Reconnecting tile drainage to riparian buffer hydrology for enhanced nitrate removal

Existing wetland conservation programs miss nutrient reduction targets

Restoring wetlands will reduce nitrogen contamination from excess fertilization but estimates of the efficacy of the strategy vary widely. The intervention is often described as effective for reducing nitrogen export from watersheds to mediate bottom-level hypoxia threatening marine ecosystems. Other research points to the necessity of applying a suite of interventions, including wetland restoration to mitigate meaningful quantities of nitrogen export. Here, we use process-based physical modeling to evaluate the effects of two hypothetical, but plausible large-scale wetland restoration programs intended to reduce nutrient export to the Gulf of Mexico. We show that full adoption of the two programs currently in place can meet as little as 10% to as much as 60% of nutrient reduction targets to reduce the Gulf of Mexico dead zone. These reductions are lower than prior estimates for three reasons. First, net storage of leachate in the subsurface precludes interception and thereby dampens the percent decline in nitrogen export caused by the policy. Unlike previous studies, we first constrained riverine fluxes to match observed fluxes throughout the basin. Second, the locations of many restorable lands are geographically disconnected from heavily fertilized croplands, limiting interception of runoff. Third, daily resolution of the model simulations captured the seasonal and stormflow dynamics that inhibit wetland nutrient removal because peak wetland effectiveness does not coincide with the timing of nutrient inputs. To improve the health of the Gulf of Mexico efforts to eliminate excess nutrient, loading should be implemented beyond the field-margin wetland strategies investigated here.

Saturated buffers: Improvements and issues

Saturated buffers are a newly developed agricultural best management practice used to redirect tile flow away from waterways, thereby mitigating nutrient losses and downstream eutrophication. This study evaluated the potential benefits of a novel saturated buffer design, which included pitchfork-shaped (PF) dispersion lines and a backflow check valve, that was installed alongside a traditional or standard (ST) buffer on a field in Moultrie County, Illinois, in the spring of 2019. Daily flow measurements and routine water samples were used to monitor the movement of water through both buffers and estimate nutrient loads. During observation days in 2020 and 2021, the PF buffer diverted 35% and 1.9% of incoming tile flow, respectively, while the ST buffer increased effluent rates by 116% and 137% over the same period. Both the PF and ST buffers experienced backflow from 30% to 47% of the monitoring period, well above the often reported 5%. Ultimately, the efficacy of saturated buffers could be improved with minimal, low-cost additions to their designs. Check valves are a simple supplement to saturated buffer design that can enhance flow diversion and potential nutrient removal. Added dispersion lines provide more opportunity for diversion of tile flow; however, they require more land to be removed from agricultural production and could increase backflow volumes, so the costs and benefits should be weighed.

Potential for managing pool levels in a flood-control reservoir to increase nitrate-nitrogen load reductions

Few strategies are available to reduce nitrate-nitrogen (NO3 -N) loads at larger landscape scales, but flood control reservoirs are known to reduce riverine loads. In this study, we evaluated the potential to increase nitrogen (N) loss at Lake Red Rock, a large reservoir located in central Iowa, by evaluating the inundation of sediments deposited at the reservoir inflow. Sediment samples were collected at 51 locations in the lower delta region and analyzed for particle size and nutrient content. Nitrogen loss rates in delta sediments were determined from laboratory assays, and satellite imagery was used to develop a rating curve to quantify land area inundated within the delta. The daily mass of NO3 -N reduced with delta inundation was estimated by applying the mean N 24-h loss rate (0.66 g N m2 day-1 ) by the area of inundation (m2 ). Results indicated that raising pool elevations to inundate more of the delta would result in greater N losses, ranging from 2 to 377 Mg per year. Potential N loss of 102 Mg achieved by increasing pool stage by 0.5 m would be equivalent to installing nearly 650 edge-of-field practices in the watershed. Although more work is needed to integrate with an existing environmental pool management plan, study results indicate that reservoir management could achieve N reductions at a novel landscape scale.

A review of ecosystem services from edge-of-field practices in tile-drained agricultural systems in the United States Corn Belt Region

Edge-of-field management practices that reduce nutrient pollution from tile drainage while contributing habitat and other ecosystem services are needed to enhance agricultural systems in the US Corn Belt Region. In this review, we identified edge-of-field and catchment scale agricultural conservation practices for intercepting and treating tile drainage. The reviewed conservation practices were (1) controlled drainage, also known as drainage water management (USDA-NRCS Code 554); (2) drainage water recycling (USDA-NRCS Code 447); (3) denitrifying bioreactors (USDA-NRCS Code 605); (4) saturated buffers (USDA-NRCS Code 604); and (5) constructed or restored wetlands designed for water quality improvement (USDA-NRCS Code 656) herein referred to as water quality wetlands. We examined 119 studies that had information on one or more of the following ecosystem services: water retention, water quality improvement (e.g., nitrate, phosphate, sediment, or pesticide retention), wetland habitat (for birds, aquatic organisms, and pollinators), crop yield improvement, and other benefits (e.g., recreation, education, aesthetic appreciation, greenhouse gas retention). We found the five edge-of-field practices were all effective at removing nitrate with varying degrees of other potential benefits and disservices (e.g., greenhouse gas production). Drainage water recycling and water quality wetlands have the potential to provide the most co-benefits as they provide surface water systems for capturing surface flows in addition to tile drainage while also potentially providing habitat and recreation opportunities. However, the following research needs are identified: 1) the disservices and benefits associated with drainage water recycling have not been adequately evaluated; 2) surface flow dynamics are understudied across all reviewed management practices; 3) a complete accounting of phosphorus species and flow pathways for all management practices is needed; 4) field evaluations of the habitat benefit of all management practices are needed; and 5) greenhouse gas dynamics are understudied across all management practices. While all management practices are expected to reduce nitrate loads, addressing these knowledge gaps will help inform holistic management decisions for diverse stakeholders across the US Corn Belt.

Woodchip bioreactors for saline leachates denitrification can mitigate agricultural impacts in mediterranean areas: The Campo de Cartagena-Mar Menor environmental issue

Leachates from intensive agriculture containing high nitrate have been identified as a major cause of the severe eutrophication crisis that impacts Mar Menor (SE Spain), the largest hypersaline coastal lagoon in the Mediterranean basin. A best management practice for removing NO₃^--N is denitrifying bioreactors. This is the first study to assess the efficiency of citrus woodchips bioreactors in treating agricultural leachates that flow to the Mar Menor via surface discharges. Denitrification capacity, woodchip degradation (by weight loss), formation of potentially harmful compounds, and greenhouse gas (GHG) emissions were assessed. Three bioreactors (6 m × 0.98 m x 1.2 m) filled with citrus woodchips (3 m³ d^-1 per bioreactor) through which the untreated ditch water over 1.5 years. Bioreactors were operated at 8 h, 16 h, and 24 h hydraulic residence time respectively, in each bioreactor. The main characteristics of the ditch water were: pH ≈ 7.5-8.0, electrical conductivity ≈ 5-8 dS m^-1, dissolved organic carbon ≈6-10 mg L^-1, and NO₃^--N ≈ 22-45 mg L^-1. Bioreactors were highly efficient in reducing NO₃^--N. The average R_NO3 in effluents was for the complete experimental period 8 g N m^-3 d^-1, 10.9 g N m^-3 d^-1, and 12.6 g N m^-3 d^-1 for 8, 16 and 24 h residence time, respectively. Nitrate reduction efficiency was modulated by seasonal changes in temperature, with an increasing efficiency in warmer periods (maximum ≈ 85-90% for all hydraulic residence time) and decreasing in colder ones (minimum ≈ 12%, 23% and 41% for hydraulic residence time 8, 16 and 24 h respectively). Woodchips degradation was greatest during the first six months (average ≈ 29% weight loss) in the material above the water level, attributable to aerobic mineralization of the organic carbon, while weight loss was ≈11% in woodchip media continuously below the water level. Dissolved organic carbon, sulfide, ammonium, and soluble phosphorus concentrations in the effluents were mostly low, although some peaks in concentrations occurred. Design consideration must be taken to avoid environmental impacts due to the occasional presence of harmful compounds in the effluents.

