Phosphorus transport through subsurface drainage and surface runoff from a flat watershed in east central Illinois, USA

Assessing the concept of control points for dissolved reactive phosphorus losses in subsurface drainage

Agricultural phosphorus (P) loss, which is highly variable in space and time, has been studied using the hot spot/hot moment concept, but increasing the rigor of these assessments through a relatively newer "ecosystem control point" framework may help better target management practices that provide a disproportionate water quality benefit. Sixteen relatively large (0.85 ha) subsurface drainage plots in Illinois were used as individual observational units to assess dissolved reactive P (DRP) concentrations and losses within a given field over four study years. Three plot-months were identified as DRP control points (one export and two transport control points), where each plot-month contributed >10% of the annual DRP load from the field. These control points occurred on separate plots and in both the growing and nongrowing seasons but were likely related to agronomic P applications. Elevated soil test P, especially near a historic farmstead, and soil clay content were spatial drivers of P loss across the field. The nongrowing season was hypothesized to be the most significant period of P loss, but this was only documented in two of the four study years. A cereal rye (Secale cereale L.) cover crop did not significantly reduce DRP loss in any year, but there was also no evidence of increased drainage P losses due to freezing and thawing of the cover crop biomass. This work confirmed annual subsurface drainage DRP losses were agronomically small (<3% of P application rate), although the range of DRP concentrations relative to eutrophication criteria still demonstrated a potential for negative environmental impact. The control point concept may provide a new lens to view drainage DRP losses, but this framework should be refined through additional within-field studies because mechanisms of P export at this field were more nuanced than just the presence of tile drainage (i.e., a transport control point).

Phosphorus availability and leaching losses in annual and perennial cropping systems in an upper US Midwest landscape

Excessive phosphorus (P) applications to croplands can contribute to eutrophication of surface waters through surface runoff and subsurface (leaching) losses. We analyzed leaching losses of total dissolved P (TDP) from no-till corn, hybrid poplar (Populus nigra X P. maximowiczii), switchgrass (Panicum virgatum), miscanthus (Miscanthus giganteus), native grasses, and restored prairie, all planted in 2008 on former cropland in Michigan, USA. All crops except corn (13 kg P ha⁻¹ year⁻¹) were grown without P fertilization. Biomass was harvested at the end of each growing season except for poplar. Soil water at 1.2 m depth was sampled weekly to biweekly for TDP determination during March–November 2009–2016 using tension lysimeters. Soil test P (0–25 cm depth) was measured every autumn. Soil water TDP concentrations were usually below levels where eutrophication of surface waters is frequently observed (> 0.02 mg L⁻¹) but often higher than in deep groundwater or nearby streams and lakes. Rates of P leaching, estimated from measured concentrations and modeled drainage, did not differ statistically among cropping systems across years; 7-year cropping system means ranged from 0.035 to 0.072 kg P ha⁻¹ year⁻¹ with large interannual variation. Leached P was positively related to STP, which decreased over the 7 years in all systems. These results indicate that both P-fertilized and unfertilized cropping systems may leach legacy P from past cropland management.

Controls on subsurface nitrate and dissolved reactive phosphorus losses from agricultural fields during precipitation-driven events

The magnitude of nitrogen (N) and phosphorus (P) exported from agricultural fields via subsurface tile drainage systems is determined by site-specific interactions between weather, soil, field, and management characteristics. Here, we used multiple regression analyses to evaluate the influence of 29 controls of precipitation event-driven discharge, nitrate (NO₃^--N) load, and dissolved reactive P (DRP) load from subsurface tile drains, leveraging a unique dataset of ~7000 precipitation events observed across 40 agricultural fields (n = 190 site years) instrumented to collect continuous water quality samples. We calculated marginal effects of significant controls and assessed the modifying influence of event rainfall, duration, and intensity, and antecedent precipitation. Tile discharge was strongly and positively influenced by previous 7-day precipitation and total rainfall and negatively influenced by daily temperature and tile spacing. Both tile NO₃^--N and DRP loads were positively influenced by transport and source variables, including event discharge and total fertilizer applied as well as soil test P (STP) in the case of tile DRP load; factors with the strongest negative influence on tile NO₃^--N and DRP loads were related to time of year. The strength and direction of both positive and negative controls also varied with precipitation characteristics. For example, the positive influence of event discharge on nutrient loads lessened as event duration, event intensity, and previous 7-day precipitation increased, while the positive influence of N and P sources strengthened, particularly in response to extreme (or maximum) events. Results here demonstrate the predominant role of transport and source controls while accounting for interactive effects among site-specific characteristics and underscore the importance of storm dynamics when managing N and P loss from agricultural fields.

High flow event induced the subsurface transport of particulate phosphorus and its speciation in agricultural tile drainage system

Subsurface storm flow of phosphorus (P), including particulate P, has been recently discussed as an important P transport path in contrast to typical surface runoff events. However, P speciation, and P concentration during storm events has not been extensively investigated; therefore, its contribution to the water quality is not clearly understood. In this study, the physicochemical properties of particulate P in tile water samples during a high flow event were investigated in Midwestern agricultural lands using wet chemical methods, 31P Nuclear Magnetic Resonance spectroscopy and P K-edge X-ray absorptions near edge structure spectroscopy. In slightly alkaline pH tile water, total P was ranging from ∼0.06 to 0.22 mg L^-1, which is significantly greater than dissolved reactive P (DRP) (∼0.02-0.08 mg L^-1). The tile water contains P enriched particulate matters (∼200-660 mg L^-1). Total P in the colloidal fraction was from 1013 to 2270 mg kg^-1. Phosphate and organic P species, especially monoesters, are sorbed in soil colloids like calcite, and iron oxides, and colloids are effective carriers of P in the subsurface transport process during storm events. The results of this study show that storm events can accelerate the subsurface transport of P with soil particles in addition to DRP.

