Most phosphorus (P) modeling studies of water quality have focused on surface runoff loses. However, a growing number of experimental studies have shown that P losses can occur in drainage water from artificially drained fields. In this review, we assess the applicability of nine models to predict this type of P loss. A model of P movement in artificially drained systems will likely need to account for the partitioning of water and P into runoff, macropore flow, and matrix flow. Within the soil profile, sorption and desorption of dissolved P and filtering of particulate P will be important. Eight models are reviewed (ADAPT, APEX, DRAINMOD, HSPF, HYDRUS, ICECREAMDB, PLEASE, and SWAT) along with P Indexes. Few of the models are designed to address P loss in drainage waters. Although the SWAT model has been used extensively for modeling P loss in runoff and includes tile drain flow, P losses are not simulated in tile drain flow. ADAPT, HSPF, and most P Indexes do not simulate flow to tiles or drains. DRAINMOD simulates drains but does not simulate P. The ICECREAMDB model from Sweden is an exception in that it is designed specifically for P losses in drainage water. This model seems to be a promising, parsimonious approach in simulating critical processes, but it needs to be tested. Field experiments using a nested, paired research design are needed to improve P models for artificially drained fields. Regardless of the model used, it is imperative that uncertainty in model predictions be assessed.

Phosphorus movement in subsurface flow from agricultural soils can be a significant pathway contributing to eutrophication of surface waters. Our study aimed to evaluate several environmental and agronomic soil P tests as indicators of dissolved reactive P (DRP) concentrations in soil-column leachate from Ontario soils. Undisturbed soil columns were collected from six major soil series, with 10 sites of each to quantitatively cover a wide range of soil test P (STP) or degree of P saturation (DPS). Split-line models described the relationships (P < 0.001) between leachate DRP concentrations and the values of In(STP) and In(DPS), with a greater slope observed above the change points than below them. Among the tested soil P measures, water-extractable P (WEP), Mehlich-3 P/(Mehlich-3 Al + Fe) (DPS^^3-1), and Mehlich-3 P/Mehlich-3 Al (DPS^,3-2) had the strongest overall relationships with leachate DRP concentration. Ontario soils were grouped into no-risk, low-risk, medium-risk, and high-risk categories based on the conditional probability of yielding leachate DRP > 0.1 mg P L"^ at a given STP as measured by WEP and Olsen P or a given DPS as measured by DPS^^j-i and DPS,^.j-2. While the Olsen P test is most commonly used for agronomic calibration in Ontario, DPS^3-2 provided the best indicator of leachate DRP concentration from Ontario soils. Regardless of the test method used, these numeric criteria could be combined with site hydrology and P management practices for a more comprehensive soil P loss assessment.Abbreviations: DPS, degree of phosphorus saturation; DPS^^,-1, Mehlich-3 phosphorus/ (Mehlich-3 aluminum + iron); DPS^¡-2, Mehlich-3 phosphorus/Mehlich-3 aluminum; DPS^3-3, Mehlich-3 phosphorus/Mehlich-3 calcium; DRR dissolved reactive phosphorus; FeO-P, iron-oxide-coated filter paper strip phosphorus; STP, soil test phosphorus; WEP, water-extractable phosphorus.

Soil phosphorus (P) cycling in agroecosystems is highly complex, with many chemical, physical, and biological processes affecting the availability of P to plants. Traditionally, P fertilizer recommendations have been made using an insurance‐based approach, which has resulted in the accumulation of P in many intensively managed agricultural soils worldwide and contributed to the widespread water quality issue of eutrophication. To mitigate further environmental degradation and because future P fertilizer supplies are threatened due to finite phosphate rock resources and associated geopolitical and quality issues, there is an immediate need to increase P use efficiency (PUE) in agroecosystems. Through cultivar selection and improved cropping system design, contemporary research suggests that sufficient crop yields could be maintained at reduced soil test P (STP) concentrations. In addition, more efficient P cycling at the field scale can be achieved through agroecosystem management that increases soil organic matter and organic P mineralization and optimizes arbuscular mycorrhizal fungi (AMF) symbioses. This review paper provides a perspective on how agriculture has the potential to utilize plant and microbial traits to improve PUE at the field scale and accordingly, maintain crop yields at lower STP concentrations. It also links with the need to tighten the P cycle at the regional scale, including a discussion of P recovery and recycling technologies, with a particular focus on the use of struvite as a recycled P fertilizer. Guidance on directions for future research is provided. Core Ideas There is an urgent need to increase P use efficiency in agroecosystems. Crop yields could be maintained at lower than recommended soil test P concentrations. Both the quantity and quality of organic matter influence P availability. Further research on ability of organic P to supply P to crops is needed. Struvite has the potential to fill an important niche in P recycling.

Phosphorus (P) loss from agricultural land in surface runoff can contribute to eutrophication of surface water. This study was conducted to evaluate a range of environmental and agronomic soil P tests as indicators of potential soil surface runoff dissolved reactive P (DRP) losses from Ontario soils. The soil samples (0- to 20-cm depth) were collected from six soil series in Ontario, with 10 sites each to provide a wide range of soil test P (STP) values. Rainfall simulation studies were conducted following the USEPA National P Research Project protocol. The average DRP concentration (DRP30) in runoff water collected over 30 min after the start of runoff increased (p < 0.001) in either a linear or curvilinear manner with increases in levels of various STPs and estimates of degree of soil P saturation (DPS). Among the 16 measurements of STPs and DPSs assessed, DPS(M3) 2 (Mehlich-3 P/[Mehlich-3 Al + Fe]) (r2 = 0.90), DPS(M3)-3 (Mehlich-3 P/Mehlich-3 Al) (r2 = 0.89), and water-extractable P (WEP) (r2 = 0.89) had the strongest overall relationship with runoff DRP30 across all six soil series. The DPS(M3)-2 and DPS(M3)-3 were equally accurate in predicting runoff DRP30 loss. However, DPS(M3)-3 was preferred as its prediction of DRP30 was soil pH insensitive and simpler in analytical procedure, ifa DPS approach is adopted.

Tile drainage systems have been identified as a significant conduit for phosphorus (P) losses to surface water, but P indices do not currently account for this transport pathway in a meaningful way. Several P indices mention tile drains, but most account for either the reduction in surface runoff or the enhanced transport through tiles rather than both simultaneously. A summary of the current state of how tile drains are accounted for within P indices is provided, and the challenges in predicting the risk of P losses through tile drains that are relative to actual losses are discussed. A framework for a component P Index is described, along with a proposal to incorporate predictions of losses through tile drains as a component within this framework. Options for calibrating and testing this component are discussed.