*7.3. Riparian Zone Impacts on Subsurface Phosphorus Transport (Qif and Qgw)*

While there is considerable RBZ-P attenuation uncertainty surrounding Qof, there is wider variation for shallow groundwater and vadose zone P attenuation [86,99]. Despite the fact that RBZs are commonly recommended for reducing P transport, information on RBZ effects on P speciation and fluxes is lacking, particularly in CC regions where hydrobiogeochemical processes and agricultural/riparian management interactions largely control nutrient fluxes [94,99]. Moreover, the hydrologic and soil processes driving P transport from upland agricultural areas to RBZs and Qsf are themselves highly spatially and temporally variable, particularly during freeze–thaw cycles with diurnally fluctuating air temperatures and soil physical conditions (frozen/partially frozen) that complicate water infiltration, subsurface water movement, and therefore P transport [45–47,52,91,94,99,105,116]. An improved understanding of coupled soil hydro-biogeochemical processes driving P transport from RBZs to Qsf in CC regions is needed, in addition to developing a broader set of predictive tools that can accommodate the multivariate and dynamic nature of subsurface P transport and subsequent movement and potential transfer to Qsf.

Hydrology, soils and vegetation are intimately linked and their interactions largely control localized physicochemical environments and biogeochemical mechanisms regulating P availability to both matrix and macropore soil water flows [117,118] (Figure 3). This observation helps partially explain studies reporting mixed efficacy for RBZ subsurface P attenuation [13,37,78,86,95,96,117–124]. In shallow Qgw of RBZs from eastern Canada, Carlyle and Hill [95] reported that RBZ shallow Qgw with lower dissolved oxygen concentrations had higher ferrous iron (Fe2+) and DRP concentrations, and suggested that Qgw redox potential was a main factor affecting the likelihood of P release to Qgw and discharging Qsf. Young and Briggs [13] monitored P concentrations in soil solution (sampled via tension lysimeters, representing Qif) and shallow Qgw for 16 paired cropland-RBZ plots for >2-yr in Central New York. Mean DRP concentrations in Qgw and Qif were lower for RBZs compared to corn and hayfields; however, poorly drained RBZs had greater particulate reactive and dissolved unreactive P concentrations in Qgw, suggesting poorly drained RBZs with elevated water tables and low to moderate labile P status were vulnerable to P release and transport in Qgw compared to more oxidizing Qgw zones. The importance of soil hydrology on P biogeochemistry was also supported by ammonium and nitrate-N patterns. Shallow Qgw zones with lower dissolved oxygen concentrations had lower nitrate-N, higher ammonium-N, and significantly greater DRP concentrations, which suggests denitrification zones could be episodic P flux hotspots [13,125].

Gu et al. [126] combined Qgw, Qif, and Qof measures in the Kervidy-Naizin catchment of Northwestern France over 4-yr with P concentrations and speciation to elucidate transport mechanisms in shallow subsurface flows (Qif + Qgw). The authors hypothesized that the main P transport mechanisms were related to soil hydrology via: (i) reductive dissolution of ferric (Fe3+) phosphates during episodic saturation events (hot moment) and, (ii) P mobilization in soil water flows associated with rainfall events following dry periods (hot moment). The degree and duration of soil saturation is a critical factor affecting P release from RBZ soils since prolonged saturation can elicit both reductive dissolution of Fe-P and Mn-P compounds and encourages dissolution of Al-P, Ca-P, and other P complexes [86,87,111,127,128]. Changes in pH during saturation also affect release of dissolved organic P and other C-P complexes that may be more vulnerable to movement in Qif and/or Qgw due to lower affinity for P sorption sites compared to free orthophosphate [4,5,13,37,78,86,103,104,111,116–118,127,128]. Shallow Qgw residence time is also an important factor influencing thermodynamic conditions and P release and retention in RBZs, particularly via the Fe-P redox cycle [86,111].