Closed depressions and soil phosphorus influence subsurface phosphorus losses in a tile-drained field in Illinois

Artificial subsurface (tile) drainage systems can convey phosphorus (P) from agricultural fields to surface waters; however, controls of subsurface dissolved reactive P (DRP) losses at the sub-field scale are not fully understood. We characterized subsurface DRP loads and flow-weighted mean concentration (FWMC) from January 2015 through September 2017 to determine seasonal (growing vs. non-growing) patterns from 36 individually monitored plots across a farm under a corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] rotation in east-central Illinois. Using linear mixed models, we investigated the effects of soil test P (STP), depression depth, and their interaction with precipitation and P fertilization on subsurface DRP losses. Dissolved reactive P loads in drainage tiles increased with precipitation and were greatest during the non-growing season (NGS) in 2016 and 2017. Annual subsurface DRP loads were positively related to STP, and during the NGS, there was a positive relationship between depression depth quantified at the plot-scale and subsurface DRP loads and FWMC. Along a depression-depth gradient, piecewise regression displayed a threshold at a depth of 0.38 m at which STP increased, indicating soil P accumulation in deeper closed depressions. Our study highlights the need to identify areas with the greatest risk of subsurface P losses to implement sub-field scale nutrient management practices.

Using Steel Slag for Dissolved Phosphorus Removal: Insights from a Designed Flow-Through Laboratory Experimental Structure

Steel slag, a byproduct of the steel making process, has been adopted as a material to reduce non-point phosphorus (P) losses from agricultural land. Although substantial studies have been conducted on characterizing P removed by steel slag, few data are available on the removal of P under different conditions of P input, slag mass, and retention time (RT). The objective of this study was to investigate P removal efficiency as impacted by slag mass and RT at different physical locations through a horizontal steel slag column. Downstream slag segments were more efficient at removing P than upstream segments because they were exposed to more favorable conditions for calcium phosphate precipitation, specifically higher Ca2+ concentrations and pH. These results showed that P is removed in a moving front as Ca2+ and slag pH buffer capacity are consumed. In agreement with the calcium phosphate precipitation mechanism shown in previous studies, an increase in RT increased P removal, resulting in an estimated removal capacity of 61 mg kg−1 at a RT of 30 min. Results emphasized the importance of designing field scale structures with sufficient RT to accommodate the formation of calcium phosphate.

Impact factors and mechanisms of dissolved reactive phosphorus (DRP) losses from agricultural fields: A review and synthesis study in the Lake Erie basin

Dissolved Reactive Phosphorus (DRP) losses from agricultural fields promote algae growth in water bodies, and may increase the risk of Harmful Algal Blooms (HABs). Using existing data from the Lake Erie Basin, we applied multiple regression analysis to better understand the impacts of both site-specific conditions (e.g., soil types/properties) and management practices (e.g., Agricultural Conservation Practices [ACP]) on annual DRP losses in subsurface and surface runoff. Results showed that soil properties significantly impact DRP losses. Greater DRP losses were associated with increased soil pH and Soil Test Phosphorus (STP). By contrast, soil organic matter (SOM) was inversely correlated with DRP losses. Soil clay content was also inversely correlated with DRP subsurface losses, but had no impact on DRP surface losses. The ACPs evaluated had varied effectiveness on DRP loss reduction. Cropping systems involving soybean could reduce DRP subsurface losses, whereas winter cover crops could cause unintended DRP subsurface losses. Cropping systems involving soybean and cover crops, however, had no impact on DRP surface losses. In addition, no-till and conservation tillage also enhanced DRP losses compared to conventional tillage, particularly for soils with high SOM and/or high clay content. Precipitation amount and fertilizer application rate significantly increased DRP surface losses as expected. Fertilizer application rate, however, had no impact on DRP subsurface losses. The impact of precipitation amount on DRP subsurface losses depends on STP levels. Precipitation amount significantly increases DRP subsurface losses when STP is higher (>41 mg kg-1 in this analysis). The optimal STP level for crop growth is 30 to 50 mg kg-1. Results from this study help us to better understand DRP losses and the effectiveness of ACPs for controlling them. We suggest taking soil surveys and soil tests into consideration when designing and/or implementing ACPs to manage DRP losses. Furthermore, the method we used for this study could be applied to other agricultural regions to investigate impacts of site-specific conditions and management practices on water quality.

Internal Phosphorus Storage in Two Headwater Agricultural Streams in the Lake Erie Basin

Internal phosphorus (P) in sediments plays an important role in the nutrient dynamics of lakes, sometimes long after external loads have been reduced. Similarly, internal P sources may drive the nutrient dynamics of small agricultural streams that drain to larger rivers and lakes, despite best management practices intended to reduce external P loads from adjacent fields. Here, internal P concentrations were measured with sequential extraction on cores collected in spring and summer from two small agricultural streams in the drainage basin of Lake Erie, a large, eutrophic lake experiencing increasing SRP loads. Average total extractable P concentrations were similar to within 5% during spring and summer, but mobile P binding fractions nearly doubled in summer, possibly due to accelerated rates of organic matter mineralization or iron reduction beneath suboxic, stagnant surface waters. One site had chronically greater internal P concentrations by 25-75%, despite the implementation of best management practices such as grass buffers. The site also had more aquatic vegetation that restricted the flow, less dissolved oxygen in surface water, and greater organic matter in sediments during both seasons, suggesting that variations in hydrology, sediment composition, and vegetation influence hot spots of P retention throughout small agricultural streams.

Midwestern cropping system effects on drainage water quality and crop yields

Grain producers are challenged to maximize crop production while utilizing nutrients efficiently and minimizing negative impacts on water quality. There is a particular concern about nutrient export to the Gulf of Mexico via loss from subsurface drainage systems. The objective of this study was to investigate the effects of crop rotation, tillage, crop residue removal, swine manure applications, and cereal rye (Secale cereale L.) cover crops on nitrate-N (NO3 -N) and total reactive phosphorus (TRP) loss via subsurface drainage. The study was evaluated from 2008 through 2015 using 36 0.4-ha plots outfitted with a subsurface drainage water quality monitoring system. Results showed that when swine manure was applied before both corn (Zea mays L.) and soybean [Glycine max (L.) Merr.], drainage water had significantly higher 8-yr-average flow-weighted NO3 -N concentrations compared with swine manure applied before corn only in a corn-soybean rotation. The lowest NO3 -N loss was 15.2 kg N ha-1  yr-1 from a no-till corn-soybean treatment with rye cover crop and spring application of urea-ammonium nitrate (UAN) to corn. The highest NO3 -N loss was 29.5 kg N ha-1  yr-1 from swine manure applied to both corn and soybean. A rye cover crop reduced NO3 -N loss, whereas tillage and residue management had little impact on NO3 -N loss. Losses of TRP averaged <32 g P ha-1  yr-1 from all treatments. Corn yield was negatively affected by both no-till management and cereal rye cover crops. Results showed that cropping management affected N leaching but impacts on P leaching were minimal.

