should have a reduced potential for P leaching unless the P accumulated in the soil is high (Reid et al., 2012).

Several studies have documented contradicting results. Beauchemin et al. (1998) measured P in tile drainage from 27 different sites across nine soil series in Quebec, Canada, and determined that greater losses were from fine-textured soils. Eastman et al. (2010) also reported greater tile P losses from a clay loam compared with a sandy loam; these authors found that particulate P losses accounted for 80% of the losses in the clay loam and only 20% in the sandy loam. In Sweden, P losses measured in leachate of finer-textured soils were >70 times greater

than losses from a sandy soil (Djodjic et al., 1999). Phosphorus leaching losses from a clay loam were also shown to be 39 times greater than from a loamy sand after liquid manure application (van Es et al., 2004). In many of these studies, greater P losses from finer-textured soils were attributed to preferential flow. Thus, although the P sorption capacity of finer-textured soils is much greater than that of sandy or organic soils, the potential for development of preferential flow paths in finely textured soils may be a more important driver of subsurface P losses, and soil sorption capacity by itself may not be a good indicator of P leaching potential.

Redox ConditionsPotential reducing conditions that occur under high water

tables may play a role in P extractability or solubility (Wright et al., 2001; Pierzynski et al., 2005). The effects of redox potential on P mobility have been primarily evaluated in bench-scale experiments and wetland studies, although there is anecdotal evidence and speculation from tile drainage research that redox potential is responsible for increased P concentrations when tile outlet elevations are raised and/or when the tile system is used for irrigation. In a laboratory experiment, Ann et al. (2000) subjected agricultural cropland soils to flooding conditions and noted changes in P solubility and increases in dissolved P concentrations in the flood waters. Indeed, the potential for P to be released under reducing conditions is not only positively correlated with P saturation but is also exacerbated with overfertilization (Scalenghe et al., 2002). Shenker et al. (2005) found similar P release in wetland soils from Israel that had been drained and cultivated for four decades before reflooding. They noted that the primary process controlling P release was reductive dissolution of ferric hydroxides where P was adsorbed. Similar findings have been reported for U.S. soils by Vadas and Sims (1998) and Sallade and Sims (1997). In Norway, Braskerud et al. (2005) studied P dynamics in a wetland for 3 yr and determined that P was extracted from the sediments under reducing conditions; however, P losses were only noted during 19% of the 3-yr period.

In tile-drained landscapes, evidence of the reducing conditions occurs when the tile system is managed, as in drainage water management (i.e., controlled drainage) or for subirrigation (Valero et al., 2007; Tan and Zhang, 2011). Valero et al. (2007) and Tan and Zhang (2011) cite significant increases in dissolved P concentrations when drainage systems are managed and/or used for subirrigation compared with free drainage, implying that reducing conditions may be responsible for P remobilization. Over a 5-yr period in southern Ontario, Canada, Tan and Zhang (2011) reported a 30% increase in flow-weighted mean dissolved P concentrations under the controlled drainage subirrigation system (0.094 mg L-1) compared with free drainage (0.057 mg L-1). Similarly, in Quebec, Canada, Valero et al. (2007) reported a 178% increase in dissolved reactive P (DRP) concentration between free drainage (0.019 mg L-1) and controlled drainage with subirrigation (0.053 mg L-1). Despite the increase in concentrations resulting from the controlled drainage/subirrigation management, P loading was reduced (Tan and Zhang, 2011) as a result of reduced discharge. In the controlled drainage/subirrigation management system, the soil profile is frequently alternating between reduced and

Journal of Environmental Quality

oxidized states; however, a soil’s minimum redox potential may be realized shortly after submergence (Savant and Ellis, 1964). Indeed, Wright et al. (2001) investigated different flooding and drainage approaches and found that soils flooded for as little as 2 mo and then drained had statistically the same P concentrations compared with soils that were flooded for 6 mo and then drained.

Drainage Depth and SpacingDrainage depth and spacing decisions are generally made

based on crop production goals (Wright and Sands, 2001) and are a function of soil permeability, crop and soil management, and extent of surface drainage (Randall and Goss, 2001). Depth is largely dependent on siting an outlet and ensuring that adequate depth can be maintained throughout the drainage network, whereas drain spacing is a balance between water table depth and crop response and requires an integration of water removal rate (i.e., drainage coefficient) and soil permeability. The two, however, are interdependent, in that shallow drains require a more narrow spacing to achieve equivalent drainage. Field and modeling investigations of drainage depth and spacing have generally focused on the hydrologic and nitrogen impacts; few have included information on P transport. As water quality concerns increase, joint optimization between production and water quality goals needs to be considered.

On a silt loam soil in Ohio, shallow drains (60-cm depth) responded more rapidly to precipitation than deep drains (90-cm depth), but the drainage volume was significantly less in the shallow drains (Goins, 1956). If positioned at the same depth, drainage volume from narrower drain spacing (9 m) was significantly greater than from wider-spaced (18 m) drains (Goins, 1956: Hoover and Schwab, 1969). Similarly, on a silt loam soil in Indiana, unit area discharges were significantly greater from drains spaced at 5-m intervals compared with those positioned at 20-m intervals (Kladivko et al., 1999). A multilevel drainage system (1.8 m depth at 20 m spacing plus 0.75 m depth at 3.3 m spacing) also proved more efficient at water removal than a single-level system (1.8 m depth at 20 m spacing) (Hornbuckle et al., 2007).

Several studies have found that P concentrations were higher from shallow drains compared with deeper drains. For instance, Culley et al. (1983) found that dissolved and total P concentrations from tile positioned at 1.0 m depth were approximately 50% less than concentrations measured from 0.6-m-deep tile. Similarly, in Switzerland, greater dissolved P concentrations were measured from tile positioned at a 0.5  m depth compared with tile placed at 1 m depth (Stamm et al., 1998). Pool et al. (2005) also reported higher dissolved P concentrations in a shallow tile system (0.75 m deep, 12.5 m spacing) versus a deep tile system (1.5 m depth, 25 m spacing). Although the placement (i.e., depth) of tile may not eliminate P leaching (Duxbury and Peverly, 1978), tiles placed deeper appear to generally reduce P concentrations. However, because drains placed deeper generally produce greater discharge, from a loading perspective, the benefits of deeper drainage may only be perceived. Indeed, total P loads from a silty loam in Ohio with drains placed at 1 m depth and 12 m spacing (1.2 kg ha-1) were greater compared with total P loads from drains positioned at 0.5 m depth and 6 m spacing (0.8 kg ha-1) (Schwab et al., 1980). Regarding drain spacing alone, Addiscott et al. (2000) did not

detect any differences in dissolved P concentrations from mole drains with 2 and 4 m spacing. It has been also shown that the lateral movement of P in subsoil decreased from the point of injection to background levels within a 1- to 1.5-m distance, indicating that P sorption in the subsoil can be significant and limit lateral transport of P to tile (Allen et al., 2012). Therefore, the general consensus in the literature is that drains placed shallower will result in greater P concentrations, whereas deeper drains will have greater mass losses.