#### *7.4. Artificial Subsurface Tile Drainage and Phosphorus Loss Potential*

Installation of subsurface agricultural tile drainage systems (a.k.a., tile drains) is relatively common in CC agricultural regions with poorly drained soils [129]. Modern tile drainage systems consist of perforated plastic drainpipe (typically 10 cm ID for lateral

field lines) typically installed at a depth of 1.0 to 1.5 m deep with variable lateral spacing and designs. Hydrologically, the main objective is lowering the seasonally high ground water table elevation, which facilitates more rapid gravitational (macropore) soil water drainage compared to an undrained condition in a similar setting. Tile drains have long been recognized for their multiple agronomic benefits (e.g., greater yields, earlier planting/harvesting) and erosion mitigation potential [36,129]. Typically, tile-drained fields outlet to some type of surface ditch or directly to streams or open waters.

While accelerated nitrate-N loss via tile drainage flows has long been recognized, P leaching and transport in tile systems has gained more attention over the last two decades [12,16,36,45,63,105]. In a recent review, King et al. [36] discuss P transport dynamics in tile drained systems and the role of soil and nutrient management factors in controlling P concentrations and fluxes to tile drained soils (mainly the US and Canada). In addition to preventing soil P accumulation to high or excessive levels, the authors stressed the importance of soil type and the propensity for macropore flow in regulating P movement to tile drain flow. Unlike matrix soil water flow characterized by advection and dispersion mechanisms, macropore flow is much more rapid, decreasing the opportunity for P sorption reactions that might otherwise bind P and reduce transfer potential to tile flows [12,16,36,41,130].

While P leaching and transport to tile flow is a concern in some settings, it is also important to recognize that tile drains in general significantly reduce Qof and as such can mitigate particulate and/or DRP transport in Qof compared to undrained conditions in some settings [12,36,45,105]. From this standpoint, tile drains are part of the set of solutions to help mitigate P transport in Qof using combinations of practices, while also potentially contributing to less P transfer to down-slope RBZs [131]. Therefore, while not considered environmentally beneficial with respect to N, tile drains may offer site-specific benefits for reducing erosion, Qof, and P transport in Qof. Early RBZ research with N suggested tile drains could lower the water table sufficiently to reduce interaction of cropland Qgw with upper RBZ soil horizons, thus contributing to lower nitrate-N attenuation in the RBZ. However, the full scope of tile drain impacts in RBZ hydrology and P transport is far from clear since few studies have explicitly investigated the impacts of tile drainage designs on P loss compared to undrained conditions.

#### **8. Future Research Considerations**

#### *Phosphorus Transport Modeling and Site Indices*

Calibrated field and watershed-scale P transport models help in allocating P load estimates to different land uses and broad scale targeting of P transport mitigation practices. Incorporating variable source area hydrology algorithms into P models and agronomic PSIs show promise for improving P transport risk predictions [10,32,69,132]. However, watershed scale models are often designed to predict P transport over long time periods and over relatively large areas using historical weather and management data, potentially limiting their effectiveness as a dynamic P loss risk tool at the field scale without substantial modification. Additionally, model routines that can better capture snowmelt runoff processes and soil freeze–thaw dynamics in relation to water flow and P mobility are needed [35,59,99]. Given these potential limitations and the fact that large runoff events tend to dominate P losses from cropland to streams, developing tools that can better predict event based and real-time P fluxes and include RBZ hydro-biogeochemical impacts on P transport will be important, especially in high priority watersheds with chronic P pollution.

Combining LiDAR-based DEMs with hydrologic models and GIS tools show promise for enhancing agroecosystem services by creating opportunities to optimize agricultural land while maintaining RBZ water quality functions. For example, Shrivastav et al. [84] and Thomas et al. [71] combined LiDAR-DEMs and GIS tools to map and ground-truth Qof pathways in cropland–riparian–stream settings (Figure 4a,b). These and other hydrologic studies have clearly demonstrated the tendency for Qof heterogeneity in agricultural areas, highlighting the critical importance of targeting RBZs at known "delivery points" to

intercept dissolved and entrained P in Qof prior to reaching Qsf. Kuglerová et al. [82] used a high resolution LiDAR-DEM and a hydrologic model to establish variable width forest RBZs based on soil and landscape characteristics, whereby recharge areas more vulnerable to solute leaching had wider RBZs (Figure 4c,d).