Study on Characteristics of Nitrogen and Phosphorus Loss under an Improved Subsurface Drainage

Agricultural drainage plays an effective role in preventing waterlogging and salinity disasters and also is the main transport pathway for agricultural non-point source pollutants into rivers and lakes. Hence, the water quality of agricultural drainage should be a point of focus. In this paper, nitrogen and phosphorus loss under improved subsurface drainage with different filter materials (gravel, layered sand-gravel, mixed sand-gravel, straw) were studied by a three-year field experiment (2016–2018) compared with the conventional subsurface drainage. The pH values, total nitrogen, ammonia nitrogen, nitrate nitrogen, total phosphorus and soluble reactive phosphate were considered. The results showed that the nitrogen and phosphorus concentrations of drain outflow under improved subsurface drainage with gravel filter were larger than that with layered sand-gravel filter and mixed sand-gravel filter. The improved subsurface drainages with layered sand-gravel filter and mixed sand-gravel filter had an effect on reducing the ammonia nitrogen, total phosphorus and soluble reactive phosphate concentrations of the outflow. Meanwhile, the characteristics of nitrogen and phosphorus loss under the improved subsurface drainage with straw filter were different from that with layered sand-gravel filter and mixed sand-gravel filter. For the improved subsurface drainage with layered sand-gravel filter outflow, the ammonia nitrogen, total phosphorus, and soluble reactive phosphate concentrations were about 13%–78%, 38%–63%, 40%–68% less, and total nitrogen, nitrate nitrogen concentrations were 24%–80%,18%–96% more than that under conventional subsurface drainage. Meanwhile, for the improved subsurface drainage with straw filter outflow, compared with conventional subsurface drainage outflow, the percentage changes of the total nitrogen, nitrate nitrogen, ammonia nitrogen, total phosphorus and the soluble reactive phosphate concentrations were about −76%–62%, −77%–78%, −152%–−274%, −103%–−400% and −221%–−291%, respectively. Additionally, in the outflow of all subsurface drainage patterns, there were much higher total nitrogen and nitrate nitrogen concentrations which should be focused on and the agricultural water management should be adopted.

Contribution of Overland and Tile Flow to Runoff and Nutrient Losses from Vertisols in Manitoba, Canada

This study quantified the contributions of overland and tile flow to total runoff (sum of overland and tile flow) and nutrient losses in a Vertisolic soil in the Red River valley (Manitoba, Canada), a region with a cold climate where tile drainage is rapidly expanding. Most annual runoff occurred as overland flow (72-89%), during spring snowmelt and large spring and summer storms. Tile drains did not flow in early spring due to frozen ground. Although tiles flowed in late spring and summer (33-100% of event flow), this represented a small volume of annual runoff (10-25%), which is in stark contrast with what has been observed in other tile-drained landscapes. Median daily flow-weighted mean concentrations of soluble reactive P (SRP) and total P (TP) were significantly greater in overland flow than in tile flow ( < 0.001), but the reverse pattern was observed for NO-N ( < 0.001). Overland flow was the primary export pathway for both P and NO-N, accounting for >95% of annual SRP and TP and 50 to 60% of annual NO-N losses. Data suggest that tile drains do not exacerbate P export from Vertisols in the Red River valley because they are decoupled from the surface by soil-ice during snowmelt, which is the primary time for P loss. However, NO-N loading to downstream water bodies may be exacerbated by tiles, particularly during spring and summer storms after fertilizer application.

Hydrograph separation of subsurface tile discharge

Baseflow is an important component of streamflow and watershed hydrologic budgets, yet quantifying the baseflow fraction of tile drainage has rarely been reported. In this study, we used two common hydrograph separation methods (local minimum method, recursive digital filter) to separate the discharge hydrographs from three drainage district tiles located in Iowa. Based on data collected from 2009 to 2013, annual baseflow ranged from 116 to 162 mm and comprised approximately 60% of the annual discharge. Baseflow was greatest during June (average of 34% of annual baseflow) and the March through August period produced 86% of the total annual baseflow. We found that the two methods of hydrograph separation produced similar results but the digital filter method was less erratic in estimating baseflow fraction. Study results can be used to better quantify hydrologic pathways in tiled landscapes and improve the design, implementation, and evaluation of nutrient reduction strategies.

Fixed bed column evaluation of phosphate adsorption and recovery from aqueous solutions using recycled steel byproducts

Excessive phosphorus loading from anthropogenic sources is a major cause of eutrophication of natural waters. Phosphorus is also a non-renewable natural resource that cannot be substituted with other sources. The objective of this study was to determine the feasibility of using recycled steel byproducts to remove and recover phosphate from aqueous solutions. Laboratory fixed bed column experiments were conducted with recycled steel chips of different sizes to evaluate phosphate adsorption characteristics and phosphate recovery efficiencies using alkaline solutions. The results showed that phosphate adsorption onto steel chip filters was characterized by an initial fast breakthrough followed by a stable removal phase. The cumulative phosphate adsorption capacities of the steel chips were 8.43-10.4 mg P/g following 4800 empty bed volumes with a 3 min contact time and an initial concentration of 10 mg P/L. The phosphate adsorption onto steel chips was favored at low flow rates, low pH values, and low organic carbon concentrations. Sodium hydroxide solutions effectively desorbed phosphate from the steel chips. The total phosphate desorption percentages were 58.9%, 64.2%, and 83.4% after 120 empty bed volumes using 0.05 M, 0.10 M, and 0.20 M NaOH solutions, respectively. Steel chips also exhibited high phosphate adsorption and desorption capacities when treating agricultural subsurface drainage water, municipal wastewater, and stormwater runoff. Overall, the results of this study suggest that recycled steel byproducts are efficient and promising low-cost phosphate capturing materials for sustainable phosphorus management.

