**1. Introduction**

Phosphorus (P) is an essential biosphere component and integral to cellular energy currency in the form of adenosine triphosphate. Phosphate molecules also form the backbone of deoxyribiose nucleic acid and other important biological molecules. In addition to imposing important limits on both terrestrial plant and crop productivity, P availability is also the main factor affecting freshwater eutrophication risk [1]. Unlike carbon (C) and nitrogen (N), P does not undergo substantial atmospheric loss. Phosphine (PH3) is the only known gaseous P form on Earth and its formation is not considered a substantial P loss mechanism from most soils or aquatic sediments [2]. In soil–water systems, pentavalent P forms appear to be most common (P5+); however, water-soluble reduced organic and inorganic P species have also been reported [3].

Once in solution, P acts as a weak Lewis acid with strong affinity for positively charged surface metal ligands, most notably aluminum (Al), iron (Fe), and manganese (Mn) hydroxides, often as organic matter-metal-P complexes [4,5]. Orthophosphate is bioavailable once in solution with maximum availability to (micro)organisms in soils and aquatic sediments near pH 7.0. Variably charged Al and Fe hydroxides are protonated at lower pH (and thus are highly soluble at lower pH), sorbing P from solution more efficiently [5]. As pH

**Citation:** Young, E.O.; Ross, D.S.; Jaisi, D.P.; Vidon, P.G. Phosphorus Transport along the Cropland–Riparian–Stream Continuum in Cold Climate Agroecosystems: A Review. *Soil Syst.* **2021**, *5*, 15. https://doi.org/ 10.3390/soilsystems5010015

Academic Editors: Rick Wilkin and Peter Leinweber

Received: 1 January 2021 Accepted: 5 March 2021 Published: 9 March 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

increases above 7.0, Ca and Mg phosphate formation is thermodynamically favorable; however, a range of metal-P species occur over a wide pH range in soils and sediments [4–7]. The term legacy P refers to accumulation of P in soils/sediments over time accelerated by anthropogenic activities including P inputs from agriculture. Part of the challenge in sustainable water quality improvement is that legacy P stocks can function as a variable but continual source of P release, hampering the efficacy of remediation efforts.

Agricultural P sources are a leading cause of water quality impairment in US rivers and lakes [8]. Managing P for the dual purpose of profitable agriculture and water quality is a major challenge and is pivotal in the water–energy–food security nexus [8–11]. Once viewed as relatively immobile and subject to mainly erosional transport, carrier-facilitated P transport as particulate or colloidal P in addition to dissolved P forms are all vulnerable to transport in Dunne and Hortonian overland flow (a.k.a., overland flow or surface runoff), interflow, subsurface tile drainage, and shallow groundwater flow [4–6,10–20]. Soil physical properties impose important physical transport constraints on P fluxes from upland agricultural and forested landscapes to riparian buffer zones (RBZs) and streams [15,17–19]. While overland flow is an important P transport mechanism in many settings, P is also mobilized in shallow subsurface flows where it has the potential to contribute P to open waters including ditches, streams, rivers, lakes, wetlands, and RBZs.

Cold climates characterize a large number of agriculturally productive regions globally and can be qualitatively defined by areas where a snowpack and frozen soils substantially influence hydrology [19]. Managing P transport in CCs is uniquely challenged by the combination of short growing seasons, high snowmelt runoff, and seasonally wet and/or partially frozen soils [20]. Recent literature highlights gaps in our current understanding of P transport in CCs, suggesting new approaches are needed to more effectively mitigate P transport from cropland to streams and better understand RBZs effects on P speciation and fluxes [21–23].

Water quality is intimately connected to the landscapes through which streams flow. RBZs are widely recognized for their stream water quality benefits, however, their impacts on P transport are variable and site-specific. Traditionally, P transport research has tended to focus on cropland or RBZs exclusively, with relatively few studies evaluating P dynamics in both cropland and RBZs and/or along their natural hydrologic gradients. Since RBZs and cropland often have a close hydrologic connection with similar processes regulating P transport, in this review we focus on factors influencing P transport in surface and subsurface runoff flows along the continuum from cropland through RBZs to streamflow, defined here as the cropland–RBZ–stream continuum. We primarily draw on studies from the USA and Canada over the last two decades.