Surface InletsSurface inlets are often placed to drain small depressional wet

areas or catchments that form where surface topography prevents a natural overland movement of excess water off the field. The depressions or catchments may range in size from areas <1 ha to areas draining several hectares. These inlets are usually placed at the low point of the depression and are connected to and routed through an existing tile network. Because this type of system is representative of surface losses rather than subsurface losses, it would be expected that P concentrations and loads would be equivalent to those found in surface runoff. In Minnesota, total P concentrations collected at the tile outlets from two large depressions (catchment areas of 44.3 and 41.5 ha) were dominated by particulate P (0.2–6.5 mg L-1), whereas dissolved P concentrations ranged from 0.1 to 0.9 mg L-1 (Ginting et al., 2000). The authors indicated that these P concentrations were considerably higher than concentrations in tile drain effluent observed in similar systems without surface inlets. Mean annual total P loss from the “potholes” was 0.1 kg ha-1, whereas dissolved P loss was 0.05 kg ha-1. In larger events when ponding occurs around a surface inlet, particulate P losses are generally reduced because of sediment deposition, but soluble losses tend to increase.

Effects of Management PracticesTillage

Conservation tillage practices, such as ridge tillage and no-till, are often recommended because they promote soil health by minimizing soil disturbance and preserving crop residues. These tillage practices substantially reduce soil erosion and sediment-bound P loss (e.g., McDowell and McGregor, 1980; Seta et al., 1993), but no-till soils are also the subject of the majority of studies reporting significant subsurface transport of P (Sims et al., 1998). Research has shown that leaching of P through soils to tile drains is dominated by preferential flow via cracks, fissures, and macropores as opposed to matrix flow. For example, Shipitalo and Gibbs (2000) identified earthworm burrows as a primary conduit for swine slurry found in tile drain effluent. Tilling the soil immediately above the tile drain decreased contamination of the tile drain when the slurry was applied, likely by destroying the continuity of the preferential flow paths between the surface soil and the tile drain. Andreini and Steenhuis (1990) also concluded in their study with undisturbed soil columns that in the no-till soil columns preferential flow short-circuited movement through the profile, whereas in the tilled columns the solute moved through the soil matrix.

Several column- and plot-scale studies have further confirmed that subsurface P transport is greater under ridge tillage and no-till compared with conventional tillage. The presence of

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preferential flow in no-till soils coupled with P stratification due to surface application is often cited as the main mechanism for the increase in P loss. Indeed, ridge tillage has been shown to result in 11 times higher dissolved P losses compared with moldboard plowing (Zhao et al., 2001). Geohring et al. (2001) found that total P concentrations were consistently and significantly greater for disked plots compared with conventional plowing. Similarly, Gaynor and Findlay (1995) observed that dissolved P loss from conventional tillage was two to three times less than ridge tillage and no-till. Tilling soils that were broadcast with manure has also been shown to significantly lower P losses in leachate relative to no-till soils (Kleinman et al., 2009). The majority of these studies were completed on clay loam or silty clay loam soil types; therefore, more work is needed to determine if the effects of tillage are consistent across a range of soil types.

The effect of tillage may only result in differences in subsurface P transport within a short timeframe after tillage occurs, as suggested by Djodjic et al. (2002) and Schelde et al. (2006). Fine-textured soils with high clay content found in fields across the midwestern United States and Canada are prone to cracking due to desiccation (Peron et al., 2009). Temperature variations around the freezing point and repeated freezing and thawing cycles during the winter may also create preferential flow paths in the topsoil (Djodjic et al., 2002). The majority of the studies documenting higher P losses from no-till soils compared with conventional tillage practices were conducted immediately after tillage, and P losses were monitored over a single or over a series of rainfall simulations or only during the growing season. Thus, when an entire year is considered or in situations where rotational tillage is practiced, cracking of the soil due to desiccation or freeze–thaw cycles as well as earthworm activity may recreate macropores in the topsoil, which would offset the tillage effects. Several studies have noted that tillage has a minimal impact on annual subsurface P loss to tile drains (Schwab et al., 1973; Djodjic et al., 2002; Algoazany et al., 2007). This is important when the impact of tillage in mitigating P loss via tile drains over an annual cycle is weighed against the benefits of no-till in reducing P loss in surface runoff. Much attention is currently being focused on no-till agriculture and the associated soil health benefits resulting from no-till as well as the promotion of cover crops with no-till to accelerate the soil health benefits. However, sufficient scientific information is lacking to include more discussion.

Cropping SystemPhosphorus transport to subsurface tile drains varies among

cropping systems (Zhu and Fox, 2003). Cropping systems have variable rooting depths, root densities, water use rates, nutrient requirement characteristics, and nutrient uptake efficiencies (Peterson and Power, 1991), which can influence the concentration and amount of P leaching. A few studies have observed significant differences in subsurface P losses between cropping systems. Many studies, however, have found that cropping system did not have an impact on subsurface P transport (Brye et al., 2002; Carefoot and Whalen, 2003; Udawatta et al., 2004; Algoazany et al., 2007; Nayak et al., 2009; King et al., unpublished data, 2014). Bolton et al. (1970) monitored P losses in tile drains under continuous corn, continuous bluegrass, and a 4-yr rotation of corn–oats–alfalfa–alfalfa. They found that

continuous corn had the highest P losses and that continuous bluegrass had the smallest losses of P. However, subsequent studies on the same plots yielded greater subsurface P losses from the bluegrass compared with continuous corn and rotational crops (Culley et al., 1983). Bottcher et al. (1981) found that subsurface P losses were nearly three times greater for corn compared with subsurface P losses from soybean. Vegetable crops have also been shown to have higher concentrations of P in drainage water compared with corn and hay crops (Sawhney, 1978).

In the majority of studies that found an effect of cropping system on P concentrations and loads, one of the crops often received greater P inputs. For instance, Kimmell et al. (2001) found that P loads were generally lower for soybean compared with grain sorghum because P was only applied to the sorghum. Hooda et al. (1999) also found that subsurface P losses were two times greater for a grass–clover pasture compared with a grass only pasture, but the grass–clover pasture had a higher P application rate. No difference was found in subsurface P loss between plots with orchard grass and corn because both plots received the same P application (van Es et al., 2004). These results are similar to those reported by Kinley et al. (2007), which suggest that, regardless of the cropping system, soil test P concentrations and the source of P determine the amount of P in drainage water. Thus, the general conclusion derived from the literature is that subsurface P concentrations and losses are likely more affected by P application rate, P source, P placement, P timing, and soil test P concentration (discussed below) rather than the cropping system.