Groundwater flow in saturated riparian buffers and implications for nitrate removal

A saturated riparian buffer (SRB) is an edge-of-field conservation practice that intercepts tile drainage and reduces nitrate flux to nearby streams by redistributing the flow as shallow groundwater. In this study, a three-dimensional, finite-difference groundwater model representative of SRBs in central Iowa was developed to assess the flow of groundwater and implications for nitrate removal during spring conditions, when flow to the SRB is highest. The model reproduces field observations of water level with Nash-Sutcliffe efficiency of 0.68, which is deemed acceptable for hydrologic models. The modeling shows that groundwater flow is three-dimensional near the distribution pipe and the stream and primarily one-dimensional in the rest of the buffer. The path the water takes in flowing toward the stream depends on where it exits the distribution pipe. When nitrate is not limiting, the potential for nitrate removal depends on the length of the path-and thus travel time-and depth because denitrification potential varies with depth. Travel time T_t can be estimated well with slight modifications to a one-dimensional approximation: T_t = 1.11L_x /v_x , where L_x is the buffer width and v_x is a one-dimensional approximation of the average linear velocity of groundwater. Refining knowledge of SRB function is an important step toward enhancing design for improving water quality.

Effect of alfalfa on subsurface (tile) nitrogen and phosphorus loss in Ohio, USA

Growing annual crops such as corn (Zea mays L.) can lead to considerable nutrient losses through subsurface drainage in agricultural fields, posing a serious threat to surface water quality in the midwestern United States. Perennial crops have the potential to reduce these nutrient losses. However, more comprehensive data are needed on the nutrient loss effect of perennial crops. We examined the effect of alfalfa (Medicago sativa L.) on nitrate-nitrogen (NO₃ ^- -N), total nitrogen (TN), dissolved reactive phosphorus (DRP), and total phosphorus (TP) in subsurface drainage using a before-after-control-impact (BACI) experimental design with one control field (with annual crops) and one impact field (with alfalfa) each on two farms (Sites A and B) located in northwestern Ohio. The "Before" period (prior to planting alfalfa at the impact field) extended for 4 yr (2013-2017) at Site A and 6 yr (2011-2017) at Site B; the "After" period extended for an additional 2 yr at both sites. Reductions in the mean monthly discharge and loads of NO₃ ^- -N, TN, DRP, and TP were significant at Site A, whereas the only significant change at site B was a reduction in the mean monthly TP load. Significant reductions in NO₃ ^- -N loads were observed during spring and winter at Site A. In addition, alfalfa reduced the variability of discharge and nutrient loads through subsurface drainage at both sites. Our findings suggest that introducing alfalfa into annual crop rotations has the potential to reduce subsurface nutrient loads and increase the resiliency of agricultural systems.

Managing nitrate-nitrogen in the intensively drained upper Mississippi River Basin, USA under uncertainty: a perennial path forward

The upper Mississippi River basin has been identified as the most significant contributor of excessive nutrients to the hypoxic zone in the Gulf of Mexico. The land-use changes from an internally drained prairie-wetland complex to an intensively managed corn-soybean production system drained by subsurface tile drainage system in the north-central Iowa and south-central Minnesota are the primary cause of nutrient loads into the Mississippi River and many other environmental stresses. The present study summarizes the water-quality degradation from land-use change and offers a fuzzy logic-based decision support for assessing degree of suitability of the four recommended perennial plant options for managing water and nitrate-nitrogen export. These options are designed based on landscape position that currently fails to produce high yielding row crops and scale: (1) marginal upland depressions for water storage by planting deep-rooted perennial grasses and fast-growing woody poplar, willow, and alder in poorly drained swales; (2) saturated buffers and/or subtle changes in landscape slope for draining high nitrate-nitrogen subsurface (through multi-species phytoremediation treatment buffers or strips of perennial vegetation); (3) two-stage ditches with linear floodplains planted with perennial grasses; and (4) riparian and in-channel ecologically engineered trees, shrubs, and grasses to better connect meander belt width to frequent peak stream flows at larger scales. When applied throughout a typical (Des Moines Lobe Till) DMLT watershed, each option can have positive cumulative environmental effects. Fuzzy logic enhanced the precision in watershed decision-making by incorporating the uncertainty associated with factors like cost effectiveness, nitrate reduction potential, water quality improvement, and level of acceptance.

Soil CO2 emissions in cropland with fodder maize (Zea mays L.) with and without riparian buffer strips of differing vegetation

Vegetated land areas play a significant role in determining the fate of carbon (C) in the global C cycle. Riparian buffer vegetation is primarily implemented for water quality purposes as they attenuate pollutants from immediately adjacent croplands before reaching freashwater systems. However, their prevailing conditions may sometimes promote the production and subsequent emissions of soil carbon dioxide (CO₂). Despite this, the understanding of soil CO₂ emissions from riparian buffer vegetation and a direct comparison with adjacent croplands they serve remain elusive. In order to quantify the extent of CO₂ emissions in such an agro system, we measured CO₂ emissions simultaneously with soil and environmental variables for six months in a replicated plot-scale facility comprising of maize cropping served by three vegetated riparian buffers, namely: (i) a novel grass riparian buffer; (ii) a willow riparian buffer, and; (iii) a woodland riparian buffer. These buffered treatments were compared with a no-buffer control. The woodland (322.9 ± 3.1 kg ha^- 1) and grass (285 ± 2.7 kg ha^- 1) riparian buffer treatments (not significant to each other) generated significantly (p = < 0.0001) the largest CO₂ compared to the remainder of the treatments. Our results suggest that during maize production in general, the woodland and grass riparian buffers serving a maize crop pose a CO₂ threat. The results of the current study point to the need to consider the benefits for gaseous emissions of mitigation measures conventionally implemented for improving the sustainability of water resources.

Paired riparian water table monitoring to quantify hydraulic loading to a saturated buffer

The use of saturated buffers for reducing NO₃-N loads from tile-drained croplands is increasing in the US Midwest and there is a need to develop options for estimating reductions at riparian sites. In this study, we present a paired water table monitoring approach to estimate hydraulic and NO₃-N loading into a saturated buffer in eastern Iowa. One well was located within the saturated buffer (treatment) and a second well was installed in the same section of the riparian buffer but without the saturated buffer (control). Over a season of monitoring, water table depths were remarkably consistent between the two wells but the water table beneath the saturated buffer was consistently 0.22 m higher than the non-saturated buffer control. The increase in water table height increased the amount of water discharged from a 162 m long buffer by 468.2 m³/year and, assuming concentration reduction of 15 mg/l, resulted in a N reduction of approximately 7 kg. Although more work is needed to document this paired monitoring approach elsewhere, the method may hold promise for inexpensively quantifying the performance of conservation practices at landowner-led sites.

Paired field and water measurements from drainage management practices in row-crop agriculture

This paper describes a multi-site and multi-decadal dataset of artificially drained agricultural fields in seven Midwest states and North Carolina, USA. Thirty-nine research sites provided data on three conservation practices for cropland with subsurface tile drainage: saturated buffers, controlled drainage, and drainage water recycling. These practices utilize vegetation and/or infrastructure to minimize off-site nutrient losses and retain water in the landscape. A total of 219 variables are reported, including 90 field measurement variables and 129 management operations and metadata. Key measurements include subsurface drain flow (206 site-years), nitrate-N load (154 site-years) and other water quality metrics, as well as agronomic, soil, climate, farm management and metadata records. Data are published at the USDA National Agricultural Library Ag Data Commons repository and are also available through an interactive website at Iowa State University. These multi-disciplinary data have large reuse potential by the scientific community as well as for design of drainage systems and implementation in the US and globally.