Sections 2 and 3 focus on the relationship among agronomic nutrient management, assessing agronomic P transport potential, and an overview of hydroclimatic and agricultural management factors influencing P transport. Sections 4 and 5 discuss the critical source area concept and the importance of soil properties for P transport modeling, mapping and risk assessment. The cropland–RBZ–stream hydrologic continuum concept is introduced in Section 6, followed by a review of RBZ impacts on P transport in overland and subsurface flow (interflow and shallow groundwater), including a Section 7 describing stream bank erosion effects on P loading to streams. Section 8 concludes with future research suggestions and some examples from the literature illustrating new approaches combining hydrologic modeling with geographic information system tools for mapping runoff flow pathways in cropland–RBZ–stream systems.

#### **2. Agricultural Nutrient Management**

#### *2.1. Agronomic Phosphorus Site Indices*

Agricultural nutrient management plans (NMPs) specify the form, method, rate, and timing of crop nutrient applications with the goal of increasing crop nutrient use efficiency while minimizing environmental losses and crop production risk. In the US, regulated livestock farms must follow nutrient management guidelines developed by

state Land Grant Universities and the USDA—Natural Resources Conservation Service (NRCS) (Figure 1). The amount of plant-available soil P (i.e., soil test P concentration) is a main driver of agronomic P recommendations. Unlike P, NMPs estimate plant-available N release from mineralization of soil organic matter, manure, and previous crops (using static rate estimates independent of in-season weather conditions). While NMPs account for total P inputs from manure applications, plant-available P release from mineralization of soil organic P is not considered. Similarly, while potentially ecologically important in some regions, atmospheric depositions of P (and N) are not considered.

**Figure 1.** US livestock farms subject to federal Clean Water Act regulations or receiving grant monies must implement cropland nutrient management plans (NMPs) to reduce nonpoint source pollutant loss to open waters. Agronomic P site indices (PSIs) capture soil and management factors affecting annual P loss potential in overland flows and are used to rank P loss potential by fields.

Agronomic NMPs specify field-by-field crop nutrient needs and must include delineation of field characteristics related to erosion and nutrient loss potential, including modeled erosion estimates, presence of concentrated overland flow areas, and proximity to streams/ditches and other landscape features that affect water and nutrient movement (tile drains, karst topography, springs, swales, surface drain inlets). In general, these are also areas where manure and fertilizer P are not recommended during times of high runoff potential and, in some cases, are not to receive any further P applications. Watershed agencies may place further restrictions on land application of manure and fertilizer if farms are in priority watersheds with public drinking water supplies (i.e., New York City watershed, US Great Lakes, Lake Champlain).

Most NMPs in the US require a formal field site assessment of P loss potential using a research based, Land Grant University and NRCS-approved agronomic P site index (PSI). Agronomic PSIs include various rubrics for quantifying P source and transport factors to assign a P loss potential for individual fields based on soil and management factors [24] (Figure 1). Whereas some PSIs include more detailed runoff processes with calibration from edge-of-field runoff P data, many remain qualitative.

Recent US national guidance indicates that agronomic PSIs must establish threshold water quality risks to identify fields not to receive further P inputs. There is also a general consensus that, despite best efforts, P management practices are underperforming with respect to necessary water quality improvement and that there is a need to better account for site-specific hydrology, farm management, and biogeochemical processes influencing P fate and transport [10,11,19,22].

## *2.2. Precision Agriculture and Phosphorus Management*

The ability to manage the timing and placement of crop nutrients in accordance with variable soil and weather conditions can help increase crop P uptake while minimizing losses in runoff. Precision agriculture takes advantage of known field spatial variability (from sampling) by using geographic information systems (GIS) to facilitate autonomous equipment navigation, real-time crop yield monitoring, and variable rate nutrient application. These tools also offer economic advantages for larger farms and are now fairly common [25]. Variable-rate fertilizer application technologies differentially apply P and other nutrients as soil and crop conditions vary across fields [26]. With variable rate application, auxiliary data important for P transport are also routinely collected including soil type boundaries, drainage features, erosion/runoff potential, and other spatially varying soil properties (soil test P, pH, organic matter content). These data can be used to refine P fertility for individual fields and used as inputs for PSIs and other P transport decision support tools aimed at better quantifying P transport potential.

