Nutrient losses from agricultural fields are the largest sources of phosphorus (P) entering the Great Lakes in North America. Stacked conservation practices (CPs) may reduce P losses from individual fields. Simple low-cost, low disturbance, commercially available filters containing wood chips and phosphorus sorbing materials (PSM) were installed on two fields already using conservation practices in midwestern Ontario (ILD and LON) to quantify their ability to remove soluble reactive P (SRP), particulate P (PP), total P (TP) and total suspended sediments (TSS) from surface runoff. Laboratory tests on unused (new) and used (field) filter materials were also conducted to estimate P sorption and remobilization potentials. During the two-year study period, the filter retained 0.018 kg ha-1 of SRP, 0.38 kg ha-1 of PP, 0.4 kg ha-1 of TP and 8.75 kg ha-1 of TSS from surface runoff at the ILD site. In contrast, although the filter at LON removed 37 kg ha-1 of TSS and 0.07 kg ha-1 of PP, it released 0.22 kg ha-1 of SRP and 0.15 kg ha-1 TP. A reduction in filter efficacy was observed over time, particularly at the site with greater cumulative surface runoff and larger runoff events (LON). The majority of the SRP retained by the filter was held in a loosely bound form, thus, susceptible to P remobilization. The results of this study demonstrate that low-cost, simple PSMs have some potential to retain P from surface runoff, but their efficacy may decline over time and their P retention capability may differ with site hydrology (e.g., runoff volumes and velocity) and P supply.

Abstract Nutrient and soil loss from agricultural areas impairs surface water quality globally. In the Great Lakes region, increases in the frequency and magnitude of harmful and nuisance algal blooms in freshwater lakes have been linked to elevated phosphorus (P) losses from agricultural fields, some of which are transported via tile drainage. This study examined whether concentrations and loads of P fractions, total suspended sediments (TSS), nitrate (NO 3 − ), and ammonium (NH 4 + ) in tile drainage in a clay soil differed between a continuous no‐till system combining cover crops and surface broadcast fertilizer (no‐till cover crop [NTCC]), and a more conventional tillage system with shallow tillage, fertilizer incorporation and limited use of cover crops (conventional conservation‐till, CT). Both sites had modest soil fertility levels. Year‐round, high‐frequency observations of tile drainage flow and chemistry are described over 4 full water years and related to management practices on the associated fields. There were similar water yields in tile drainage between the two systems; however, losses of TSS, particulate P (PP), and NO 3 − were consistently greater from the CT site, which received larger quantities of fertilizer. In contrast, dissolved reactive P (DRP) losses were considerably greater from the NTCC site, offsetting the lower PP losses, such that there was little difference in TP losses between sites. Approximately 60% of the DRP losses from the NTCC site over the 4 years were associated with incidental losses following surface application of fertilizer in fall. This study provides insight into trade‐offs in controlling losses of different nutrient fractions using different management systems.

Agricultural fields in the Red River Valley of the Northern Great Plains are located on flat clay soils, often drained by shallow, roadside ditches that are not graded and lacking relief. These conditions can result in flow reversals and subsequent flooding of adjacent fields during large runoff events, which can mobilize phosphorus (P). Surface runoff from two agricultural fields and their adjacent ditches was monitored from 2015 to 2017 in southern Manitoba, Canada. Overbank flooding of fields adjacent to ditches was observed in 5 of 21 hydrologic events, and such events dominated annual runoff and P budgets (>83% of losses over the 3-year study period). Flooding events were often dominated by soluble P fractions (57–67%) relative to events where flooding was not observed (39–63%). Concentrations of soluble reactive P in water standing on fields increased with time during flooding events, suggesting that P was mobilized during such events; however, the source of the soluble reactive P is not clear. This study has highlighted temporal differences in hydrologic and biogeochemical interactions between fields and ditches and demonstrated the need for an improved understanding of mechanisms of P mobilization in the landscape, which has direct implications for predicting P mobility in agricultural watersheds.