Measurement(s)	subsurface drain flow • Nitrate-N concentration • water table • soil nitrate concentration • precipitation • grain yield
Technology Type(s)	pressure transducers within drainage control structure/outlet • automated flow-proportional sampling • pressure transducers installed inside deep wells of perforated PVC pipes • dry combustion • on-site weather station • mechanical harvest with combine
Factor Type(s)	controlled drianage • drainage water recylcing • saturated buffers
Sample Characteristic - Organism	Zea mays • Glycine max
Sample Characteristic - Environment	agricultural field
Sample Characteristic - Location	United States

Saturated buffer design flow and performance in Illinois

There are few peer-reviewed studies documenting saturated buffer annual nitrate (NO₃ ) removal or that have assessed the federal practice standard design criteria. Drainage flow, NO₃ , and dissolved reactive phosphorus (DRP) were monitored at three saturated buffers in Illinois, USA, for a combined 10 site-years. Nitrate loss reduction averaged 48 ± 19% with removals of 3.5-25.2 kg NO₃ -N ha^-1 annually. Median DRP concentrations at all sampling locations were at the analytical detection limit of 0.01 mg L^-1 . The current design paradigm (i.e., USDA practice standard) prescribes there should be no flow bypassing the saturated buffer at flow rates that are ≤5% of the peak drainage system flow rate. The drainage coefficient-based and Manning's equation-based peak flow estimates were higher and lower, respectively, than the observed annual peaks in all years. This illustrated inherent uncertainty introduced early in the design process, which can be further compounded by dynamic in-buffer hydrology. The percentage of the observed peak flow rate at which bypass initiated ranged across an order of magnitude between sites (4.4-8.1% of peak flow rate at one site and 42-49% of peak at another) despite the buffers providing relatively similar NO₃ removal. Bypass at one site (SB2) was related to the concept of "antecedent buffer capacity filled," which was defined as the 5-d average water depth in the middle control structure chamber expressed as a relative percentage of the bypass stop log height. This design flow analysis serves as a call to further evaluate predictive relationships and design models for edge-of-field practices.

The interactions of site-specific factors on riparian buffer effectiveness across multiple pollutants: A review

Following decades of riparian buffer zone (RBZ) studies there remains a need to look across individual site data for collective evidence on the site-specific pollution mitigation and river water quality. We explored primary study evidence on runoff, sediment, P, N, coliforms and pesticides using complimentary styles of metadata interpretation. A quantitative assessment of pollution retention (75 studies, 474 data rows) derived relationships for retention versus width, including significant covariates of clay particle size and buffer slope for sediment, total and dissolved P. Total N and coliforms related to texture and slope but were independent of width. Other factors across pollutants were inconsistently reported. With limitations on quantitative studies a second approach examined factor significance (formal testing versus inferred; 93 studies) across source pressure, transport/physical, vegetation and soil biogeochemical factors on pollution effectiveness. The RBZ evidence showed considerable disagreement and bias in shorter term study implications on longer-term processes. Screening for stronger evidence by study number and agreement left fifteen factors informing on at least one pollutant, whereas only rainfall intensity, preferential deposition, tree planting, soil infiltration remained addressing three or more pollutants. Key messages were that: data complexities, from short-term trapping in upper buffer edges and so-called 'negative effectiveness' associated with internal recycling and/or errors in constraining mass inputs for dissolved pollutant and subsurface transport require careful interpretation; RBZ intervention and study durations were limited compared to effect times (particularly vegetation management and changing soil conditions); factors affect pollutants with particulate and dissolved phases differently and must be understood to limit RBZ pollution swapping. Buffer functioning is highly site-specific. To understand this better attention should be given to revisiting studies of vegetation management to extend timeframes, wider study of belowground (soil biogeochemical and transport) processes and studies should document site contexts across source pressures, riparian hydrological, soil and vegetation factors.

Slope stability of streambanks at saturated riparian buffer sites

Saturated riparian buffers (SRBs) reduce nitrate export from agricultural tile drainage by infusing drainage water into carbon-rich riparian soils where denitrification and plant uptake occur. The water quality benefits from SRBs are well documented, but uncertainties about their effect on streambank stability have led to design standards that limit the maximum bank height and minimum buffer width, thus reducing the number of suitable candidate sites. In this study, the relationship between SRB design and streambank stability was examined through numerical slope stability modeling and validated using field sites. At the study sites, the addition of SRB flow increased the probability of failure by less than 3% for both simulated dry and rainfall scenarios. Furthermore, the simulations provide no evidence to support excluding potential sites based on bank height alone. Multivariate analysis of dimensionless parameters developed for SRB flow conditions was used to predict the factor of safety as a function of the SRB site and design conditions. The equation presented allows designers to assess the stability of a potential site where bank failure poses a heightened risk. The results of this study alleviate the need for extensive geotechnical evaluations at future SRB sites and could increase SRB implementation by expanding the range of eligible sites.

Quantifying the effectiveness of a saturated buffer to reduce tile NO3-N concentrations in eastern Iowa

Agricultural drainage tiles are primary contributors to NO₃-N export from Iowa croplands. Saturated buffers are a relatively new conservation practice that diverts tile water into a distribution tile installed in a riparian buffer parallel to a stream with the intent of enhancing NO₃-N processing within the buffer. In this study, tile NO₃-N concentration reductions were characterized through two different saturated buffers at a working farm site in eastern Iowa. Study objectives were to (1) evaluate the hydrogeology and water quality patterns in the saturated buffer and (2) quantify the reduction in tile NO₃-N concentration from the saturated buffer installation. Results showed that the two saturated buffers are reducing NO₃-N concentrations in tile drainage water from input concentrations of approximately 15 mg/l to levels < 1.5 mg/l at the streamside well locations. The reduction occurs rapidly in the fine-textured and organic-rich alluvial soils with most of the reduction occurring within 1.5 m of the distribution line. Denitrification is hypothesized as being primarily responsible for the concentration reductions based on soil and water chemistry conditions, completion of a geophysical survey (quantifying low potential for N loss to deeper aquifers), and comparisons to other similar Iowa sites. The study provides more assurance to new adopters that this practice can be installed in many areas throughout the Midwestern Cornbelt region.

Mapping of Agricultural Subsurface Drainage Systems Using Unmanned Aerial Vehicle Imagery and Ground Penetrating Radar

Agricultural subsurface drainage systems are commonly installed on farmland to remove the excess water from poorly drained soils. Conventional methods for drainage mapping such as tile probes and trenching equipment are laborious, cause pipe damage, and are often inefficient to apply at large spatial scales. Knowledge of locations of an existing drainage network is crucial to understand the increased leaching and offsite release of drainage discharge and to retrofit the new drain lines within the existing drainage system. Recent technological developments in non-destructive techniques might provide a potential alternative solution. The objective of this study was to determine the suitability of unmanned aerial vehicle (UAV) imagery collected using three different cameras (visible-color, multispectral, and thermal infrared) and ground penetrating radar (GPR) for subsurface drainage mapping. Both the techniques are complementary in terms of their usage, applicability, and the properties they measure and were applied at four different sites in the Midwest USA. At Site-1, both the UAV imagery and GPR were equally successful across the entire field, while at Site-2, the UAV imagery was successful in one section of the field, and GPR proved to be useful in the other section where the UAV imagery failed to capture the drainage pipes’ location. At Site-3, less to no success was observed in finding the drain lines using UAV imagery captured on bare ground conditions, whereas good success was achieved using GPR. Conversely, at Site-4, the UAV imagery was successful and GPR failed to capture the drainage pipes’ location. Although UAV imagery seems to be an attractive solution for mapping agricultural subsurface drainage systems as it is cost-effective and can cover large field areas, the results suggest the usefulness of GPR to complement the former as both a mapping and validation technique. Hence, this case study compares and contrasts the suitability of both the methods, provides guidance on the optimal survey timing, and recommends their combined usage given both the technologies are available to deploy for drainage mapping purposes.

The utility of spatial data to delineate river riparian functions and management zones: A review

Riparian zones of rivers are transitional environments between land and water ecosystems with distinct hydrological gradients, soils and habitats strongly related to their functioning. When these functions are intact, they integrate multi-directional processes across the land-river channel (e.g. canopy shade effects on the stream, flood inundation effects on the land) with mutual beneficial effects. In many managed landscapes these functions have been degraded. To restore them, considerable efforts have been directed over the last 20 years to understand and place effective riparian 'buffer' zones, particularly to enhance water quality and biodiversity. Since water quality targets are not easily met by current practices in many managed landscapes (as additive pressures increase), catchment managers will have to increasingly restore riparian functions to enhance aquatic ecosystem resilience to land and climate change. Targeting effective restoration within site-specific contexts requires availability of spatial data, in combinations that inform on individual and multiple functions. There are accelerating developments with spatial data, arising from increased spatial resolution of key underlying datasets, availability of soil and landcover data and increasing secondary derived attributes. Hence, a review is timely into the best practices in the use of these data for delineating riparian functions and management zones for rivers. Our review evaluates the application of spatial data and is structured around three conceptual methods of riparian delineation; fixed width, variable width by river corridor features and variable width by context of local pressures or required outcomes. We explore process representation and incorporation into management across main riparian functions (hydrological connectivity, water quality, shading, resource transfers and habitat provision). Translating spatial data into functions informs the ability to go beyond contemporary, generally fixed width approaches using basic structural components towards planning to better target functional attributes to optimise ecosystem protection.