Catchment-scale Phosphorus Export through Surface and Drainage Pathways

The site-specific nature of P fate and transport in drained areas exemplifies the need for additional data to guide implementation of conservation practices at the catchment scale. Total P (TP), dissolved reactive P (DRP), and total suspended solids (TSS) were monitored at five sites-two streams, two tile outlets, and a grassed waterway-in three agricultural subwatersheds (221.2-822.5 ha) draining to Black Hawk Lake in western Iowa. Median TP concentrations ranged from 0.034 to 1.490 and 0.008 to 0.055 mg P L for event and baseflow samples, respectively. The majority of P and TSS export occurred during precipitation events and high-flow conditions with greater than 75% of DRP, 66% of TP, and 59% of TSS export occurring during the top 25% of flows from all sites. In one subwatershed, a single event (annual recurrence interval < 1 yr) was responsible for 46.6, 84.0, and 81.0% of the annual export of TP, DRP, and TSS, respectively, indicating that frequent, small storms have the potential to result in extreme losses. Isolated monitoring of surface and drainage transport pathways indicated significant P and TSS losses occurring through drainage; over the 2-yr study period, the drainage pathway was responsible for 69.8, 59.2, and 82.6% of the cumulative TP, DRP, and TSS export, respectively. Finally, the results provided evidence that particulate P losses in drainage were greater than dissolved P losses. Understanding relationships between flow, precipitation, transport pathway, and P fraction at the catchment scale is needed for effective conservation practice implementation.

Tillage and Fertilizer Management Effects on Phosphorus Runoff from Minimal Slope Fields

Phosphorus fertilization can increase P losses in surface runoff, but limited information is available for fields with <2% slopes in the US Midwest. Our objectives were to determine the effects of tillage-fertilizer placement (no-till-broadcast, strip till-broadcast; or strip till-deep placement, -15-cm subsurface band) and fertilizer rate applied in the fall (0, 52, or 90 kg PO ha yr) on runoff P concentrations and loads in fields with <2% slopes near Pesotum, IL, during fall and spring simulation runoff events, and to measure corn ( L.) and soybean [ (L.) Merr.] grain yield. Across four simulated runoff events, deep placement reduced dissolved reactive P (DRP) loads by 69 to 72% compared with the broadcast treatments. A tillage-fertilizer placement × P rate interaction showed that DRP and total P (TP) concentrations remained low when P was deep placed, regardless of P rate, whereas concentrations increased with increasing P rate for the broadcast treatments, but no differences existed for bioavailable P (BAP) (α = 0.05). At one site, rainfall simulation in the spring versus fall increased runoff volumes but sharply decreased BAP concentrations. During fall runoff simulations, deep placement reduced TP loads, and greater TP loads occurred with the 90- than the 52-kg PO ha yr rate. Similarly, when P was broadcast in the fall, DRP and TP concentrations were greater than deep-placed P, but no treatment differences occurred in the spring. Deep banding P and K did not reduce crop yield but reduced runoff losses of P from flat fields compared with broadcast P applications, particularly at high rates of P application.

Exploring the effects of nitrogen fertilization management alternatives on nitrate loss and crop yields in tile-drained fields in Illinois

It is vital to manage the excessive use of nitrogen (N) fertilizer in corn production, the single largest consumer of N fertilizer in the United States, in order to achieve more sustainable agroecosystems. This study comprehensively explored the effects of N fertilization alternatives on nitrate loss and crop yields using the Root Zone Water Quality Model (RZWQM) in tile-drained fields in central Illinois. The RZWQM was tested for the prediction of tile flow, nitrate loss, and crop yields using eight years (1993-2000) of observed data and showed satisfactory model performances from statistical and graphical evaluations. Our model simulations demonstrated the maximum return to nitrogen (MRTN) rate (193 kgha^-1), a newly advised N recommendation by the Illinois Nutrient Loss Reduction Strategy (INLRS), can be further reduced. Nitrate loss was reduced by 10.3% and 29.8%, but corn yields decreased by 0.3% and 1.9% at 156 and 150 kgha^-1 of N fertilizer rate in the study sites A and E, respectively. Although adjustment of N fertilization timing presented a further reduction in nitrate loss, there was no optimal timing to ensure nitrate loss reduction and corn productivity. For site A, 100% spring application was the most productive and 40% fall, 10% pre-plant, and 50% side dress application generated the lowest nitrate loss. For site E, the conventional N application timing was verified as the best practice in both corn production and nitrate loss reduction. Compared to surface broadcast placement, injected N fertilizer in spring increased corn yield, but may also escalate nitrate loss. This study presented the need of an adaptive N fertilizer management due to the heterogeneity in agricultural systems, and raised the importance of timing and placement of N fertilizer, as well as further reduction in fertilizer rate to devise a better in-field N management practice.

Limited Dissolved Phosphorus Runoff Losses from Layered Double Hydroxide and Struvite Fertilizers in a Rainfall Simulation Study

The enrichment of P in surface waters has been linked to P runoff from agricultural fields amended with fertilizers. Novel slow-release mineral fertilizers, such as struvite and P-exchanged layered double hydroxides (LDHs), have received increasing attention for P recycling from waste streams, and these fertilizers may potentially reduce the risk of runoff losses. Here, a rainfall simulation experiment was performed to evaluate P runoff associated with the application of recycled slow-release fertilizers relative to that of a soluble fertilizer. Monoammonium phosphate (MAP), struvite, and LDH granular fertilizers were broadcasted at equal total P doses on soil packed in trays (5% slope) and covered with perennial ryegrass ( L.). Four rainfall simulation events of 30 min were performed at 1, 5, 15, and 30 d after the fertilizer application. Runoff water from the trays was collected, filtered, and analyzed for dissolved P. For the MAP treatment, P runoff losses were high in the first two rain events and leveled off in later rain events. In total, 42% of the applied P in the MAP treatment was lost due to runoff. In the slow-release fertilizer treatments, P runoff losses were limited to 1.9 (struvite) and 2.4% (LDH) of the applied doses and were more similar over the different rain events. The use of these novel P fertilizer forms could be beneficial in areas with a high risk of surface water eutrophication and a history of intensive fertilization.