Soil Test Phosphorus ConcentrationResearch has shown that dissolved P concentration in surface

runoff is related to soil test phosphorus (STP) concentration in the topsoil (e.g., Pote et al., 1996). Several studies have also indicated that STP is an important factor in determining P concentrations in drainage water. Maguire and Sims (2002) found strong correlations between surface STP concentration and dissolved P concentrations in subsurface drainage. In general, elevated levels of STP lead to greater and more strongly correlated concentrations of dissolved P in subsurface drainage. The STP concentration at which the impact on subsurface concentrations and correlation strength change is often referred to as a “change point.” Smith et al. (1995) monitored changes in STP and dissolved P in drainage water over a 9-yr period. They found that over that period, STP increased by 9 mg Olsen-P  kg-1 (from 42.25 to 51.25 kg P ha-1) and that mean dissolved P concentration increased by 0.01 mg L-1 (from 0.045 to 0.055  mg L-1). On flat clayey soils, STP concentrations >112 mg Mehlich-3 P kg-1 were found to have a greater risk of high P losses in subsurface drainage (Beauchemin et al., 1998). In comparison, Heckrath et al. (1995) observed that total P concentrations in drainage water were small from plots with <60 mg Olsen-P kg-1 (~127 mg Mehlich-3 P kg-1). Above 60 mg kg-1, total P concentrations in drainage water substantially increased. Similar results were reported by Hesketh and Brooks (2000) and Smith et al. (1998), in which dissolved P concentrations in tile drainage increased sharply at Olsen-P concentrations >57  mg kg-1 (~121  mg Mehlich-3 P kg-1) and 70 mg kg-1 (~149 mg Mehlich-3 P kg-1), respectively. In contrast, McDowell and

Journal of Environmental Quality

Sharpley (2001) found a change point at 193 mg Mehlich-3 P kg-1, above which the P concentration in subsurface drainage started to increase with increasing STP.

There is agreement among these studies that there exists a STP threshold beyond which a unit increase in STP results in higher P concentrations and losses in drainage water. There is, however, no consensus on the value of the STP concentration threshold; values in the literature range from 112 to 193 mg Mehlich-3 P kg-1. The agronomic STP level for corn is around 30 mg Mehlich-3 P kg-1 (Vitosh et al., 1995); thus, as Klatt et al. (2002) suggest, significant increases in subsurface P concentration are more likely to occur when STP levels are nearly four times greater than agronomic levels. An evaluation of STP levels from commercial soil laboratories in five agricultural watersheds in Ohio (Western Lake Erie Basin, Upper Wabash, Great Miami, Little Miami, and Scioto River) found that between 9 and 23% of >1 million soil samples submitted between 2000 and 2012 had STP levels >100 mg Mehlich-3 P kg-1 (King et al., unpublished data, 2013). However, that data were not adjusted for duplication across site years. Another survey indicates that approximately 5% of Ohio soils test above a Bray P1 level of 100 mg L-1 (IPNI, 2010).

Several studies have also reported that STP concentrations were not an adequate indicator of the potential risk for P movement beyond the edge of the field (Reid et al., 2012). Young and Briggs (2008) observed that extractable P concentrations in topsoil were not effective in predicting dissolved P concentrations in shallow groundwater. Kinley et al. (2007) showed that across study sites in Nova Scotia, Canada, lower STP levels generally resulted in lower and more stable dissolved P concentrations in subsurface drainage, but high dissolved P concentrations in drainage water were found from sites with low STP in some instances. Enright and Madramootoo (2003) monitored P losses in subsurface drains from two fields with different STP concentrations (140 and 360 mg Mehlich-3 P kg-1) and found that the field with a lower STP had significantly higher P concentrations and losses compared with the field with a higher STP. They concluded the sandy clay loam soil in the field with a lower STP was more prone to P transport in preferential flow paths compared with the field with a high STP and sandy loam soil. Thus, variability in STP threshold concentrations among studies may be due to differences in soil type, and STP threshold values may be lower for soils with high clay content compared with other soil types.

Phosphorus Source: Organic vs. InorganicThere are two categories of P that are applied to agricultural

lands: organic (e.g., manure) and inorganic (e.g., fertilizer). Research suggests that the concentration and losses of P transported in subsurface pathways may be dependent on the source of P, with losses from organic sources generally being greater than losses from inorganic sources. For instance, Eghball et al. (1996) compared long-term application of cattle manure to a similar rate of inorganic fertilizer and found that P moved deeper into the soil profile and in greater concentrations in the manure treatment. Swine manure, applied at nitrogen-based rates, also significantly increased dissolved P concentration in subsurface water compared with inorganic fertilizer (Nayak et al., 2009). McDowell et al. (2005) and Delgado et al. (2006) found that P

losses in drainage water were two times greater on plots receiving manure compared with plots receiving like or greater amounts of P as superphosphate and phosphogypsum, respectively. In comparison, results from a study by Zhao et al. (2001) suggested that solid beef manure had four times higher total P losses in subsurface tile drainage compared with inorganic fertilizer when applied at nitrogen-based rates. Macrae et al. (2007) also examined the effect of P source across a small watershed in Ontario, Canada. They observed that fields receiving exclusively manure had greater dissolved P concentrations in subsurface drainage than fields receiving a mixture of manure and inorganic fertilizer or only inorganic fertilizer.

Greater total P concentrations and losses from organic sources compared with inorganic sources have been attributed to differences in source characteristics. Several studies indicate that P losses are often greater at a site that has been fertilized with an organic source of P because organic P is sorbed less strongly than inorganic P and, as a result, may be leached (Frossard et al., 1989; Simard et al., 1995; McDowell et al., 2005). For example, the study by McDowell et al. (2005) analyzed the forms of P in sites receiving equal rates of P from irrigated dairy effluent and in sites receiving only inorganic fertilizer. They concluded that more P was in the dissolved organic form for the irrigated dairy effluent site compared with the inorganic fertilizer site, which resulted in higher losses of P from the dairy effluent site. Other research suggests that organic sources of P increase STP more than inorganic sources of P, which ultimately results in more P in drainage water (Kinley et al., 2007; Nayak et al., 2009). The potential for subsurface P transport among organic sources may also be due to differences in the amount of water-extractable P of the source materials (Sharpley and Moyer, 2000). Indeed, Elliott et al. (2002) showed that the amount of water-extractable P in eight biosolid materials varied widely and was significantly correlated to the leaching potential of the source material. Kleinman et al. (2005) did a survey of 140 different organic P sources and found that water-extractable P varied by animal (swine > poultry > dairy > beef ) and general storage system (liquid > dry).