Vegetation Type Does not Affect Nitrous Oxide Emissions from Riparian Zones in Agricultural Landscapes

Riparian zones provide multiple benefits in agricultural landscapes, but nitrogen (N) loading can cause N₂O emissions. There is a knowledge gap on how different types of riparian vegetation influence N₂O emissions. This study quantified N₂O emissions from a rehabilitated riparian zone with deciduous trees (RH), a herbaceous (grassed) riparian zone (GRS), a natural forested riparian zone with deciduous trees (UNF-D), a natural forested riparian zone with coniferous trees (UNF-C), and an agricultural field (AGR). N₂O fluxes were not significantly different (p > 0.05) among riparian zones (11-17 µg N₂O-N m^-2 h^-1) and were not significantly different (p > 0.05) when comparing riparian zones to the AGR field (34 µg N₂O-N m^-2 h^-1). Despite high N-loading, cumulative N₂O emissions (1989 µg N₂O-N m^-2) in the riparian zones was significantly lower (p > 0.05) than AGR (13,278 µg N₂O-N m^-2). The main predictors of N₂O fluxes were soil temperature and soil NO₃^--N for the riparian zones and the AGR field. We found that environmental conditions played a greater role than the type of riparian vegetation or age in predicting N₂O emissions. We suggest that soil environmental factors created an anaerobic environment that favored N₂O consumption via complete denitrification.

Improving the effectiveness of saturated riparian buffers for removing nitrate from subsurface drainage

A saturated riparian buffer (SRB) is an edge-of-field conservation practice that reduces nitrate export from agricultural lands by redistributing tile drainage as shallow groundwater and allowing for denitrification and plant uptake. We propose an approach to improve the design of SRBs by analyzing a tradeoff in choosing the SRB width, and we apply the approach to six sites with SRBs in central Iowa. A larger width allows for more residence time, which increases the opportunity for removing nitrate that enters the buffer. However, because the SRBs considered here treat only a portion of the tile flow when it is large, for the same difference in hydraulic head, a smaller width allows more of the total tile flow to enter the buffer and therefore treats more of the drainage. By maximizing the effectiveness of nitrate removal, defined as the ratio of total nitrate removed by the SRB to total nitrate leaving the field in tile drainage, an equation for the optimal width was derived in terms of soil properties, denitrification rates, and head difference. All six sites with existing SRBs considered here have optimal widths smaller than the current width, and two are below the minimum width listed in current design standards. In terms of uncertainty, the main challenges in computing the optimal width for a site are estimating the removal coefficient for nitrate and determining the saturated hydraulic conductivity. Nevertheless, including a width that accounts for site conditions in the design standards would improve water quality locally and regionally.

Efficiency of mitigation measures targeting nutrient losses from agricultural drainage systems: A review

Diffusive losses of nitrogen and phosphorus from agricultural areas have detrimental effects on freshwater and marine ecosystems. Mitigation measures treating drainage water before it enters streams hold a high potential for reducing nitrogen and phosphorus losses from agricultural areas. To achieve a better understanding of the opportunities and challenges characterising current and new drainage mitigation measures in oceanic and continental climates, we reviewed the nitrate and total phosphorus removal efficiency of: (i) free water surface constructed wetlands, (ii) denitrifying bioreactors, (iii) controlled drainage, (iv) saturated buffer zones and (v) integrated buffer zones. Our data analysis showed that the load of nitrate was substantially reduced by all five drainage mitigation measures, while they mainly acted as sinks of total phosphorus, but occasionally, also as sources. The various factors influencing performance, such as design, runoff characteristics and hydrology, differed in the studies, resulting in large variation in the reported removal efficiencies.

Mapping of Agricultural Subsurface Drainage Systems Using a Frequency-Domain Ground Penetrating Radar and Evaluating Its Performance Using a Single-Frequency Multi-Receiver Electromagnetic Induction Instrument

Subsurface drainage systems are commonly used to remove surplus water from the soil profile of a poorly drained farmland. Traditional methods for drainage mapping involve the use of tile probes and trenching equipment that are time-consuming, labor-intensive, and invasive, thereby entailing an inherent risk of damaging the drainpipes. Effective and efficient methods are needed in order to map the buried drain lines: (1) to comprehend the processes of leaching and offsite release of nutrients and pesticides and (2) for the installation of a new set of drain lines between the old ones to enhance the soil water removal. Non-invasive geophysical soil sensors provide a potential alternative solution. Previous research has mainly showcased the use of time-domain ground penetrating radar, with variable success, depending on local soil and hydrological conditions and the central frequency of the specific equipment used. The objectives of this study were: (1) to test the use of a stepped-frequency continuous wave three-dimensional ground penetrating radar (3D-GPR) with a wide antenna array for subsurface drainage mapping and (2) to evaluate its performance with the use of a single-frequency multi-receiver electromagnetic induction (EMI) sensor in-combination. This sensor combination was evaluated on twelve different study sites with various soil types with textures ranging from sand to clay till. While the 3D-GPR showed a high success rate in finding the drainpipes at five sites (sandy, sandy loam, loamy sand, and organic topsoils), the results at the other seven sites were less successful due to the limited penetration depth of the 3D-GPR signal. The results suggest that the electrical conductivity estimates produced by the inversion of apparent electrical conductivity data measured by the EMI sensor could be a useful proxy for explaining the success achieved by the 3D-GPR in finding the drain lines.

Utility of 'Diffusive Gradients in Thin-Films' for the measurement of nitrate removal performance of denitrifying bioreactors

The increase in environmental nutrient availability as a result of human activities has necessitated the development of mitigation strategies for nutrient removal, such as nitrate. Current methods for determining the efficiency of different mitigation strategies required measurement of changes in nitrate concentrations, however, these methods can be expensive or do not account fully for the temporal variability of nitrate concentration. This study evaluated the utility of Diffusive Gradients in Thins-Films (DGT) for determining nitrate removal in two denitrifying bioreactors, and compared DGT performance to traditional approaches for determining performance, including high and low frequency water grab sampling. The binding layer was produced using the Purolite® A520E anion exchange resin. The uptake and elution efficiencies were 98.8% and 93.4% respectively. DGTs of three material diffusion layer thicknesses were placed in piezometers along longitudinal transects, to enable calculation of the diffusive boundary layer and provide replicates. These were removed after 16, 24 and 36 h, and the accumulated nitrate masses were extracted and quantified to calculate nitrate concentration. Concentrations were subsequently utilised to calculate nitrate removal rates in both bioreactors. Grab samples were taken at 30 and 60 min intervals over those periods, nitrate concentrations were also measured to determine nitrate removal. DGTs provided nitrate removal rates at bioreactor site one (controlled flow, wastewater treatment) of 14.83-30.75 g N m^-3 d^-1, and 1.22-3.63 g N m^-3 d^-1 at site two (variable flow, agricultural run-off). DGT determined nitrate concentrations and removal rates were in strong accordance with high frequency grab sampling, but data collection via DGTs was considerably easier. Utilising DGTs for the measurement of bioreactor performance overcame many of the challenges associated with high frequency grab sampling, and other methods, such as accounting for temporal variation in nitrate concentration and reduced analytical requirements.

On the potential for saturated buffers in northwest Ohio to remediate nutrients from agricultural runoff

Nutrient loading from nonpoint source runoff in the Midwest has emerged as one of the largest threats to water quality as the frequency of harmful algal blooms, hypoxic zones, and issues associated with human-resource interactions have risen abruptly over the past several decades. In this study, a saturated buffer ~500 m in length located in the western basin of the Lake Erie watershed was evaluated for its potential to reduce edge of field runoff and nutrient loading. Saturated buffers reduce runoff by routing subsurface tile drainage water into the riparian zone, providing an opportunity for drainage volume as well as nutrient reduction of runoff waters. Over a 12-month study period, controlled drainage was used to redirect nearly 25% of the total tile flow into the riparian zone from a subwatershed in corn/soybean rotation with near complete reductions of dissolved nitrogen and phosphorus from tile inflows averaging 4.7 and 0.08 mg/L, respectively, as well as total reduction of suspended sediments (average 10.4 mg/L). This study provides additional evidence that riparian areas are an important part of nutrient reduction strategies as they can act as both controlled drainage points by raising water tables in fields as well as nutrient sinks which couple to help mitigate nutrient runoff in the region.