Delineating the relative solubility of soil phosphorus (P) in agricultural landscapes is essential to predicting potential P mobilization in the landscape and can improve nutrient management strategies. This study describes spatial patterns of soil extractable P (easily, moderately, and poorly soluble P) in agricultural landscapes of the Red River basin and the southern Great Lakes region. Surface soils (0-30 cm) and select deeper cores (0-90 cm) were collected from 10 cropped fields ranging in terrain (near-level to hummocky), soil texture (clay to loam), composition (calcareous to noncalcareous), and climate across these differing glacial landscapes. Poorly soluble P dominated (up to 91%) total extractable P in the surface soils at eight sites. No differences in the relative solubilities of soil extractable P with microtopography were apparent in landscapes without defined surface depressions. In contrast, in landscapes with pronounced surface depressions, increased easily soluble P (Sol-P), and decreased soil P sorption capacity were found in soil in wetter, low-slope zones relative to drier upslope locations. The Sol-P pool was most important to soil P retention (up to 28%) within the surface depressions of the Red River basin and at sites with low-carbonate soils in the southern Lake Erie watershed (up to 28%), representing areas at elevated risk of soil P remobilization. This study demonstrates interrelationships among soil extractable P pools, soil development, and soil moisture regimes in agricultural glacial landscapes and provides insight into identifying potential areas for soil P remobilization and associated P availability to crops and runoff.

The eutrophication of freshwater systems is a pervasive issue in North America and elsewhere, which has been linked to elevated phosphorus (P) loading from watersheds. Most excess P is thought to originate from non-point agricultural sources, and less attention has been given to small rural point sources, such as bunker silos on livestock farms. Sophisticated management practices are rarely used to attenuate nutrients from bunker silo effluent, leaving simple vegetated buffer strips or riparian zones to protect surface water; however, the efficacy of these systems or larger constructed treatment systems is unclear. This study compared two systems receiving bunker silo effluent, one a natural riparian system with a vegetated buffer strip that is the most common practice and the other a constructed treatment system with a forebay, slag filter, and swale. The study quantified P retention within various subsections of each system and characterized the forms of stored P to infer the potential for remobilization. Results indicate that soils receiving bunker silo effluent represent considerable stores of legacy P in the landscape (750 and 3400 kg ha−1), the majority of which is stored in labile forms that may be vulnerable to remobilization under the waterlogged conditions that often occur in management practices and riparian zones. Some areas of the systems were able to store considerably more P than others, with the slag filter showing the greatest treatment efficacy. Spatial variability in stored P was apparent, where sections of the systems that directly received effluent retained more P than sections located farther away from bunker silos (indirect inputs). Results indicate that passive treatment systems become P saturated over time, limiting their longterm P removal efficacy. The efficacy of these systems may be improved with the inclusion of sorptive materials as a slag filter within the constructed treatment system significantly increased the life expectancy of that system. Greater understanding of both quantity and forms of P retained in systems and soils receiving bunker silo effluent will help develop management strategies that are more effective and longer-lasting for reducing excess P losses to surface water bodies.

Nutrient losses from agricultural operations are a major contributor to the eutrophication of freshwaters. Although many studies have quantified diffuse nutrient losses, less is known about agricultural point-source contributions, such as bunker silos, to watershed phosphorus (P) loads. This study examined the contributions of a dairy farm bunker silo effluent to watershed soluble reactive P (SRP) and total P (TP) losses. The bunker silo effluent discharged to an adjacent stream via a riparian soakaway for ca. 15 years. Prior to the annual refilling of the bunker silo, flow weighted mean concentrations of SRP (TP) were similar between stream locations up and downstream of the farm. After the bunker silo was refilled, flow-weighted SRP (TP) concentrations in the stream increased by factors of 1.5(2.2) during events and 3.1(2.3) during baseflow. Higher P concentrations occurred in the riparian soils receiving bunker silo effluent (525–3125 mg/kg TP, and 0.1–9.9 mg/kg water extractable P (WEP), compared with 525–939 mg/kg TP, and 0.11–1.43 mg/kg WEP on the opposite side of the stream with no bunker silo effluent. Riparian soils impacted by the bunker silo were near P-saturation, and the riparian zone did little to reduce P transfer in shallow groundwater. The net contributions of bunker silo effluent to annual watershed P losses were 32% (SRP) and 22% (TP). This study highlights the importance of agricultural point sources, and the need to quantify their contributions to watershed P budgets to target P remediation effectively.

Agricultural P losses are a global economic and water quality concern. Much of the current understanding of P dynamics in agricultural systems has been obtained from rainfall-driven runoff, and less is known about cold-season processes. An improved understanding of the magnitude, form, and transport flow paths of P losses from agricultural croplands year round, and the climatic drivers of these processes, is needed to prioritize and evaluate appropriate best management practices (BMPs) to protect soil-water quality in cold regions. This study examines multiyear, year-round, high-frequency edge-of-field P losses (soluble reactive P and total P [TP]) in overland flow and tile drainage from three croplands in southern Ontario, Canada. Annual and seasonal budgets for water, P, and estimates of field P budgets (including fertilizer inputs, crop uptake, and runoff) were calculated for each site. Annual edge-of-field TP loads ranged from 0.18 to 1.93 kg ha yr (mean = 0.59 kg ha yr) across the region, including years with fertilizer application. Tile drainage dominated runoff across sites, whereas the contribution of tiles and overland flow to P loss differed regionally, likely related to site-specific topography, soil type, and microclimate. The nongrowing season was the dominant period for runoff and P loss across sites, where TP loss during this period was often associated with overland flow during snowmelt. These results indicate that emphasis should be placed on BMPs that are effective during both the growing and nongrowing season in cold regions, but that the suitability of various BMPs may vary for different sites.