Evaluation of bioenergy crop growth and the impacts of bioenergy crops on streamflow, tile drain flow and nutrient losses in an extensively tile-drained watershed using SWAT

Large quantities of biofuel production are expected from bioenergy crops at a national scale to meet US biofuel goals. It is important to study biomass production of bioenergy crops and the impacts of these crops on water quantity and quality to identify environment-friendly and productive biofeedstock systems. SWAT2012 with a new tile drainage routine and improved perennial grass and tree growth simulation was used to model long-term annual biomass yields, streamflow, tile flow, sediment load, and nutrient losses under various bioenergy scenarios in an extensively agricultural watershed in the Midwestern US. Simulated results from bioenergy crop scenarios were compared with those from the baseline. The results showed that simulated annual crop yields were similar to observed county level values for corn and soybeans, and were reasonable for Miscanthus, switchgrass and hybrid poplar. Removal of 38% of corn stover (3.74Mg/ha/yr) with Miscanthus production on highly erodible areas and marginal land (17.49Mg/ha/yr) provided the highest biofeedstock production (279,000Mg/yr). Streamflow, tile flow, erosion and nutrient losses were reduced under bioenergy crop scenarios of bioenergy crops on highly erodible areas and marginal land. Corn stover removal did not result in significant water quality changes. The increase in sediment and nutrient losses under corn stover removal could be offset with the combination of other bioenergy crops. Potential areas for bioenergy crop production when meeting the criteria above were small (10.88km²), thus the ability to produce biomass and improve water quality was not substantial. The study showed that corn stover removal with bioenergy crops both on highly erodible areas and marginal land could provide more biofuel production relative to the baseline, and was beneficial to water quality at the watershed scale, providing guidance for further research on evaluation of bioenergy crop scenarios in a typical extensively tile-drained watershed in the Midwestern U.S.

Evaluation of Phosphorus Filter Media for an Inline Subsurface Drainage Treatment System

Subsurface drainage from agricultural land has been identified as a contributor of both N and P into surface waters, leading to water quality degradation and eutrophication. This study evaluates the ability of P sorption media (PSM; expanded shale, expanded clay, furnace slag, and natural soil) to sorb P in both batch and column tests. Batch sorption tests estimated sorption of 3.4, 1.2, and 0.5 g P kg for expanded shale, expanded clay, and natural soil, respectively. Furnace slag sorption was evaluated for fine (FS), small (FS), and large (FS) particle sizes, with estimated sorption of 6.8, 5.1, and 3.8 g P kg, respectively. Phosphorus removal for the three furnace slag particle sizes and natural soil were tested in flow-through columns operated at residence times of 50, 17, and 7 s. A decrease in residence time reduced P removal in all columns evaluated. Following all trials, the average P removal from influent was 50% for FS, followed by 27% for FS (furnace slag-coated pea gravel), 22% for FS, and 6% for sandy loam-coated pea gravel. The data from this study provides crucial information for developing and sizing an inline tile drainage treatment system to remove P from tile drainage outlets before reaching surface waters.

Nitrate and phosphate removal from agricultural subsurface drainage using laboratory woodchip bioreactors and recycled steel byproduct filters

Woodchip bioreactors have been increasingly used as an edge-of-field treatment technology to reduce the nitrate loadings to surface waters from agricultural subsurface drainage. Recent studies have shown that subsurface drainage can also contribute substantially to the loss of phosphate from agricultural soils. The objective of this study was to investigate nitrate and phosphate removal in subsurface drainage using laboratory woodchip bioreactors and recycled steel byproduct filters. The woodchip bioreactor demonstrated average nitrate removal efficiencies of 53.5-100% and removal rates of 10.1-21.6 g N/m(3)/d for an influent concentration of 20 mg N/L and hydraulic retention times (HRTs) of 6-24 h. When the influent nitrate concentration increased to 50 mg N/L, the bioreactor nitrate removal efficiency and rate averaged 75% and 18.9 g N/m(3)/d at an HRT of 24 h. Nitrate removal by the woodchips followed zero-order kinetics with rate constants of 1.42-1.80 mg N/L/h when nitrate was non-limiting. The steel byproduct filter effectively removed phosphate in the bioreactor effluent and the total phosphate adsorption capacity was 3.70 mg P/g under continuous flow conditions. Nitrite accumulation occurred in the woodchip bioreactor and the effluent nitrite concentrations increased with decreasing HRTs and increasing influent nitrate concentrations. The steel byproduct filter efficiently reduced the level of nitrite in the bioreactor effluent. Overall, the results of this study suggest that woodchip denitrification followed by steel byproduct filtration is an effective treatment technology for nitrate and phosphate removal in subsurface drainage.

Assessment and Synthesis of 50 Years of Published Drainage Phosphorus Losses

The prevalence of anthropogenic drainage systems in intensively cropped areas across North America combined with the degradation of important freshwater resources in these regions has created a critical intersection where understanding phosphorus (P) transport in drainage waters is vital. In this study, drainage-associated nutrient load data were retrieved and quantitatively analyzed to develop a more comprehensive understanding of the P loading and crop yield impacts of agronomic management practices within drained landscapes. Using the Drain Load table in the MANAGE (Measured Annual Nutrient loads from AGricultural Environments) database, the effect of factors such as soil characteristics, tillage, and nutrient management on P loading were analyzed. Across site-years, generally less than 2% of applied P was lost in drainage water, which corroborates the order of magnitude difference between agronomic P application rates and P loadings that can cause deleterious water quality impacts. The practice of no-till significantly increased drainage dissolved P loads compared with conventional tillage (0.12 vs. 0.04 kg P ha). The timing and method of P application are both known to be important for P losses, but these conclusions could not be verified due to low site-year counts. Findings indicate there is a substantial need for additional field-scale studies documenting not only P losses in drainage water but also important cropping management, nutrient application, soil property, and drainage design impacts on such losses.