Combining Tools from Edge-of-Field to In-Stream to Attenuate Reactive Nitrogen along Small Agricultural Waterways

Reducing excessive reactive nitrogen (N) in agricultural waterways is a major challenge for freshwater managers and landowners. Effective solutions require the use of multiple and combined N attenuation tools, targeted along small ditches and streams. We present a visual framework to guide novel applications of ‘tool stacking’ that include edge-of-field and waterway-based options targeting N delivery pathways, timing, and impacts in the receiving environment (i.e., changes in concentration or load). Implementing tools at multiple locations and scales using a ‘toolbox’ approach will better leverage key hydrological and biogeochemical processes for N attenuation (e.g., water retention, infiltration and filtering, contact with organic soils and microbes, and denitrification), in addition to enhancing ecological benefits to waterways. Our framework applies primarily to temperate or warmer climates, since cold temperatures and freeze–thaw-related processes limit biologically mediated N attenuation in cold climates. Moreover, we encourage scientists and managers to codevelop N attenuation toolboxes with farmers, since implementation will require tailored fits to local hydrological, social, and productive landscapes. Generating further knowledge around N attenuation tool stacking in different climates and landscape contexts will advance management actions to attenuate agricultural catchment N. Understanding how different tools can be best combined to target key contaminant transport pathways and create activated zones of attenuation along and within small agricultural waterways will be essential.

Using Spatially Targeted Conservation to Evaluate Nitrogen Reduction and Economic Opportunities for Best Management Practice Placement in Agricultural Landscapes

The US Cornbelt leads North American production of intensively managed, row-crop corn and soybeans. While highly productive, agricultural management in the region is often linked with nonpoint source nutrient pollution that negatively impacts water quality. Presently, conservation programs designed to install best management practices (BMPs) to mitigate agricultural nonpoint source pollution have not been targeted to those areas of the landscape that contribute disproportionately to surface water quality concerns. We used an innovative spatially targeted conservation protocol coupled with a GIS-based landscape planning tool to evaluate the cost and effect on water quality from nitrate-nitrogen loss under alternative landscape scenarios in an Iowa watershed. Outputs indicate large reductions in watershed-level nitrate-nitrogen loss could be achieved through coordinated placement of BMPs on high-contributing parcels with limited reduction of cultivated land, resulting in improved surface water quality at relatively low economic costs. For example, one scenario, which added wetlands, cover crops, and saturated buffers in the watershed, required the removal of <5% of cultivated area to reduce nitrate-nitrogen loss by an estimated 49%, exceeding the Iowa Nutrient Reduction Strategy goal for enhancing water quality. Annualized establishment and management costs of landscape scenarios that met the nonpoint source nitrogen reduction goal varied from $3.16 to $3.19 million (2017 US dollars). These results support our hypothesis that water quality can be improved by targeting high-contributing parcels, and highlights the potential to minimize tradeoffs by coupling targeted conservation and planning tools to help stakeholders achieve water quality outcomes within agricultural landscapes.

Nitrous oxide and methane production from denitrifying woodchip bioreactors at three hydraulic residence times

Denitrifying bioreactors remove nitrate (NO₃^-) from agricultural drainage and are slated to be an integral part of nitrogen reduction strategies in the Mississippi River Basin. However, incomplete denitrification can result in nitrous oxide (N₂O) production and anaerobic conditions within bioreactors may be conducive to methane (CH₄) production via methanogenesis. Greenhouse gas production has the potential to trade excess NO₃^- in surface water with excess greenhouses gases in the atmosphere. Our study examined N₂O and CH₄ production from pilot scale (6.38 m³) bioreactors across three hydraulic residence times (HRTs), 2, 8, and 16 h. Production was measured from both the surface of the bioreactors and dissolved in the bioreactor effluent. Nitrous oxide and CH₄ was produced across all HRTs, with the majority dissolved in the effluent. Nitrous oxide production was significantly greater (P < 0.05) from 2 h HRTs (478.43 mg N₂O m^-3 day^-1) than from 8 (29.95 mg N₂O m^-3 day^-1) and 16 (36.61 mg N₂O m^-3 day^-1) hour HRTs. Methane production was significantly less (P < 0.05) from 2 h HRTs (0.51 g C m³ day^-1) compared to 8 (1.50 g C m³ day^-1) and 16 (1.69 g C m³ day^-1) hour HRTs. The 2 h HRTs had significantly greater (P = 0.05) impacts to climate change compared to 8 and 16 h HRTs. Results from this study suggest managing HRTs between 6 and 8 h in field bioreactors could minimize total greenhouse gas production and maximize NO₃^- removal.

A new approach to generalizing riparian water and air quality function across regions

There is growing interest in generalizing the impact of hydrogeomorphology and weather variables on riparian functions. Here, we used RZ-TRADEOFF to estimate nitrogen, phosphorus, water table (WT) depth, and greenhouse gas (GHG: N₂O, CO₂, CH₄) functions for 80 riparian zones typical of the North American Midwest, Northeast (including Southern Ontario, Canada), and Mid-Atlantic. Sensitivity to weather perturbations was calculated for temperature and precipitation-dependent functions (CO₂, phosphate concentration, and water table), and multivariate statistical analysis on model outputs was conducted to determine trade-offs between riparian functions. Mean model estimates were 93.10 cm for WT depth, 8.45 mg N L^-1 for field edge nitrate concentration, 51.57% for nitrate removal, 0.45 mg PO₄^3- L^-1 for field edge phosphate concentration, 1.5% for subsurface phosphate removal, 91.24% for total overland phosphorus removal, 0.51 mg N m^-2 day^-1 for N₂O flux, 5.5 g C m^-2 day^-1 for CO₂ fluxes, and - 0.41 mg C m^-2 day^-1 and 621.51 mg C m^-2 day^-1 for CH₄ fluxes in non-peat sites and peat sites, respectively. Sites in colder climates were most sensitive to weather perturbations for CO₂, sites with deep water tables estimates had the highest sensitivity for WT, and sites in warm climates and/or with deep confining layers had the lowest sensitivity for phosphate concentration. Slope, confining layer depth, and temperature were the primary characteristics influencing similarities and trade-offs between sites. This research contributes to understanding how to optimize riparian restoration and protection in watersheds based on both water (nitrogen, phosphorus) and air quality (GHG) goals.

RZ-TRADEOFF: A New Model to Estimate Riparian Water and Air Quality Functions

Riparian zones are often used as best management practices due to their ability to remove nitrate (NO3−) from subsurface flow. Research suggests that beyond local biogeochemical controls, the impact of riparian zones on nitrogen removal and other functions, such as phosphorus dynamics and greenhouse gas emissions, largely depends on land-use/land-cover, hydrogeomorphology, and weather. In this study, we therefore present RZ-TRADEOFF, a novel and easily applicable model that connects multiple riparian functions and characteristics (NO3− and phosphate (PO43−), concentration and removal in subsurface flow, total phosphorus (TP) removal in overland flow, nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) emissions, water table) to landscape hydrogeomorphic characteristics, weather, and land-cover/land-use. RZ-TRADEOFF was developed with data from past studies and digital databases, and validated with data collected from the literature. Three functions (water table, PO43− and CO2) were observed to be significantly influenced by climate/weather, while the others were primarily influenced by hydrogeomorphology and land use. The percent bias and normalized root mean square error respectively were −3.35% and 0.28 for water table, 16.00% and 0.34 for NO3− concentration, −7.83% and 20.82 for NO3− removal, 6.64% and 0.35 for PO43− concentration, 2.55% and 0.17 for TP removal, 40.33% and 0.23 for N2O, 72.68% and 0.18 for CH4, and −34.98% and 0.91 for CO2. From a management standpoint, RZ-TRADEOFF significantly advances our ability to predict multiple water and air quality riparian functions using easily accessible data over large areas of the landscape due to its scalability.