Phosphorus Placement: Broadcast vs. IncorporatedIn addition to the source of P, the placement of applied P can

affect P transport in subsurface pathways. Turtola and Jaakkola (1995) showed that when applied at a similar rate, dissolved P losses in drainage water were greater with broadcast application (untilled grass) compared with incorporation of P (tilled barley system). Kleinman et al. (2009) also observed nearly two times greater dissolved and total P losses in leachate from broadcast manure compared with incorporated manure. Similarly, light disking of dairy slurry after broadcast application resulted in greater P transfers to tile drains than did moldboard plowing (Geohring et al., 2001). In contrast, Feyereisen et al. (2010) investigated P leaching losses from poultry litter after three application methods (i.e., surface broadcast, surface broadcast with disking [8–10 cm], and subsurface [direct injection 4–8  cm] incorporation) on Delmarva Peninsula in Maryland. The soils were a mixture of the poorly drained Othello series and well-drained Matapeake series. They found that the average cumulative total P loss was greatest for the subsurface litter incorporation (0.48 kg ha-1), followed by broadcast

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(0.32  kg  ha-1) and broadcast then disked (0.29 kg ha-1) approaches. However, these differences were not statistically significant. Injected liquid swine manure had slightly greater dissolved P losses in tile drainage compared with surface-applied manure (Ball-Coelho et al., 2007). Soils in the Ball-Coelho et al. (2007) study would not be expected to develop significant preferential flow paths. Thus, greater P losses observed in the studies by Feyereisen et al. (2010) and Ball-Coelho et al. (2007) for the subsurface application method compared with broadcast application are likely due to direct injection of manure into the soil with little mixing between the soil and manure. Previous studies have suggested that incorporation with mixing of the P and soil generally decreases P loss in surface and subsurface flow pathways as P is adsorbed to the soil (Kleinman et al., 2009). All studies, however, noted that differences in P concentration and loss among treatments (i.e., broadcast vs. incorporated) were greatest in the first precipitation events after application. The differences among broadcast and incorporated treatments tended to become smaller in tile drainage after several precipitation/discharge events.

Phosphorus Application RateRegardless of the source of P or method of P application,

P application rate can significantly influence the amount of P transported in subsurface pathways (Philips et al., 1981; Elliott et al., 2002; Algoazany et al., 2007; Kleinman et al., 2009). Of the literature reviewed, all studies indicate that the potential for P leaching increases with an increase in P application rate. For example, Hergert et al. (1981) monitored P concentrations in drainage water from plots receiving manure at either 35 or 200 Mg ha-1 (40.9 or 225.6 kg total P ha-1, respectively). They found that flow-weighted mean P concentrations were between 0.10 and 0.44 mg L-1 for the 200 Mg ha-1 treatment, compared with P concentrations of 0.01 to 0.02 mg L-1 for the 35 Mg ha-1 treatment. Higher manure application rates to grass and grass–clover plots also resulted in a 1.5 to 2.0 times increase in P losses in drainage water (Hooda et al., 1999). Increasing the rate of liquid swine manure application rate from 0 to 93.5  m3  ha-1 resulted in dissolved P concentrations that were 40 times greater at the highest application rate compared with the control (Ball-Coelho et al., 2007). Similar results have been reported for inorganic fertilizers. Indeed, Bolton et al. (1970) observed higher P concentrations and losses on plots receiving 336 kg ha-1 of 5–20–10 fertilizer compared with plots receiving no fertilizer inputs. Turtola and Jaakkola (1995) also showed that higher inorganic P application rates resulted in higher dissolved P losses in drainage water. Thus, when P source and application method are similar, increasing the application rate of P to fields is likely to increase the amount of P transported in subsurface pathways. Furthermore, P application at rates greater than crop removal rates also increase subsurface P losses.

Phosphorus Application TimingThe timing of P application has also been shown to be a

significant factor affecting P loss (Sharpley et al., 2001). The aspects of application timing include when the fertilizer is applied relative to crop planting and how soon after application before the onset of precipitation. In P-risk indices, greater risk is assigned to applications occurring during autumn and winter

compared with spring and summer (Sharpley et al., 2001) and has been verified in field studies (van Es et al., 2004; Algoazany et al., 2007). Indeed, Algoazany et al. (2007) collected tile drainage discharge and water quality from four corn–soybean agricultural production fields in Illinois and observed greater P concentrations and losses in drainage water from fields where P was applied after soybean harvest in the fall compared with fields where P was applied just before planting. Furthermore, the authors determined that there was no significant difference in tile drainage P concentrations using a split application (i.e., applying P before each crop) compared with applying all the P necessary for the 2-yr rotation with the corn crop. In New York, greater P losses were also reported when liquid manure was applied in the fall compared with spring applications (van Es et al., 2004). However, unusual weather patterns may help to explain greater early fall losses reported in the van Es et al. (2004) study. Differences among sites, discharge, and application rates confound the comparison of these studies. Also, greater P losses in the winter suggest that soil temperature may affect vertical movement of P (Williams et al., 2012).

Precipitation soon after P application significantly increases the risks of P movement (Sharpley et al., 2001). Soluble P concentrations in tile drainage discharge were greater after rainfall/discharge events occurring 1 to 2 d after application compared with concentrations in rainfall/discharge events despite a smaller peak flow in the first event (Stamm et al., 1998). Likewise, in England, increased P concentrations and loads were observed in the first rainfall event after application, and approximately 60% of the total P loss over the drainage season was measured during the first 7 d after application (Withers et al., 2003). In New York, simulated rainfall immediately after manure application significantly increased tile drainage P concentrations compared with concentrations collected from plots where irrigation was delayed for 6 d (Geohring et al., 2001). However, increased P losses have also been observed with rainfall events occurring 2 and 3 wk after cattle slurry application (Ulen and Mattsson, 2003).

Effect of Hydrology and ClimatePhosphorus transport via tile drainage can vary temporally

between baseflow and event flow as well as throughout and among different events and with season/climate. The episodic nature of P loss in tile drainage has made it challenging to obtain reliable estimates of P loads in tile drains in many regions. The variation in data sets produced by weather patterns (e.g., seasonal, interannual) can also lead to challenges in identifying the effects of various treatments (e.g., tillage, cropping system, etc.) in scientific studies.