#### **3. Evaluating Cropland Phosphorus Transport Potential**

#### *3.1. Agricultural and Hydroclimatic Factors*

Managing P inputs from manure and fertilizers for optimal crop production while protecting water quality is a challenge in CC agroecosystems. Livestock manure is an important source of C, N, and P for crops and has beneficial physicochemical effects on soil quality, however, P from manure can contribute to excessive soil P concentrations over time and can be readily transported by overland flow, particularly if not incorporated via tillage or injected beneath the soil surface [27–29]. Dairy manure contains relatively high P content with speciation and total P content dependent on animal species, age, diet, and other farm-specific factors [29]. However, once applied to soils, research indicates that much of the organic P transforms fairly rapidly to inorganic P [30,31] and subject to transport in runoff [10,18,20,22]. Recent research suggests that dairy manure application can be associated with larger and more variable overland flow P losses compared to fields receiving similar rates of fertilizer P [32].

It is clear that a range of P forms can be transported in both overland and shallow subsurface flow in a variety of crop production systems receiving a mix of fertilizer and organic P mainly in the form of livestock manure [5,10,12–22,27–49]. While agricultural operations often account for a major nonpoint P source in the watershed via the combination of land disturbance and P applications, it is also important to recognize that streambank erosion and runoff from forested lands can contribute to loading to streams [50–53]. Irrespective of original source, landscape position, or form, P transfer risk to streams is greater during the non-growing season, when much of the annual runoff occurs in CC regions [15,18– 22,34,35,45–47,52,54–60]. Biogeochemical reactions removing P from solution (sorption and plant and microbial assimilation) also diminish during the non-growing season, contributing to greater overall P mobility and the non-growing season is also a period of elevated overland flow potential. Frozen surface soil layers all but eliminate surface water infiltration and exacerbate overland flows during snow melting or mixed precipitation events. Additionally, decreased soil–water interaction in frozen or partially frozen soils contributes to lower P sorption and greater P mobility in overland flow compared to unfrozen soils. On the other hand, when soils are not frozen and infiltration is possible, greater soil–water interaction increases P removal from solution via sorption reactions and metabolic uptake prior to overland flow reaching streamflow.

Climate and the amount, form, and intensity of precipitation are important factors affecting overland flow, erosion, and P transport potential, and varies regionally in CCs. Hoffman et al. [35] monitored overland flow from five small agricultural watersheds (4 to 30 ha) over a 12-yr period in southwestern Wisconsin (WI) and showed that mixed precipitation events had greater mean dissolved reactive P (DRP; assumed to be mainly orthophosphate and bioavailable) concentrations (2.2 mg L−1) than snow (1.9 mg L−1) or rainfall events (1.2 mg L−1). They also reported that snow (74%) and mixed (84%) events

had nearly two-fold greater proportions of DRP in overland flow compared to rainfall (39%), stressing the importance of field-specific interactions among precipitation types and soil physical conditions, temperature, and depth of frozen layers.

Vadas et al. [54] used 108 site years of edge-of-field overland flow data from WI and a calibrated P transport model (SurPhos) to evaluate P loss potential with differing soil hydrologic and P management. Unlike many current P transport models, SurPhos attempts to simulate snowmelt runoff dynamics and processes regulating DRP transfer from soil, fertilizer, and manure P sources using daily weather data. Their simulations indicated site hydrology was the overriding factor influencing P loss with winter application increasing P loss potential by 2.5 to 3.6 times relative to unfrozen soils. They reported that P loss potential was greatest in late January and early February (from melting events) and that P loss potential was reduced by a factor of 3.4 to 7.5-fold by applying manure to fields with a lower overland flow potential.