Yield stability analysis reveals sources of large-scale nitrogen loss from the US Midwest

Loss of reactive nitrogen (N) from agricultural fields in the U.S. Midwest is a principal cause of the persistent hypoxic zone in the Gulf of Mexico. We used eight years of high resolution satellite imagery, field boundaries, crop data layers, and yield stability classes to estimate the proportion of N fertilizer removed in harvest (NUE) versus left as surplus N in 8 million corn (Zea mays) fields at subfield resolutions of 30 × 30 m (0.09 ha) across 30 million ha of 10 Midwest states. On average, 26% of subfields in the region could be classified as stable low yield, 28% as unstable (low yield some years, high others), and 46% as stable high yield. NUE varied from 48% in stable low yield areas to 88% in stable high yield areas. We estimate regional average N losses of 1.12 (0.64–1.67) Tg N y⁻¹ from stable and unstable low yield areas, corresponding to USD 485 (267–702) million dollars of fertilizer value, 79 (45–113) TJ of energy, and greenhouse gas emissions of 6.8 (3.4–10.1) MMT CO₂ equivalents. Matching N fertilizer rates to crop yield stability classes could reduce regional reactive N losses substantially with no impact on crop yields, thereby enhancing the sustainability of corn-based cropping systems.

Quantifying the contribution of tile drainage to basin-scale water yield using analytical and numerical models

The Des Moines Lobe (DML) of north-central Iowa has been artificially drained by subsurface drains and surface ditches to provide some of the most productive agricultural land in the world. Herein we report on the use of end-member mixing analysis (EMMA) models and the numerical model Soil and Water Assessment Tool (SWAT) to quantify the contribution of tile drainage to basin-scale water yields at various scales within the 2370 km² Boone River watershed (BRW), a subbasin within the Des Moines River watershed. EMMA and SWAT methods suggested that tile drainage provided approximately 46 to 54% of annual discharge in the Boone River and during the March to June period, accounted for a majority of flow in the river. In the BRW subbasin of Lyons Creek, approximately 66% of the annual flow was sourced from tile drainage. Within the DML region, tile drainage contributes to basin-scale water yields at scales ranging from 40 to 16,000 km², with downstream effects diminishing with increasing watershed size. Developing a better understanding of water sources contributing to river discharge is needed if mitigation and control strategies are going to be successfully targeted to reduce downstream nutrient export.

Life in the slow drain: Landscape structure affects farm ditch water quality

Agrichemical contamination is a major threat to aquatic ecosystems in farmland. There is a need to better understand the influence of the surrounding landscape on farm wetlands to recommend land management options that minimize water quality impacts from agricultural practices. We tested hypothesized relationships between landscape structure and multiple water quality measures in farm drainage ditches in a multi-landscape study in Eastern Ontario, Canada. We measured physicochemical water quality (levels of atrazine, glyphosate, neonicotinoid insecticides, inorganic nitrogen, and dissolved oxygen), and biological water quality indicators (aquatic macroinvertebrate richness, leaf litter decomposition, and Ceriodaphnia dubia population responses) in 27 farm ditches, and measured the amounts of forest cover and high-intensity crop cover (landscape composition), and field edge cover (landscape configuration) in 1-km radius landscapes surrounding each ditch sampling site. We used confirmatory path analysis to simultaneously model the direct and indirect relationships between the landscape predictors and water quality variables. Landscape composition measures were the strongest predictors of water quality: pesticides decreased as surrounding forest cover increased, and nitrogen increased with increasing amounts of high-intensity crop cover. Crop cover was also indirectly negatively related to macroinvertebrate richness via its effects on nitrogen and dissolved oxygen. We found no effects of landscape configuration on agrichemical levels, but there was some support for a positive relationship between macroinvertebrate richness and field edge cover. Our results indicate that aquatic macroinvertebrate richness is strongly impacted by fertilizer use in our region, and that macroinvertebrate richness is a more sensitive biotic indicator of farmland water quality than leaf litter decomposition or C. dubia responses. We conclude that, in our region, landscape management to improve farmland water quality should focus primarily on landscape composition. Such management should aim to increase amounts of non-crop cover such as forest, and reduce amounts of crop cover with high agrichemical inputs.

An Assessment of the Multifunctionality of Integrated Buffer Zones in Northwestern Europe

Integrated buffer zones (IBZs) have recently been introduced in the Northwestern Europe temperate zone to improve delivery of ecosystem services compared with the services associated with long-established vegetated buffer zones. A common feature of all the studied IBZ sites is that tile drainage, which previously discharged directly into the streams, is now intercepted within the IBZ. Specifically, the design of IBZs combines a pond, where soil particles present in drain water or surface runoff can be deposited, and a planted subsurface flow infiltration zone. Together, these two components should provide an optimum environment for microbial processes and plant uptake of nutrients. Nutrient reduction capacities, biodiversity enhancement, and biomass production functions were assessed with different emphasis across 11 IBZ sites located in Denmark, Great Britain, and Sweden. Despite the small size of the buffer zones (250-800 m) and thus the small proportion of the drained catchment (mostly <1%), these studies cumulatively suggest that IBZs are effective enhancements to traditional buffer zones, as they (i) reduce total N and P loads to small streams and rivers, (ii) act as valuable improved habitats for aquatic and amphibian species, and (iii) offer economic benefits by producing fast-growing wetland plant biomass. Based on our assessment of the pilot sites, guidance is provided on the implementation and management of IBZs within agricultural landscapes.

In Situ Denitrification in Saturated Riparian Buffers

Excess NO leaching from the agricultural Midwest via tile drainage water has contributed to both local drinking water and national Gulf of Mexico benthic hypoxia concerns. Both in-field and edge-of-field practices have been designed to help mitigate NO flux to surface waters. Edge-of-field practices focus on maximizing microbial denitrification, the conversion of NO to N gas. This study assessed denitrification rates from two saturated riparian buffers (SRBs) for 2 yr and a third SRB for 1 yr, for a total of five sample years. These SRBs were created by diverting NO-rich tile drainage water into riparian buffers soils. The SRBs in this study removed between 27 and 96% of the total diverted NO load. Measured cumulative average denitrification rate for each SRB sample year accounted for between 3.7 and 77.3% of the total NO removed. Both the cumulative maximum and 90% confidence interval denitrification rates accounted for all of the NO removed by the SRBs in three of the five sample years, indicating that denitrification can be a dominant NO removal mechanism in this edge-of-field practice. When adding the top 20 cm of each core to the cumulative denitrification rates for each SRB, denitrification accounted for between 33 and over 100% of the total NO removed. Buffer age (time since establishment) was speculated to enhance denitrification rates, and there was a trend of the soil closer to the surface making up the majority of the total denitrification rate. Finally, both NO and C could limit denitrification in these SRBs.

Performance of Saturated Riparian Buffers in Iowa, USA

Nitrate from artificial drainage pipes (tiles) underlying agricultural fields is a major source of reactive N, especially NO, in surface waters. A novel approach for reducing NO loss is to intercept a field tile where it crosses a riparian buffer and divert a fraction of the flow as shallow groundwater within the buffer. This practice is called a saturated riparian buffer (SRB), and although it is promising, little data on the performance of the practice is available. This research investigated the effectiveness of SRBs in removing NO at six sites installed across Iowa, resulting in a total of 17 site-years. Water flow and NO in the tile outlets, diverted into the buffers, and NO concentration changes within the buffers were monitored throughout the year at each site. Results showed that all the SRBs were effective in removing NO from the tile outlet, with the average annual NO load removal ranging from 13 to 179 kg N for drainage areas ranging from 3.4 to 40.5 ha. This is NO that would have otherwise discharged directly into the adjoining streams. The annual removal effectiveness, which is the total NO removed in the SRB divided by the total NO draining from the field, ranged from 8 to 84%. This corresponds to an average removal rate of 0.040 g N m d with a range of 0.004 to 0.164 g N m d. Assuming a 40-yr life expectancy for the structure and a 4% discount rate, we computed a mean equal annual cost for SRBs of US$213.83. Given the average annual removal of 73 kg for all site-years, this cost equates to $2.94 kg N removed, which is very competitive with other field-edge practices such as denitrification bioreactors and constructed wetlands. Thus, SRBs continue to be a promising practice for NO removal in tile-drained landscapes.