Hydrology: Baseflow and Event FlowTile discharge tends to increase during events compared with

low flow rates (or no flow) observed during baseflow periods. The hydrological response of tile drainage during a given event varies based on event characteristics, such as rainfall amount and intensity (Vidon and Cuadra, 2011), and antecedent moisture conditions before the event (Kung et al., 2000a; Dolezal et al., 2001; Macrae et al., 2007). The hydrogeology of the drained land is important in determining discharge patterns during baseflow periods because tile drains may be ephemeral or may

Journal of Environmental Quality

drain an aquifer (Dolezal et al., 2001). In comparison, under very wet conditions, tile drainage may be influenced by the hydraulic capacity of the drainage systems (Dolezal et al., 2001); thus, flow through a given tile may be related to contributions from the soil above it but also to what is flowing through adjacent and downgradient tiles. Differences in tile discharge rates and the source of tile discharge between baseflow and event flow, as well as among events, also influence P concentrations and loads.

A majority of P loss through tile drainage, in the dissolved and particulate forms, has generally been observed during periods of elevated flow (Ulen, 1995; Sims et al., 1998; Dils and Heathwaite, 1999; Ulen and Persson, 1999; Kleinman and Sharpley, 2003; Macrae et al., 2007; Schilling and Helmers, 2008; Bende-Michl et al., 2013). Event-related P losses have frequently been attributed to preferential transport between surface soils and drainage tiles during storms (Stamm et al., 1998; Ulen and Persson, 1999; Uusitalo et al., 2001; Poirier et al., 2012). Several studies have found strong correlations between P concentrations and tile discharge rates (Vidon and Cuadra, 2011; Bende-Michl et al., 2013; King et al., 2014b), with P concentrations increasing with increases in tile flow. Other studies, however, have reported poor or no relationships between P concentration and tile discharge rates (Baker et al., 1975; Madramootoo et al., 1992; Macrae et al., 2007).

Although P concentrations increase during high discharge periods, they are often not consistent throughout and among different events (Gachter et al., 1998; Geohring et al., 2001; Macrae et al., 2010; Brauer et al., 2009; Vidon and Cuadra, 2011). Many studies have shown a pulse of P early in an event (e.g., Stamm et al., 1998; Stone and Krishnappan, 2002). Other studies have found that as tile discharge increases, dissolved and particulate P also increase but peak before the discharge peak (Scott et al., 1998; Zhao et al., 2001; Gentry et al., 2007; Ball-Coelho et al., 2010). Gentry et al. (2007) reported that particulate P concentrations in tile discharge decreased more rapidly than dissolved P concentrations after events. In a study of a series of spring storms in Indiana, P loss through tile drainage after large, intense precipitation events was due to enhanced macropore activity (Vidon and Cuadra, 2011). Similarly, Kung et al. (2000a, 2000b) demonstrated that the macropore connectivity between surface soils and tile drains was greater during wet antecedent moisture conditions, and the authors related this to the potential for P loss in tile drain effluent under variable hydrologic conditions. Indeed, annual total P loads from tile-drained agricultural watersheds were shown to be disproportionately higher during wet years than dry years (Gentry et al., 2007). The supply of P can also decline throughout successive storm events, leading to reduced P concentrations in tile drainage. For example, Bende-Michl et al. (2013) reported that P concentrations were greatest during the early events of higher-flow seasons. Similarly, Heathwaite and Dils (2000) reported higher P concentrations in storms after a dry summer. In general, greater P losses in tile drainage have been observed during periods of greater flow, usually coincident with large precipitation events.

SeasonSeason has been demonstrated to play a significant role in

P transport across the globe, with increased tile drainage and

associated P loss during the cooler, wetter months compared with P loss during hot, dry periods. Although only a few studies have measured tile discharge and P export during the nongrowing season (e.g., cool or cold winters, with or without a snowmelt period), they have all demonstrated that this period represents a significant proportion of annual discharge and P loss. A majority of annual precipitation generally occurs between the months of October and May across most of the midwestern United States and Canada (Schwab et al., 1961; Baker et al., 1975), which produces larger volumes of subsurface drainage relative to warmer, drier months due to a generally wetter soil profile (Bottcher et al., 1981; Macrae et al., 2007; Macrae et al., 2010; Tan et al., 2002). Indeed, tile discharge and P loss at the field (Macrae et al., 2007; King et al., 2014a, 2014b; King et al., unpublished data, 2014) and at the watershed scale (Macrae et al., 2010; King et al., 2014b) have been shown to be greater during the late autumn, winter, and early spring periods in comparison to other periods of the year. Tan et al. (2002) also reported that 64% of annual tile drainage occurred during the nongrowing season. In a study of 11 drained systems across Europe, Hirt et al. (2011) reported that 70% of precipitation events yielded a tile drain response; however, the tile response rate in the summer (56%) was substantially lower than in the winter (84%). Due to climatic differences, temporal variation in tile discharge and associated P loss would be expected to vary across both a latitudinal and longitudinal gradient. Additionally, the nongrowing season appears to be the critical period for greater tile discharge and P loss.

Phosphorus concentrations have also been shown to vary seasonally, with some studies reporting greater P concentrations in fall and winter relative to spring and summer (Bolton et al., 1970) and others reporting the opposite (Dils and Heathwaite, 1999; Bende-Michl et al., 2013; King et al., 2014b; King et al., unpublished data, 2014). There is also inconsistency regarding the speciation of P across different seasons. For example, Heathwaite and Dils (2000) and Chapman et al. (2001) reported increased concentrations of particulate P in tile and overland flow in winter. In contrast, Heathwaite and Dils (2000) and Macrae et al. (2007) found that dissolved P dominated in winter and particulate P dominated in summer. All of these studies reported low concentrations of particulate P during winter baseflow and suggested that most of the P transfer was associated with events.

Drainage Management Practices to Control Subsurface Phosphorus Loss

Phosphorus losses in subsurface drainage are projected to increase because the replacement of aging subsurface systems and the installation of new drainage networks in previously undrained areas are driven by economic and climatic factors (Blann et al., 2009). To combat the delivery of P to surface waters, several practices have been suggested as a means to mitigate P transport through subsurface tile drainage. These practices can be classified as (i) practices that disconnect fast flow pathways between surface runoff and subsurface drainage, (ii) drainage water management, (iii) in-stream or end-of-tile treatments, and (iv) ditch design and management.