Subsurface Drainage Nitrate and Total Reactive Phosphorus Losses in Bioenergy-Based Prairies and Corn Systems

We compare subsurface-drainage NO-N and total reactive phosphorus (TRP) concentrations and yields of select bioenergy cropping systems and their rotational phases. Cropping systems evaluated were grain-harvested corn-soybean rotations, grain- and stover-harvested continuous corn systems with and without a cover crop, and annually harvested reconstructed prairies with and without the addition of N fertilizer in an Iowa field. Drainage was monitored when soils were unfrozen during 2010 through 2013. The corn-soybean rotations without residue removal and continuous corn with residue removal produced similar mean annual flow-weighted NO-N concentrations, ranging from 6 to 18.5 mg N L during the 4-yr study. In contrast, continuous corn with residue removal and with a cover crop had significantly lower NO-N concentrations of 5.6 mg N L when mean annual flow-weighted values were averaged across the 4 yr. Prairies systems with or without N fertilization produced significantly lower concentrations below <1 mg NO-N L than all the row crop systems throughout the study. Mean annual flow-weighted TRP concentrations and annual yields were generally low, with values <0.04 mg TRP L and <0.14 kg TRP ha, and were not significantly affected by any cropping systems or their rotational phases. Bioenergy-based prairies with or without N fertilization and continuous corn with stover removal and a cover crop have the potential to supply bioenergy feedstocks while minimizing NO-N losses to drainage waters. However, subsurface drainage TRP concentrations and yields in bioenergy systems will need further evaluation in areas prone to higher levels of P losses.

Applicability of models to predict phosphorus losses in drained fields: a review

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.

Effect of replacing surface inlets with blind or gravel inlets on sediment and phosphorus subsurface drainage losses

Open surface inlets that connect to subsurface tile drainage systems provide a direct pathway for movement of sediment, nutrients, and agrochemicals to surface waters. This study was conducted to determine the reduction in drainage effluent total suspended sediment (TSS) and phosphorus (P) concentrations and loads when open surface inlets were replaced with blind (in gravel capped with 30 cm of soil) or gravel (in very coarse sand/fine gravel) inlets. In Indiana, a pair of closed depressions in adjacent fields was fitted with open inlet tile risers and blind inlets in 2005 and monitored for flow and water chemistry. Paired comparisons on a storm event basis during the growing season for years 2006 to 2013 showed that TSS loads were 40.4 and 14.4 kg ha event for tile risers and blind inlets, respectively. Total P (TP) and soluble reactive P (SRP) loads were 66 and 50% less for the blind inlets, respectively. In Minnesota, TSS and SRP concentrations were monitored for 3 yr before and after modification of 24 open inlets to gravel inlets in an unreplicated large-field on-farm study. Median TSS concentrations were 97 and 8.3 mg L and median SRP concentrations were 0.099 and 0.064 mg L for the open inlet and gravel inlet periods, respectively. Median TSS and SRP concentrations were elevated for snowmelt vs. non-snowmelt seasons for open and gravel inlets. Both replacement designs reduced suspended sediment and P concentrations and loads. The Indiana study suggests blind inlets will be effective beyond a 10-yr service life.

Phosphorus transport in agricultural subsurface drainage: a review

Phosphorus (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.

Contributions of systematic tile drainage to watershed-scale phosphorus transport

Phosphorus (P) transport from agricultural fields continues to be a focal point for addressing harmful algal blooms and nuisance algae in freshwater systems throughout the world. In humid, poorly drained regions, attention has turned to P delivery through subsurface tile drainage. However, research on the contributions of tile drainage to watershed-scale P losses is limited. The objective of this study was to evaluate long-term P movement through tile drainage and its manifestation at the watershed outlet. Discharge data and associated P concentrations were collected for 8 yr (2005-2012) from six tile drains and from the watershed outlet of a headwater watershed within the Upper Big Walnut Creek watershed in central Ohio. Results showed that tile drainage accounted for 47% of the discharge, 48% of the dissolved P, and 40% of the total P exported from the watershed. Average annual total P loss from the watershed was 0.98 kg ha, and annual total P loss from the six tile drains was 0.48 kg ha. Phosphorus loads in tile and watershed discharge tended to be greater in the winter, spring, and fall, whereas P concentrations were greatest in the summer. Over the 8-yr study, P transported in tile drains represented <2% of typical application rates in this watershed, but >90% of all measured concentrations exceeded recommended levels (0.03 mg L) for minimizing harmful algal blooms and nuisance algae. Thus, the results of this study show that in systematically tile-drained headwater watersheds, the amount of P delivered to surface waters via tile drains cannot be dismissed. Given the amount of P loss relative to typical application rates, development and implementation of best management practices (BMPs) must jointly consider economic and environmental benefits. Specifically, implementation of BMPs should focus on late fall, winter, and early spring seasons when most P loading occurs.

Surface runoff and tile drainage transport of phosphorus in the midwestern United States

The midwestern United States offers some of the most productive agricultural soils in the world. Given the cool humid climate, much of the region would not be able to support agriculture without subsurface (tile) drainage because high water tables may damage crops and prevent machinery usage in fields at critical times. Although drainage is designed to remove excess soil water as quickly as possible, it can also rapidly transport agrochemicals, including phosphorus (P). This paper illustrates the potential importance of tile drainage for P transport throughout the midwestern United States. Surface runoff and tile drainage from fields in the St. Joseph River Watershed in northeastern Indiana have been monitored since 2008. Although the traditional concept of tile drainage has been that it slowly removes soil matrix flow, peak tile discharge occurred at the same time as peak surface runoff, which demonstrates a strong surface connection through macropore flow. On our research fields, 49% of soluble P and 48% of total P losses occurred via tile discharge. Edge-of-field soluble P and total P areal loads often exceeded watershed-scale areal loadings from the Maumee River, the primary source of nutrients to the western basin of Lake Erie, where algal blooms have been a pervasive problem for the last 10 yr. As farmers, researchers, and policymakers search for treatments to reduce P loading to surface waters, the present work demonstrates that treating only surface runoff may not be sufficient to reach the goal of 41% reduction in P loading for the Lake Erie Basin.

Reducing phosphorus loss in tile water with managed drainage in a claypan soil

Installing subsurface tile drain systems in poorly drained claypan soils to improve corn ( L.) yields could potentially increase environmental phosphorus (P) loss through the tile drainage system. The objectives of the study were to quantify the average concentration and loss of ortho-P in tile drain water from a claypan soil and to determine whether managed subsurface drainage (MD) could reduce ortho-P loss in tile water compared with free subsurface drainage (FD). Flow-weighted ortho-P concentration in the tile water was significantly lower with MD (0.09 mg L) compared with that of FD (0.15 mg L). Ortho-P loss in the tile water of this study was reduced with MD (36 g ha) by 80% compared with FD (180 g ha). Contrary to previous research, reduced ortho-P loss observed over the 4-yr study was not solely due to the reduced amount of water drained annually (63%) with MD compared with FD. During the spring period, when flow was similar between MD and FD, the concentration of ortho-P in the tile water generally was lower with MD compared with FD, which resulted in significantly less ortho-P loss with MD. We speculate that MD's ability to conserve water during the dry summer months increased corn's uptake of water and P, which reduced the amount of P available for leaching loss in the subsequent springs.