Effects of vegetation types on water-extracted soil organic matter (WSOM) from riparian wetland and its impacts on riverine water quality: Implications for riparian wetland management

Riparian wetlands play important roles in the enhancement of water quality by controlling nonpoint source pollution and protecting aquatic ecosystems. In the present study, we surveyed and identified vegetation types in riparian wetlands, evaluated how vegetation types influence spatial patterns of water-extracted soil organic matter (WSOM) from riparian wetland, and probed the impacts of riparian fluorescent WSOM on fluorescent dissolved organic matter (FDOM) and water trophic states in river ecosystems. We used absorption and excitation-emission-matrix (EEM) fluorescence spectroscopy to characterize the optical properties of riparian WSOM and riverine DOM from Chongming Island, China, the largest alluvial plain island in the world. Our results showed that fifty-eight spermatophytes in riparian wetland were clustered into five vegetation types, including warm coniferous forest (WCF), deciduous broad leaf forest (DBF), evergreen broad leaf forest (EBF), aquatic plants (AP) and herbaceous plants (HP). Absorption spectra revealed the effects of vegetation types on riparian chromophoric WSOM quantity. Although no difference in water-extracted soil organic carbon (WSOC) contents was observed, deciduous broad leaf forest (DBF) and evergreen broad leaf forest (EBF) fed more fluorescent WSOM quantity than did the other vegetation (AP, HP and WCF), and deciduous broad leaf forest (DBF) and aquatic plants (AP) provided more humic-like (RC.1 and RC.2) and fulvic-like (RC.3) substances into riparian wetland (P<0.05 or P<0.01). In addition, we noted that humic-like and protein-like substances (RC.4) transported from riparian wetland into a river water body, and riverine terrestrial-originated components (FC.1 and FC.2) were significantly related to the four riparian fluorescent WSOM components (P<0.05). Furthermore, the riverine trophic state was significantly higher when the fluvial DOM and its component quantity increased (P<0.05). We concluded that riparian wetland can control the quantity and quality of riparian WSOM and reshaped riverine DOM compositions and riverine water quality with important management implications.

Beyond the Edge: Linking Agricultural Landscapes, Stream Networks, and Best Management Practices

Despite much research and investment into understanding and managing nutrients across agricultural landscapes, nutrient runoff to freshwater ecosystems is still a major concern. We argue there is currently a disconnect between the management of watershed surfaces (agricultural landscape) and river networks (riverine landscape). These landscapes are commonly managed separately, but there is limited cohesiveness between agricultural landscape-focused research and river science, despite similar end goals. Interdisciplinary research into stream networks that drain agricultural landscapes is expanding but is fraught with problems. Conceptual frameworks are useful tools to order phenomena, reveal patterns and processes, and in interdisciplinary river science, enable the joining of multiple areas of understanding into a single conceptual-empirical structure. We present a framework for the interdisciplinary study and management of agricultural and riverine landscapes. The framework includes components of an ecosystems approach to the study of catchment-stream networks, resilience thinking, and strategic adaptive management. Application of the framework is illustrated through a study of the Fox Basin in Wisconsin, USA. To fully realize the goal of nutrient reduction in the basin, we suggest that greater emphasis is needed on where best management practices (BMPs) are used within the spatial context of the combined watershed-stream network system, including BMPs within the river channel. Targeted placement of BMPs throughout the riverine landscape would increase the overall buffering capacity of the system to nutrient runoff and thus its resilience to current and future disturbances.

Simulation of targeted pollutant-mitigation-strategies to reduce nitrate and sediment hotspots in agricultural watershed

About 50% of U.S. water pollution problems are caused by non-point source (NPS) pollution, primarily sediment and nutrients from agricultural areas, despite the widespread implementation of agricultural Best Management Practices (BMPs). However, the effectiveness of implementation strategies and type of BMPs at watershed scale are still not well understood. In this study, the Soil and Water Assessment Tool (SWAT) ecohydrological model was used to assess the effectiveness of pollutant mitigation strategies in the Raccoon River watershed (RRW) in west-central Iowa, USA. We analyzed fourteen management scenarios based on systematic combinations of five strategies: fertilizer/manure management, changing row-crop land to perennial grass, vegetative filter strips, cover crops and shallower tile drainage systems, specifically aimed at reducing nitrate and total suspended sediment yields from hotspot areas in the RRW. Moreover, we assessed implications of climate change on management practices, and the impacts of management practices on water availability, row crop yield, and total agricultural production. Our results indicate that sufficient reduction of nitrate load may require either implementation of multiple management practices (38.5% with current setup) or conversion of extensive areas into perennial grass (up to 49.7%) to meet and maintain the drinking water standard. However, climate change may undermine the effectiveness of management practices, especially late in the 21st century, cutting the reduction by up to 65% for nitrate and more for sediment loads. Further, though our approach is targeted, it resulted in a slight decrease (~5%) in watershed average crop yield and hence an overall reduction in total crop production, mainly due to the conversion of row-crop lands to perennial grass. Such yield reductions could be quite spatially heterogeneously distributed (0 to 40%).

Feasibility of Recycling Excess Agricultural Nitrate with Electrodialysis

One of the most serious environmental problems associated with agriculture is excessive nitrate N in waters leaving fields. It is a health hazard in drinking water and a primary cause of hypoxia in ocean waters receiving drainage from agricultural regions. Recent mitigation efforts have focused on techniques that promote denitrification-conversion of excess agricultural nitrate to N. This seems inherently wasteful since industrial production of nitrate fertilizer from N requires a substantial input of energy and is a major source of greenhouse gas emissions. Thus, it is desirable to develop methods to recycle nitrate, keeping it in a form suitable for reuse as fertilizer. One possibility is electrodialysis, in which direct current is passed through alternating cation- and anion-permeable membranes, creating separate streams of dilute and concentrated water. We tested the concept under controlled conditions in a greenhouse and in a field setting on a contaminated trout stream with nitrate N concentrations consistently above 20 mg L. The solar-powered field system removed 42% of the nitrate from water passing through it and concentrated it in a tank for subsequent application as fertilizer. The upper limit of concentration was approximately 520 mg L, above which precipitation of calcite limited operation. Economic analysis indicates that in comparison to denitrification methods such as bioreactors, electrodialysis is likely to be more expensive per unit of nitrate removed. The approach will be most feasible for situations in which nitrate concentrations are well above environmental standards for extended periods, to maximize operating time and nitrate removal rate.

Prairie strips improve biodiversity and the delivery of multiple ecosystem services from corn-soybean croplands

Loss of biodiversity and degradation of ecosystem services from agricultural lands remain important challenges in the United States despite decades of spending on natural resource management. To date, conservation investment has emphasized engineering practices or vegetative strategies centered on monocultural plantings of nonnative plants, largely excluding native species from cropland. In a catchment-scale experiment, we quantified the multiple effects of integrating strips of native prairie species amid corn and soybean crops, with prairie strips arranged to arrest run-off on slopes. Replacing 10% of cropland with prairie strips increased biodiversity and ecosystem services with minimal impacts on crop production. Compared with catchments containing only crops, integrating prairie strips into cropland led to greater catchment-level insect taxa richness (2.6-fold), pollinator abundance (3.5-fold), native bird species richness (2.1-fold), and abundance of bird species of greatest conservation need (2.1-fold). Use of prairie strips also reduced total water runoff from catchments by 37%, resulting in retention of 20 times more soil and 4.3 times more phosphorus. Corn and soybean yields for catchments with prairie strips decreased only by the amount of the area taken out of crop production. Social survey results indicated demand among both farming and nonfarming populations for the environmental outcomes produced by prairie strips. If federal and state policies were aligned to promote prairie strips, the practice would be applicable to 3.9 million ha of cropland in Iowa alone.