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Disconnecting Flow Pathways between Surface Soils and Subsurface Drainage

Preferential flow through soil fissures, cracks, and macropores often results in faster movement of water, sediment, and nutrients through the soil than would be expected from the soil matrix properties (de Rooij, 2000; Lin and Zhou, 2008; Allaire et al., 2009). For instance, Kung et al. (2000a) showed that solute transport to subsurface drains can occur quickly (<1 h) through soil macropores in a study where rainfall simulations were used to track the mobility of adsorbing and nonadsorbing tracers. Periodically disrupting soil macropores through deep tillage is a practice that has been suggested to help reduce P concentrations in subsurface drainage from sites with excessively high STP. No-tillage practices often preserve soil macropores and therefore increase P losses by preferential flow (Djodjic et al., 2002). Gaynor and Findlay (1995) investigated P losses in surface runoff and subsurface drainage as the result of conservation and conventional tillage. They concluded that conservation tillage reduced soil erosion but increased losses of DRP in subsurface drainage. In a soil column study, Andreini and Steenhuis (1990) showed that nearly the entire soil profile was short-circuited by preferential flow under no-till, but in the tilled columns, the solute passed slowly through the mixed, unstructured plow layer. In addition, total P concentrations in tile drainage were found to be higher under ridge tillage than under moldboard plowing because the ridge tillage did not result in the disturbance of the macropore network (Simard et al., 2000).

In many areas of midwestern United States, closed basins or prairie potholes are also common. Traditionally, closed depressions have been drained with tile risers, which provide a direct connection between surface runoff and subsurface drainage. Tile risers therefore function similar to soil macropores because water, sediment, and nutrients rapidly bypass the soil profile. Closed depressions with tile risers can be a significant contributor to water quality problems; direct correlations have been observed between the extent of closed depressions and P loadings to streams (Smith et al., 2008). For example, in one storm event, 50% of the P loss from a watershed in Iowa was reported to be the result of discharge from tile risers (Tomer et al., 2010). Rock inlets and blind inlets have recently been considered as a practice to drain closed depressions, but this practice reduces sediment and P loads to subsurface drainage compared with tile risers. Replacing two tile risers with blind inlets in Indiana resulted in 68 and 65% reductions in DRP and total P, respectively (Smith and Livingston, 2013). Gieseke (2000) showed that a gravel inlet reduced DRP loads by 72 to 78% compared with open tile inlets. In a controlled laboratory experiment, Wilson et al. (1999) evaluated the effectiveness of five surface inlet designs to remove sediment and P. They found that all the inlet prototypes tested removed between 83 and 95% of sediment compared with a traditional tile riser, but the blind inlet was the most effective because it was backfilled with the finest material. In addition, they concluded that all inlet prototypes removed P, with trapping efficiencies between 65 and 88%.

Drainage Water ManagementDrainage water management (DWM), or controlled drainage,

is the practice of artificially adjusting the outlet elevation to a specified depth by restricting flow from the subsurface drainage system. The outlet elevation can be set at any level between the ground surface and the drainage depth through installation of a control structure, which is typically comprised of stackable boards or stop logs. Considerable attention has been given to DWM in recent years as a potential best management practice (Skaggs et al., 2012), with several national and regional initiatives, such as the Agricultural Drainage Management Task Force, aimed at expanding its adoption in the United States (Strock et al., 2010). Drainage water management can be used to provide adequate drainage during critical tillage, planting, and harvesting operations (Drury et al., 1999) and to prevent excessive drainage of the crop root zone after planting (Cooper et al., 1991; Drury and Tan, 1995). In addition to the water quantity benefits, the implementation of DWM has shown the potential to provide an array of water quality benefits (Evans et al., 1990; Evans et al., 1995; Catt et al., 1998; Addiscott et al., 2000; Wesström and Messing, 2007; Tan and Zhang, 2011; Frey et al., 2013).

Reductions in flow volume of 20 to 95% with DWM have been reported by many studies compared with conventional or free drainage (FD) systems (e.g., Gilliam et al., 1979), and reductions in flow volume are often accompanied by similar decreases in P loss (Evans et al., 1995). For example, at 14 study sites in North Carolina, Evans et al. (1991) found that total P losses in drainage water were 30% less with DWM compared with losses from FD. In Illinois, Cooke et al. (2004) found that implementation of DWM would reduce P losses by 83%. Similarly, reductions in total P losses in subsurface drainage from field plots with DWM in Sweden ranged from 65 to 88% compared with FD (Wesström and Messing, 2007). However, water quality trade-offs must be considered because DWM has the potential to increase overland flow from drained landscapes (Tan et al., 2002; Riley et al., 2009). Using a computer simulation model (DRAINMOD) designed to assess the impacts of drainage and water management practices on poorly drained soils, Gilliam and Skaggs (1986) predicted that overall P losses (surface runoff + subsurface drainage) would increase under DWM due to more surface runoff than FD. Although few studies have investigated both surface and subsurface P losses from DWM and FD, Tan and Zhang (2011) found that, over a 5-yr study, total P losses in surface runoff were significantly greater from a field plot with a DWM subirrigation system than from a field plot with FD, but overall P losses were still greater from the plot with FD.

The altered hydrology created by DWM may also influence the solubility and mobility of P (McDowell et al., 2012). Compared with FD, higher water table depths resulting from DWM can cause anoxic conditions to form within the soil profile. In contrast, other studies have reported that the bulk soil does not become reduced long enough or to the extent necessary with DWM to cause reducing conditions in the soil that would increase P mobility (Catt et al., 1998; Wahba et al., 2001).

Incubation experiments have shown that as soils become reduced, soluble P released from sediments increases (Sallade and Sims, 1997; Vadas and Sims, 1998). In acid soils, this soluble P increase has been attributed to the reduction of Fe and Al oxides

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and the subsequent release of P bound with these compounds (Holford and Patrick, 1979). The potential for P loss based on changes in P solubility and extractability during soil reduction and reoxidation is therefore greater in P-saturated topsoils than in soils deeper in the profile (Vadas and Sims, 1998). Feser et al. (2010) found that DWM decreased DRP and total P losses compared with FD, but P concentrations significantly increased under DWM. Valero et al. (2007) found that DRP concentrations were 0.053 and 0.019 mg L-1 for DWM and FD, respectively. Dissolved reactive P also comprised a larger proportion of total P in DWM (80%) compared with FD (67%), which suggests that DWM increased P forms that are readily available. In general, dissolved P concentrations from DWM are greater than dissolved P concentrations from FD; however, due to the reduction in discharge resulting from DWM, P loads are less when compared with FD.