Phosphorus (P) loss in agricultural discharge has typically been associated with surface runoff; however, tile drains have been identified as a key P pathway due to preferential transport. Identifying when and where these pathways are active may establish high-risk periods and regions that are vulnerable for P loss. A synthesis of high-frequency, runoff data from eight cropped fields across the Great Lakes region of North America over a 3-yr period showed that both surface and tile flow occurred year-round, although tile flow occurred more frequently. The relative timing of surface and tile flow activation was classified into four response types to infer runoff-generation processes. Response types were found to vary with season and soil texture. In most events across all sites, tile responses preceded surface flow, whereas the occurrence of surface flow prior to tile flow was uncommon. The simultaneous activation of pathways, indicating rapid connectivity through the vadose zone, was seldom observed at the loam sites but occurred at clay sites during spring and summer. Surface flow at the loam sites was often generated as saturation-excess, a phenomenon rarely observed on the clay sites. Contrary to expectations, significant differences in P loads in tiles were not apparent under the different response types. This may be due to the frequency of the water quality sampling or may indicate that factors other than surface-tile hydrologic connectivity drive tile P concentrations. This work provides new insight into spatial and temporal differences in runoff mechanisms in tile-drained landscapes.

Phosphorus (P) mobilization in agricultural landscapes is regulated by both hydrologic (transport) and biogeochemical (supply) processes interacting within soils; however, the dominance of these controls can vary spatially and temporally. In this study, we analyzed a 5-yr dataset of stormflow events across nine agricultural fields in the lower Great Lakes region of Ontario, Canada, to determine if edge-of-field surface runoff and tile drainage losses (total and dissolved reactive P) were limited by transport mechanisms or P supply. Field sites ranged from clay loam, silt loam, to sandy loam textures. Findings indicate that biogeochemical processes (P supply) were more important for tile drain P loading patterns (i.e., variable flow-weighted mean concentrations ([]) across a range of flow regimes) relative to surface runoff, which trended toward a more chemostatic or transport-limited response. At two sites with the same soil texture, higher tile [] and greater transport limitations were apparent at the site with higher soil available P (STP); however, STP did not significantly correlate with tile [] or P loading patterns across the nine sites. This may reflect that the fields were all within a narrow STP range and were not elevated in STP concentrations (Olsen-P, ≤25 mg kg). For the study sites where STP was maintained at reasonable concentrations, hydrology was less of a driving factor for tile P loadings, and thus management strategies that limit P supply may be an effective way to reduce P losses from fields (e.g., timing of fertilizer application).

Abstract Phosphorus (P) losses from agricultural soils are a growing economic and water-quality concern in the Lake Erie watershed. While recent studies have explored edge-of-field and watershed P losses related to land-use and agricultural management, the potential for soils developed from contrasting parent materials to retain or release P to runoff has not been examined. A field-based study comparing eight agricultural fields in contrasting glacial landscapes (hummocky coarse-textured till-plain, lacustrine and fine-textured till-plain) showed distinct physical and geochemical soil properties influencing inorganic P (Pi) partitioning throughout the soil profile between the two regions. Fields located on the coarse-textured till-plain in mid-western Ontario, Canada had alkaline calcareous soils with the highest Total-Pi concentrations and the majority of soil Pi stored in an acid-soluble pool (up to 91%). In contrast, loosely to moderately soluble Pi concentrations were higher in soils of the lacustrine and fine-textured till-plain in southwestern Ontario, northeast Indiana and northwestern Ohio, US. Overall, soils on the lacustrine and fine-textured till-plain had a greater shrink swell-capacity, likely creating preferential flow to minimize Pi interaction with the more acidic, lower carbonate and lower sorption capacity soils. These differences in soil Pi retention and transport pathways demonstrate that in addition to management, the natural landscape may exert a significant control on how Pi is mobilized throughout the Lake Erie watershed. Further, results indicate that careful consideration of region-specific hydrology and soil biogeochemistry may be required when designing appropriate management strategies to minimize Pi losses across the lower Great Lakes region.