**Figure 4.** Model estimates of overland runoff flow (Qof) pathways using light detection and ranging (LiDAR) based digital elevation models from Shrivastav et al. [84] (**a**) and Thomas et al. [71] (**b**) illustrating the tendency for non-uniform Qof, and highlights the critical importance of targeting riparian buffers at known Oof delivery points to intercept sediment and phosphorus prior to reaching streams. Yellow and red in 4b indicate Qof areas and blue circles indicate Qof delivery points. Variable width riparian forest buffer zones predicted by a LiDAR based groundwater hydrology model by Kuglerová et al. [82] (**c**,**d**). Panel 4c shows a sunlit LiDAR image of a stream section; blue lines are small streams and red stars are runoff collection points. In panel 4d, blue layers indicate model predicted groundwater discharge zones (darker blues indicate greater fluxes) and red areas are intermittent streams. All figures reproduced with permission.

While existing watershed P transport models and PSIs will remain important tools, simplified process-based models that can readily integrate LiDAR and other digital data will be important for simulating site-specific hydrologic and P transport processes. In a review of nutrient dynamics in CC agricultural catchments, Costa et al. [99] suggested that more parsimonious P transport models that simulate major soil and hydroclimatic processes governing runoff generation and P transport may be more advantageous than larger, more complex models. Several investigators have combined process-based model outputs with Bayesian networks, machine learning, and other artificial intelligence algorithms to develop predictive hydrologic and nutrient flux models along with uncertainty estimates [133–137].

In addition to innovative predictive tools that can account for more of the weather driven and seasonal dynamics of P transport, longer term management practices aimed at reducing P imbalances and soil P accumulation are needed. As previously indicated, current efforts are not sufficiently attenuating P transport or eutrophication risk in the US and other countries, and that new tools and practices are needed to further curb P transport from cropland to streams. While RBZs will remain an important practice, modifications may be needed to improve soluble P removal efficiencies. Not unlike cropland, RBZs must also be managed for optimal performance if P removal is a desired ecosystem service [138,139]. To this end, more widespread and routine soil P testing of RBZs is suggested (similar to P testing for NMPs). Routinely testing RBZs for soil P status as part of agronomic NMPs could be a simple and cost-effective way to provide a baseline indicator of labile P status. Additionally, soil P data could be combined with hydrologic data to further characterize P transport potential.

Given the strong relationship between pH and P availability in soils and legacy sediments [140,141], lowering or raising pH to decrease P availability (something commonly done on cropland to increase soil P availability) in RBZ soils offers a way to further decrease DRP transport from RBZs to Qsf. However, altering soil pH has implications for plant communities, organic C cycling, and other ecological considerations. Careful research is necessary to evaluate potential tradeoffs between enhancing P sorption in RBZs via pH alterations and maintaining overall ecological integrity.

Riparian vegetation plays an important but poorly understood role in P transfer to Qsf. More research to better understand RBZ soil-vegetation interactions and their impacts on P transport is another area of need [115]. Several studies suggest the periodic harvesting of RBZ vegetation to reduce labile soil P concentrations, remove P, and presumably reduce DRP release and transport potential [86,94,97,101,115,123,138,139]. While it is clear that RBZ vegetation can affect P biogeochemistry and physical transport, it is far from clear what the optimum soil-vegetation combinations are for maximizing P attenuation. More research dedicated to soil-vegetation interactions with the goal of maximizing DRP attenuation is needed (particularly for P sensitive watersheds) to enable prescriptive management combinations to mitigate P transport. Lastly, where appropriate, a broader set of predictive approaches should be considered (i.e., Bayesian neural networks, artificial intelligence, machine learning algorithms and various hybrid models) to P loss prediction and develop real-time, dynamic P transport prediction tools that can simultaneously quantify risk and uncertainty.

**Author Contributions:** This article was contributed to by the authors in the following way: conceptualization, E.O.Y.; methodology, E.O.Y., D.S.R., D.P.J., P.G.V.; formal analysis, E.O.Y., D.S.R., D.P.J., P.G.V.; investigation, E.O.Y.; writing—original draft preparation, E.O.Y.; review and editing, D.S.R., D.P.J., P.G.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported in part by the U.S. Department of Agriculture, Agricultural Research Service.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable

**Data Availability Statement:** Data are contained within the article or cited articles in the review.

**Acknowledgments:** The authors would like to thank Barbara C. Storandt for her assistance with formatting references and proofreading the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