In-stream and End-of-Tile TreatmentsOver the past 25 yr, flow-through filter cells, cartridges, and

structures installed in-line on the drainage outlet or in surface ditches have shown promise as a method to remove P from drainage waters (Westholm, 2006; Penn et al., 2007; Vohla et al., 2011; Allred, 2010; Bird and Drizo, 2010). Flow-through cartridges and structures are filled with natural materials and industrial byproducts that facilitate P removal via sorption and precipitation (Brady and Weil, 2002). Well over 100 different types of materials have been tested for P retention capacity in laboratory and bench-scale studies, including blast furnace slag, iron oxide mine tailings, coconut shell–activated carbon, flue gas desulfurization gypsum, zeolite, goethite, bone char, and dolomite sand. Materials are generally chosen because they are rich in Ca, Fe, Al, and their respective oxides/hydrous oxides, which promote P sorption and removal (Agrawal et al., 2011). Some industrial byproducts may contain concentrations of heavy metals (e.g., As, Cd, and Hg) that exceed permitted levels for land disposal (McDowell et al., 2008); therefore, the environmental toxicity of the material or byproduct must be considered before use.

In a laboratory study, Allred (2010) investigated the P removal capacity of five materials over a range of P concentrations (0.1–1.0 mg L-1). Results showed that surfactant-modified zeolite and high calcium oxide–high carbon fly ash materials reduced P concentrations by >50% under all treatment conditions. A clinoptilolite/alumina/activated carbon cartridge in a laboratory-scale simulation study decreased DRP losses by 52% (King et al., 2010). Similarly, Pant et al. (2001) found that dolomite sand reduced P concentrations by 81% in a batch-sorption experiment. Although a wide variety of materials have been shown to be capable of removing P in a laboratory setting (Westholm, 2006; Vohla et al., 2011), they may not be applicable for field-scale use because high flow conditions are typically observed during storms and under seasonal variations in precipitation. For example, Brix et al. (2001) showed that P sorption by a calcite substrate was 10 times higher in a laboratory-scale experiment compared with field-scale operation. A flow-through structure that used acid mine residual in Maryland removed 99% of P during low-flow periods, but during storm events it was only able to treat 9% of the water moving through the structure (Penn et al., 2007). In contrast, McDowell et al.

(2008) found that using a slag mix in a porous sock inserted into the outlet of a tile drain decreased DRP losses by 70% compared with control tiles. The long-term performance of the material has also been shown to be important under field conditions. Indeed, in the study by McDowell et al. (2008), the authors calculated that the practice had a life expectancy of 72 yr. On the other hand, Bryant et al. (2012) found that the hydraulic conductivity of flue gas desulfurization gypsum decreased over a 4-yr study, which resulted in P removal of only 22%. However, it is difficult to directly compare P removal among studies because the chemical composition and P-sorbing capacity of the materials used, the site-specific P loadings, and the environmental conditions vary greatly.

Constructed wetlands are another end-of-tile practice that has been proposed to reduce P losses from subsurface drainage. Most investigations into the use of constructed wetlands for the treatment of agricultural nonpoint-source pollution have focused on surface runoff, where the main removal mechanism is the settling of sediments and associated P ( Jansson et al., 1994; Jordan et al., 1999; Braskerud, 2001). There is less information on wetland performance in treating subsurface drainage (Tanner et al., 2005). Kovacic et al. (2000) monitored three constructed wetlands receiving cropland tile drainage in Illinois and found that overall removal efficiencies were 22% for DRP but only 2% for total P. Annual total P removal from these wetlands, however, ranged from -64 to 80%. At another site in Illinois, Mitsch et al. (1995) studied P retention in four constructed wetlands and found 53 to 99% annual removal of total P. Alternatively, total P losses increased by 101% after passage through a wetland in New Zealand during the first year after construction but decreased by 12% in the second year (Tanner et al., 2005). Variations in constructed wetland P removal among studies are likely related to temporal variability in seasonal hydraulic loading patterns, establishment and maturation of wetland vegetation, and the P sorption characteristics of soils and organic matter in the wetlands (Tanner et al., 2005).

Ditch Design and ManagementAgricultural drainage ditches are unique landscape features in

that they integrate the characteristics of streams and wetlands. Whereas some drainage ditches are simply straightened streams, others are intermittent wetlands with diverse vegetation and accumulations of soil organic matter (Needelman et al., 2007). Historically, the management of agricultural drainage ditches has focused on water conveyance. Recently, however, there has been increased interest in improving the management of ditches for water quality benefits. For example, drainage ditch soils can serve as large reservoirs of P, especially in intensively managed agricultural landscapes (Dunne et al., 2007; Vaughan et al., 2007). The removal of DRP from flowing ditch water has also been shown to be a function of a soil’s P sorption capacity and clay content (Sharpley et al., 2007). Thus, decreasing P loss from agricultural landscapes with drainage ditches requires consideration of the role of ditch design and management on P transport and transformations (Shigaki et al., 2008).

Dredging is one management practice that is performed periodically and often routinely in agricultural drainage ditches to increase water flow and to facilitate adequate drainage capacity. Smith et al. (2006) conducted laboratory experiments

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with sediments representing conditions before and after dredging to evaluate P sorption of high P concentration ditch water. The authors found that DRP removal was significantly greater (32%) for sediments before dredging compared with the sediments exposed after dredging. Similarly, Shigaki et al. (2008) found that sediments exposed to ditch flow before dredging were able to remove 19% more P from the water column than sediments representing bed materials after dredging. Because dredging removes biomass (standing vegetation as well as biomass associated with ditch sediments), freshly exposed sediments may contain different abundances, diversity, and species composition than dredged sediments (Koel and Stevenson, 2002). Indeed, biological removal of P can be important in agricultural drainage ditches (Sharpley et al., 2007; Shigaki et al., 2008). Newly exposed mineral sediments also likely have lower P buffering capacities than the sediments that were removed through dredging (Sharpley et al., 2007). The long-term impacts of dredging on P transport have also been investigated by Smith and Huang (2010) in Indiana. In contrast to previous findings, the authors observed a significant reduction in P transport during the 12-mo period after dredging. Their results indicated that the greater-than-normal risk of P loss that is typically associated with sediments freshly exposed by dredging was temporary (<1 mo). Therefore, they concluded that P entering a ditch during the months after dredging may have a greater opportunity to be retained, adsorbed, or biologically removed from the water column than before dredging.