Phosphorus losses from monitored fields with conservation practices in the Lake Erie Basin, USA

Conservation practices are implemented on farm fields in the USA through Farm Bill programs; however, there is a need for greater verification that these practices provide environmental benefits (e.g., water quality). This study was conducted to assess the impact of Farm Bill eligible conservation practices on soluble P (SP) and total P (TP) losses from four fields that were monitored between 2004 and 2013. No-tillage doubled SP loading compared to rotational tillage (e.g., tilled only before planting corn); however, no-tillage decreased TP loading by 69 % compared to rotational tillage. Similarly, grassed waterways were shown to increase SP loads, but not TP loads. A corn-soybean-wheat-oat rotation reduced SP loads by 85 % and TP loads by 83 % compared to the standard corn-soybean rotation in the region. We can potentially attain TP water quality goals using these Farm Bill practices; however, additional strategies must be employed to meet these goals for SP.

Topsoil and subsoil properties influence phosphorus leaching from four agricultural soils

Eutrophication, a major problem in many fresh and brackish waters, is largely caused by nonpoint-source pollution by P from agricultural soils. This lysimeter study examined the influence of P content, physical properties, and sorption characteristics in topsoil and subsoil on P leaching measured during 21 mo in 1-m-long, undisturbed soil columns of two clay and two sandy soils. Total P losses during the period varied between 0.65 and 7.40 kg ha. Dissolved reactive P was the dominant form in leachate from the sandy soils and one clay soil, varying from 48 to 76%. Particulate P dominated in leachate from the other clay soil, where low pH (5.2) in the subsoil decreased aggregate stability and thereby probably increased the dispersion of clay particles. Phosphorus leaching was small from soils with high P sorption index (PSI) and low P saturation (<10% of PSI) in the subsoil, even though extractable P (Olsen P) in the topsoil was high, and large from a soil with low sorption capacity and high P saturation (>35% of PSI) in the profile. High sorption capacity in the subsoil was more important for P leaching in sandy soils than in clay soils with macropore flow, where the effect of high sorption capacity was reduced due to less interaction between percolating water and the soil matrix. The results suggest that P leaching is greatly affected by subsoil properties and that topsoil studies, which dominate current research, are insufficient for assessing P leaching in many soils.

Phosphorus losses from agricultural watersheds in the Mississippi Delta

Phosphorus (P) loss from agricultural fields is of environmental concern because of its potential impact on water quality in streams and lakes. The Mississippi Delta has long been known for its fish productivity and recreational value, but high levels of P in fresh water can lead to algal blooms that have many detrimental effects on natural ecosystems. Algal blooms interfere with recreational and aesthetic water use. However, few studies have evaluated P losses from agricultural watersheds in the Mississippi Delta. To better understand the processes influencing P loss, rainfall, surface runoff, sediment, ortho-P (orthophosphate, PO(4)-P), and total P (TP) were measured (water years 1996-2000) for two subwatersheds (UL1 and UL2) of the Deep Hollow Lake Watershed and one subwatershed of the Beasley Lake Watershed (BL3) primarily in cotton production in the Mississippi Delta. Ortho-P concentrations ranged from 0.01 to 1.0 mg/L with a mean of 0.17 mg/L at UL1 (17.0 ha), 0.36 mg/L at UL2 (11.2 ha) and 0.12 mg/L at BL3 (7.2 ha). The TP concentrations ranged from 0.14 to 7.9 mg/L with a mean of 0.96 mg/L at UL1, 1.1 mg/L at UL2 and 1.29 mg/L at BL3. Among the three sites, UL1 and UL2 received P application in October 1998, and BL3 received P applications in the spring of 1998 and 1999. At UL1, ortho-P concentrations were 0.36, 0.25 and 0.16 for the first, second and third rainfall events after P application, respectively; At UL2, ortho-P concentrations were 1.0, 0.66 and 0.65 for the first, second and third rainfall events after P application, respectively; and at BL3, ortho-P concentrations were 0.11, 0.22 and 0.09 for the first, second and third rainfall events after P application, respectively. P fertilizer application did influence P losses, but high P concentrations observed in surface runoff were not always a direct result of P fertilizer application or high rainfall. Application of P in the fall (UL1 and UL2) resulted in more ortho-P losses, likely because high rainfall often occurred in the winter months soon after application. The mean ortho-P concentrations were higher at UL1 and UL2 than those at BL3, although BL3 received more P application during the monitoring period, because P was applied in spring at BL3. However, tillage associated with planting and incorporating applied P in the spring (BL3) may have resulted in more TP loss in sediment, thus the mean TP concentration was the highest at BL3. Ortho-P loss was correlated with surface runoff; and TP loss was correlated with sediment loss. These results indicate that applying P fertilizer in the spring may be recommended to reduce potential ortho-P loss during the fallow winter season; in addition, conservation practices may reduce potential TP loss associated with soil loss.

Accounting for the risks of phosphorus losses through tile drains in a phosphorus index

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.

Water and nutrient transport on a heavy clay soil in a fluvial plain in the Netherlands

In flat areas, transport of dissolved nutrients by water through the soil matrix to groundwater and drains is assumed to be the dominant pathway for nutrient losses to ground- and surface waters. However, long-term data on the losses of nutrients to surface water and the contribution of various pathways is limited. We studied nutrient losses and pathways on a heavy clay soil in a fluvial plain in The Netherlands during a 5-yr period. Average annual nitrogen (N) and phosphorus (P) losses to surface water were 15.1 and 3.0 kg ha(-1) yr(-1), respectively. Losses were dominated by particulate N (50%) and P (70%) forms. Rapid discharge through trenches was the dominant pathway (60-90%) for water and nutrient transport. The contribution of pipe drains to the total discharge of water and nutrients was strongly related to the length of the dry period in the preceding summer. This relationship can be explained by the very low conductivity of the soil matrix and the formation of shrinkage cracks during summer. Losses of dissolved reactive P through pipe drains appear to be dominated by preferential flow based on the low dissolved reactive P concentration in the soil matrix at this depth. Rainfall occurring after manure application played an important role with respect to the annual losses of N and P in spring when heavy rainfall occurred within 2 wk after manure application.