In a similar geographic region, Zopp et al. [60] used regression tree analysis to determine factors affecting flow-weighted mean total P (TP) and dissolved P concentrations/loads in the upper Midwest using a large regional edge-of-field overland flow and P export data set from WI and Minnesota with 26 fields, 123 site-yr of data, and >20 additional hydroclimatic and management variables. They reported that, when soils were frozen, the majority of overland flow TP was dissolved. Overall, labile soil P concentration at 0–5 cm was the most important predictor of flow weighted mean TP and DRP concentrations in frozen conditions. Soil labile P content is often highly correlated with overland runoff flow DRP concentrations [61] and a critical input for P transport models and PSIs. Additionally, recent edge-of-field runoff research suggests that surface soil P concentration is a main factor affecting DRP transfer to overland flows [62,63], emphasizing the need for NMP strategies to consider practices that slow down the rate of P accumulation in surface soils in addition to focusing on applying manure/fertilizer to fields under low P loss risk conditions (i.e., when soils are unfrozen).

#### *3.2. Cropping System Impacts on Phosphorus Loss Potential*

Soil erosion and total P loss in overland runoff flows are both generally greater under annually tilled crops compared to perennial forage crops or pasture due to mechanical disturbance of tillage operations and lack of continuous vegetative cover [55]. Despite this effect, dissolved P loss can still be substantial in overland flow from perennial forage and no-till systems due to P accumulation in surface soils [55,56]. On the other hand, in annually tilled systems, there is a wide range of impacts on erosion, overland flow, and P loss potential. Besides greater aeration and other potential agronomic benefits, tillage can decrease overland runoff flows compared to no-till by increasing surface roughness in finer-textured soils [57–59]. While there are well-known tradeoffs between greater erosion/particulate P loss with tillage versus lower erosion/particulate P loss with no-till, it is important to note that in some soils, tillage can decrease overland runoff potential, however, this effect is site-specific and depends on several other variables including the consistency and duration of no-till practices. Pasture land often comprises a substantial fraction of agricultural land and generally results in less erosion and particulate P transport compared to row crops; dissolved P forms can still be vulnerable to transport in overland flow (see Sections 4, 5 and 7 for more discussion). While beyond the scope of this review, it is important to recognize that pastured livestock with direct stream access can pose serious water quality challenges [55].

#### **4. Critical Source Areas of Phosphorus**

#### *Source and Transport Factors*

The critical source area concept assumes P transport potential is a function of hydrologic loss mechanisms interacting with P sources on the landscape at any given time [22,32]. Agricultural P sources subject to transfer in runoff pathways and streams along the cropland–RBZ–stream continuum include soil, manure, and fertilizer. From a watershed

biogeochemical perspective, RBZ sources must also be considered as potential P sources to streams in the form of overland and subsurface flows or via stream bank erosion [50–53]. Determining where and when P sources interact with hydrologic flow paths to physically transfer P to RBZs and streamflow is integral to critical source area and watershed "hotspot/hot moment" approaches and derives from distributed hydrologic modeling theory, now more commonly known as variable source area hydrology [64–67]. Variable source area hydrology posits that the amount and timing of overland flows are driven by topographic and soil moisture gradients [66–70]. Studies indicate that incorporating variable source area hydrology routines into watershed P transport models show promise for improving overland runoff flow P fluxes [15,22,32,66–70]. Overland flow sources to streamflow include cropland areas but also near-stream areas subject to variable soil moisture regimes and overland flow generation (i.e., RBZs, swales, springs/seeps, and other wetlands) [68,70,71]. Since topographic features are an important control on both overland flow generation and groundwater hydrology, accurate characterization of cropland–RBZ– stream topographic complexity is critical for developing realistic models and indices of P transport that can better account for RBZs impacts on P transport.

Both spring snowmelt and storm events are important times for P transfer from cropland to surface waters and from variable sources areas to streams [10,14,18,32,35]. Part of the difficulty of controlling CC cropland P transport resides in the seasonal asymmetry between greater non-growing season runoff potential and concomitant decreased P sorption potential and biological assimilation driven by lower soil temperatures, effectively increasing dissolved P availability to overland flow. Recognizing this asymmetry between elevated runoff potential and diminished P removal capacity is a critical aspect for NMPs to consider in CC regions to better manage cropland P loss risk and more effectively target P-specific best practices for mitigating P transport to streams.