Improved ditch design, such as the two-stage ditch, is another practice that has the potential to influence P losses in agricultural landscapes. Traditionally, agricultural drainage ditches are trapezoidal in shape (Ward et al., 2004). As described by Powell et al. (2007), the two-stage ditch approach creates a first stage (i.e., lower stage) that is an inlet channel associated with the channel forming discharge. The second stage (i.e., upper stage) is related to a maximum size to provide stability, the position of the subsurface drainage outlets, and the desired conveyance capacity to prevent frequent flooding of adjacent areas. During high flows, water is able to spread out onto the benches, which decreases the velocity of the water. Although reductions in water velocity have been shown to promote sedimentation and significantly reduce P losses (Kronvang et al., 2007; Kröger et al., 2009), there is no published literature on the effect of the two-stage ditch design on P losses.

Summary and Future Research NeedsThe objective of this paper was to critically review research

on P transport in subsurface tile drains and to determine factors that control P losses in tile drain effluent. Although a significant number of studies have been conducted and much has been learned, P transport in tile drainage remains a challenge. Phosphorus concentrations and losses are often highly variable in space and time and are dependent on a combination of factors, including soil characteristics, drainage design, management practices, and climatic variables. Results of this review indicate that P concentrations and losses are typically greater during storm events compared with baseflow and that the majority of P loss occurs during the nongrowing season. Phosphorus losses can be substantial in both sandy and organic soils due to low

sorption affinity, but elevated P losses from fine-textured soils are also possible due to the development of preferential flow paths. Fields with high STP concentrations, fields receiving P application rates above recommended levels, and fields with broadcast P application have a greater potential for P leaching to tile drains. In addition, fields with shallow tile drains are likely to have greater P concentrations relative to deep drains, but deeper drains may have greater P loads because they flow for longer periods of time. The effects of typical tile spacing on P transport are not as pronounced. Drainage systems with surface inlets are also expected to have elevated P concentrations and losses compared with drainage systems without surface inlets.

Many local, regional, and national entities throughout the world are attempting to find ways to significantly decrease P loads to surface waters. This review has highlighted several approaches that have shown promise to reduce P concentrations and loads from tile-drained landscapes. Although these practices will undoubtedly reduce P concentrations and loads, we have identified three research areas where more information is needed to better understand the role of tile drains in P transport and develop management practices that decrease P transport in tile drains.

Understanding the Hydrology of Tile Drained Landscapes

Despite several subsurface drainage studies that present hydrology data, the hydrologic processes of tile drained landscapes are not well understood. Because P transport is governed by hydrologic processes, it is paramount that these processes be thoroughly explored and evaluated under different soils and management practices to address P transport in tile-drained landscapes. Specifically, preferential flow is often cited as a primary pathway for P transport to subsurface tile drainage; yet, only a few studies have attempted to quantify the amount of water and P delivered to subsurface drains through the preferential flow paths. Research is needed on the development, extent, and behavior of preferential flow paths under an array of different soils and management practices (e.g., intensive grain production vs. pasturelands). Specific questions related to preferential flow include: When and under what conditions do preferential flow paths develop? What fraction of P is delivered to tile drains via preferential flow compared with matrix flow? Is P transport in preferential flow paths more prevalent in certain soils or under different antecedent conditions? Do management practices such as tillage affect P transport in preferential flow paths, and how long do these effects last?

Similarly, the understanding of hydrology and P transport under different climates requires further exploration, particularly for cold climates. Seasonal differences in P loss, particularly in regions with significant winter frost and snow accumulation, are poorly represented in the literature, and further research is needed in this area. Studies in regions with cold climates have rarely included the winter months and spring thaw in their sampling regime but have instead extrapolated data collected at other times of year. The few studies that have measured runoff and P export (in either surface or subsurface runoff ) during the nongrowing season (e.g., cool or cold winters, with or without a snowmelt period) have demonstrated that this period represents a

Journal of Environmental Quality

significant proportion of annual discharge and P loss. Hydrology and subsequent P transport in cold climates represents a major gap in the scientific understanding of P in tile drain effluent in these regions and is therefore a research area that should be developed.

Development and Enhancement of Phosphorus Routines in Deterministic Models

Individual field and watershed assessments are valuable and provide real-world data on P transport as a result of existing or prevailing practices. Although it is physically and fiscally impossible to monitor or measure the impacts of practices in all locations, simulation technologies or modeling can be used to project the impacts of differing management and treatment strategies on water quality. Historically, much success was found with P risk indices to identify agricultural field areas most prone to P loss. These indices proved to be an effective screening tool for an individual grower trying to assess the relative risk associated with a specific field. Current P risk indices, however, are limited because they do not permit the quantitative assessment of single or multiple practices or the extension of those practices across a farm or watershed, and few attempts have been made to integrate subsurface drainage into the calculation.

Deterministic models (e.g., Agricultural Policy/Environmental eXtender [Williams and Izaurralde, 2000] and the Soil and Water Assessment Tool [Arnold et al., 1998]) range from edge-of-field to basin scale and are often used to identify knowledge gaps in theory, to guide field experiments to help understand governing processes, to evaluate specific management, and to provide input assessment for developing agricultural policy. Subsurface drainage routines in many of the currently available models, however, are a simplified version of drainage theory. Current approaches are also focused primarily on hydrology and do not account for P transport or do so crudely. To accurately represent P movement in tile-drained landscapes, significant improvement in the model technology is required. Deterministic models use mathematical functions to describe natural processes; therefore, data across a wide range of soils, management practices, and climates are required. If achieved, the confidence with which these models are used will be greatly enhanced and will permit the investigation of conservation management as it relates to agricultural nutrient transport in surface discharge and tile drainage.

Holistic Evaluation of Management and Treatment Practices

Various approaches to mitigate subsurface P transport have demonstrated the potential to reduce P loads from tile drain effluent. However, their performance across a range of soil types, management practices, and climate has not been widely tested. A holistic evaluation of these approaches is needed to ensure that unintended consequences do not occur. Practices that decrease P concentrations and loads may increase N transport or increase the solubility of heavy metals present in the soil. Emphasis has recently been placed on surface amendments (e.g., gypsum), DWM, cover crops, and soil health to address vertical P movement, but only a limited amount of data is available to support the use of these practices. Additional practices that

target P transported in subsurface drainage water also need to be explored. For example, tillage directly over the tile lines or even no P application over tile lines has been suggested, which are both possible with real-time kinematic navigation technology.

In addition to the assessment of individual management practices, information is needed on the cumulative effect of multiple in-field and edge-of-field practices on P transport. This could lead to identifying a set of practices that result in the greatest decreases in P delivery to surface waters. Similarly, P losses in surface runoff and subsurface drainage need to be measured simultaneously to determine the partitioning of P losses between flow paths and to ensure that practices that decrease P loads in drainage water (e.g., DWM) do not increase P loads in surface runoff. Although the general consensus is that these practices are directionally correct, comprehensive assessments of P loss in surface runoff and subsurface drainage are required.

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