Use of industrial byproducts to filter phosphorus and pesticides in golf green drainage water

Golf courses are vulnerable to phosphate (PO) and pesticide loss by infiltration of the sandy, porous grass rooting media used and through subsurface tile drainage. In this study, an effort was made to remove PO, chlorothalonil, mefenoxam, and propiconazole in a golf green's drainage water with a filter blend comprised of industrial byproducts, including granulated blast furnace slag, cement kiln dust, silica sand, coconut shell-activated carbon, and zeolite. To test this filter media, two 6-h storm events were simulated by repeat irrigation of the golf green after PO and pesticide application. Drainage flows ranged from 0.0034 to 0.6433 L s throughout the course of the simulations. A significant decrease in the chlorothalonil load for the experimental run (with filter media) was observed compared with the control (without filter media) ( < 0.05). In general, percent reductions in chlorothalonil were very high (>80%) near peak flows. In contrast, filter media was not effective in removing PO, mefenoxam, or propiconazole ( > 0.05). Instead, it appears that the filter blend added PO to the effluent above flow rates of 0.037 L s. Overall, flow rate, the amount of filter media used, and contaminant properties may have influenced the filter media's ability to remove contaminants. More research is needed to determine the optimal blend and configuration for the filter media to remove significant amounts of all contaminants investigated.

Assessing the long term impact of phosphorus fertilization on phosphorus loadings using AnnAGNPS

High phosphorus (P) loss from agricultural fields has been an environmental concern because of potential water quality problems in streams and lakes. To better understand the process of P loss and evaluate the effects of different phosphorus fertilization rates on phosphorus losses, the USDA Annualized AGricultural Non-Point Source (AnnAGNPS) pollutant loading model was applied to the Ohio Upper Auglaize watershed, located in the southern portion of the Maumee River Basin. In this study, the AnnAGNPS model was calibrated using USGS monitored data; and then the effects of different phosphorus fertilization rates on phosphorus loadings were assessed. It was found that P loadings increase as fertilization rate increases, and long term higher P application would lead to much higher P loadings to the watershed outlet. The P loadings to the watershed outlet have a dramatic change after some time with higher P application rate. This dramatic change of P loading to the watershed outlet indicates that a “critical point” may exist in the soil at which soil P loss to water changes dramatically. Simulations with different initial soil P contents showed that the higher the initial soil P content is, the less time it takes to reach the “critical point” where P loadings to the watershed outlet increases dramatically. More research needs to be done to understand the processes involved in the transfer of P between the various stable, active and labile states in the soil to ensure that the model simulations are accurate. This finding may be useful in setting up future P application and management guidelines.

Nutrient transport through a Vegetative Filter Strip with subsurface drainage

The transport of nutrients and soil sediments in runoff has been recognized as a noteworthy environmental issue. Vegetative Filter Strips (VFS) have been used as one of the best management practices (BMPs) for retaining nutrients and sediments from surface runoff, thus preventing the pollutants from reaching receiving waters. However, the effectiveness of a VFS when combined with a subsurface drainage system has not been investigated previously. This study was undertaken to monitor the retention and transport of nutrients within a VFS that had a subsurface drainage system installed at a depth of 1.2 m below the soil surface. Nutrient concentrations of NO(3)-N (Nitrate Nitrogen), PO(-)(4) (Orthophosphorus), and TP (Total Phosphorus) were measured in surface water samples (entering and leaving the VFS), and subsurface outflow. Soil samples were collected and analyzed for plant available Phosphorus (Bray P1) and NO(3)-N concentrations. Results showed that PO(-)(4), NO(3)-N, and TP concentrations decreased in surface flow through the VFS. Many surface outflow water samples from the VFS showed concentration reductions of as much as 75% for PO(-)(4) and 70% for TP. For subsurface outflow water samples through the drainage system, concentrations of PO(-)(4) and TP decreased but NO(3)-N concentrations increased in comparison to concentrations in surface inflow samples. Soil samples that were collected from various depths in the VFS showed a minimal buildup of nutrients in the top soil profile but indicated a gradual buildup of nutrients at the depth of the subsurface drain. Results demonstrate that although a VFS can be very effective in reducing runoff and nutrients from surface flow, the presence of a subsurface drain underneath the VFS may not be environmentally beneficial. Such a combination may increase NO(3)-N transport from the VFS, thus invalidating the purpose of the BMP.

Relationships between benthic sediments and water column phosphorus in Illinois streams

Sediments can be important in regulating stream water P concentrations, and this has implications for establishing nutrient standards that have not been fully investigated. We evaluated abiotic and biotic processes to better understand the role of sediments in determining stream water dissolved P concentrations. Sediment and stream water samples were collected during low discharge from 105 streams across Illinois and analyzed for equilibrium P concentration at zero release or retention (EPC(0)), P sorption characteristics, stream water P concentration, and sediment particle size. In addition, four east-central Illinois streams were repeatedly sampled to examine temporal patterns in sediment P retention and biotic processing of P. Median dissolved reactive P (DRP) and total P concentrations across the state were 0.081 and 0.168 mg L(-1), respectively. Sediment EPC(0) concentrations were related to stream water DRP concentrations (r(s) = 0.75). Sediment silt+clay (and co-correlated organic matter) was related to sorbed P (r(s) = -0.49) and the reactive sediment pool of P (r(s) = 0.76). However, for most sites this pool was small given the coarse textures present (median silt+clay was 5.7%). Repeated sampling at the four intensive sites showed little variation in EPC(0) values or alkaline phosphatase activity, suggesting overall stream conditions regulated the biotic processing. Biotic retention of P was 32% of short-term P removal. We conclude that sediments in Illinois streams are a reflection of and partially affected by stream water P concentrations through both abiotic and biotic processes. Sediments seem unlikely to alter annual stream P loads, but may affect concentrations at low discharge.