Intra-annual variation of the association between agricultural best management practices and stream nutrient concentrations

Temporal variation may influence the ability of best management practices (BMPs) to mitigate the loss of agricultural pollutants to streams. Our goal was to assess variation in mitigation effects of BMPs by examining the associations between instream nutrient concentrations and the abundance and location of four structural BMPs over a hydrologic year. Water samples were collected monthly (Nov. 2013-Oct. 2014) in 15 headwater streams representing a gradient of BMP use in Southern Ontario, Canada. Partial least squares (PLS) regression models were used to associate two groups of collinear nutrient forms with the abundance and location of BMPs, antecedent precipitation and time of year. BMP metrics in PLS models were associated with instream concentrations of major phosphorus forms and ammonium throughout the year. In contrast, total nitrogen and nitrate-nitrite were only associated with BMPs during snowmelt. BMP metrics associated with reductions of phosphorus and ammonium included greater abundances of riparian buffers and manure storage structures, but not livestock restriction fences. Likewise, the abundance and location riparian vegetation in areas capturing more surface runoff were associated with decreased stream nitrogen concentrations during snowmelt. However, the amount of tile drainage was associated with increased nitrogen concentrations following snowmelt, as well as with greater phosphorus and ammonium concentrations throughout the year. Overall, our findings indicate that increasing the abundance of riparian buffers and manure storage structures may decrease instream nutrient concentrations in agricultural areas. Additionally, the implementation of these structural BMPs appear to be an effective year-round strategy to assist management objectives in reducing phosphorus concentrations in small agricultural streams and thus loadings to downstream tributaries. Further mitigation measures, such as managerial BMPs and controlled tile drainage, may be required to further reduce instream nutrient concentrations during baseflow periods and snowmelt events.

Interactions between Bt crops and aquatic ecosystems: A review

The term Bt crops collectively refers to crops that have been genetically modified to include a gene (or genes) sourced from Bacillus thuringiensis (Bt) bacteria. These genes confer the ability to produce proteins toxic to certain insect pests. The interaction between Bt crops and adjacent aquatic ecosystems has received limited attention in research and risk assessment, despite the fact that some Bt crops have been in commercial use for 20 yr. Reports of effects on aquatic organisms such as Daphnia magna, Elliptio complanata, and Chironomus dilutus suggest that some aquatic species may be negatively affected, whereas other reports suggest that the decreased use of insecticides precipitated by Bt crops may benefit aquatic communities. The present study reviews the literature regarding entry routes and exposure pathways by which aquatic organisms may be exposed to Bt crop material, as well as feeding trials and field surveys that have investigated the effects of Bt-expressing plant material on such organisms. The present review also discusses how Bt crop development has moved past single-gene events, toward multigene stacked varieties that often contain herbicide resistance genes in addition to multiple Bt genes, and how their use (in conjunction with co-technology such as glyphosate/Roundup) may impact and interact with aquatic ecosystems. Lastly, suggestions for further research in this field are provided. Environ Toxicol Chem 2016;35:2891-2902. © 2016 SETAC.

Soybean Area and Baseflow Driving Nitrate in Iowa's Raccoon River

Improved understanding of the drivers of stream nitrate is necessary to improve water quality. This is particularly true for Iowa, a large contributor to Mississippi River Basin nitrate loads. Here, we focus on the Raccoon River at Des Moines, Iowa, and develop statistical models to describe the monthly (from March to August) nitrate concentrations in terms of eight drivers representing monthly climate, monthly hydrology, and yearly cropping practices. We consider six two-parameter distributions, linear and nonlinear dependencies between the predictors, and the distributions' parameters. Model selection was performed by penalizing more complex models. Our results show that the Weibull and Gumbel distributions are the only two selected distributions. Baseflow and the previous year's soybean [ (L.) Merr.] area were the two predictors most often identified as important. Our modeling results imply that increases in soybean area have led to increasing nitrate concentrations. Moreover, nitrate concentrations are related to baseflow in a nonlinear way, with effects strongest when baseflow is near or below the average condition. Additional relevant predictors were precipitation and, to a lesser extent, temperature. We conclude that best management practices and improved conservation targeting soybean in a corn ( L.)-soybean rotation will improve water quality in this artificially drained system.

Temperature and Substrate Control Woodchip Bioreactor Performance in Reducing Tile Nitrate Loads in East-Central Illinois

Tile drainage is the major source of nitrate in the upper Midwest, and end-of-tile removal techniques such as wood chip bioreactors have been installed that allow current farming practices to continue, with nitrate removed through denitrification. There have been few multiyear studies of bioreactors examining controls on nitrate removal rates. We evaluated the nitrate removal performance of two wood chip bioreactors during the first 3 yr of operation and examined the major factors that regulated nitrate removal. Bioreactor 2 was subject to river flooding, and performance was not assessed. Bioreactor 1 had average monthly nitrate removal rates of 23 to 44 g N m d in Year 1, which decreased to 1.2 to 11 g N m d in Years 2 and 3. The greater N removal rates in Year 1 and early in Year 2 were likely due to highly degradable C in the woodchips. Only late in Year 2 and in Year 3 was there a strong temperature response in the nitrate removal rate. Less than 1% of the nitrate removed was emitted as NO. Due to large tile inputs of nitrate (729-2127 kg N) at high concentrations (∼30 mg nitrate N L) in Years 2 and 3, overall removal efficiency was low (3 and 7% in Years 2 and 3, respectively). Based on a process-based bioreactor performance model, Bioreactor 1 would have needed to be 9 times as large as the current system to remove 50% of the nitrate load from this 20-ha field.

Tile Drainage Density Reduces Groundwater Travel Times and Compromises Riparian Buffer Effectiveness

Strategies to reduce nitrate-nitrogen (nitrate) pollution delivered to streams often seek to increase groundwater residence time to achieve measureable results, yet the effects of tile drainage on residence time have not been well documented. In this study, we used a geographic information system groundwater travel time model to quantify the effects of artificial subsurface drainage on groundwater travel times in the 7443-ha Bear Creek watershed in north-central Iowa. Our objectives were to evaluate how mean groundwater travel times changed with increasing drainage intensity and to assess how tile drainage density reduces groundwater contributions to riparian buffers. Results indicate that mean groundwater travel times are reduced with increasing degrees of tile drainage. Mean groundwater travel times decreased from 5.6 to 1.1 yr, with drainage densities ranging from 0.005 m (7.6 mi) to 0.04 m (62 mi), respectively. Model simulations indicate that mean travel times with tile drainage are more than 150 times faster than those that existed before settlement. With intensive drainage, less than 2% of the groundwater in the basin appears to flow through a perennial stream buffer, thereby reducing the effectiveness of this practice to reduce stream nitrate loads. Hence, strategies, such as reconnecting tile drainage to buffers, are promising because they increase groundwater residence times in tile-drained watersheds.

Long-Term Observations of Nitrogen and Phosphorus Export in Paired-Agricultural Watersheds under Controlled and Conventional Tile Drainage

Controlled tile drainage (CTD) regulates water and nutrient export from tile drainage systems. Observations of the effects of CTD imposed en masse at watershed scales are needed to determine the effect on downstream receptors. A paired-watershed approach was used to evaluate the effect of field-to-field CTD at the watershed scale on fluxes and flow-weighted mean concentrations (FWMCs) of N and P during multiple growing seasons. One watershed (467-ha catchment area) was under CTD management (treatment [CTD] watershed); the other (250-ha catchment area) had freely draining or uncontrolled tile drainage (UCTD) (reference [UCTD] watershed). The paired agricultural watersheds are located in eastern Ontario, Canada. Analysis of covariance and paired tests were used to assess daily fluxes and FWMCs during a calibration period when CTD intervention on the treatment watershed was minimal (2005-2006, when only 4-10% of the tile-drained area was under CTD) and a treatment period when the treatment (CTD) watershed had prolific CTD intervention (2007-2011 when 82% of tile drained fields were controlled, occupying >70% of catchment area). Significant linear regression slope changes assessed using ANCOVA ( ≤ 0.1) for daily fluxes from upstream and downstream monitoring sites pooled by calibration and treatment period were -0.06 and -0.20 (stream water) (negative values represent flux declines in CTD watershed), -0.59 and -0.77 (NH-N), -0.14 and -0.15 (NO-N), -1.77 and -2.10 (dissolved reactive P), and -0.28 and 0.45 (total P). Total P results for one site comparison contrasted with other findings likely due to unknown in-stream processes affecting total P loading, not efficacy of CTD. The FWMC results were mixed and inconclusive but suggest physical abatement by CTD is the means by which nutrient fluxes are predominantly reduced at these scales. Overall, our study results indicate that CTD is an effective practice for reducing watershed scale fluxes of stream water, N, and P during the growing season.

Agricultural conservation planning framework: 1. Developing multipractice watershed planning scenarios and assessing nutrient reduction potential

Spatial data on soils, land use, and topography, combined with knowledge of conservation effectiveness, can be used to identify alternatives to reduce nutrient discharge from small (hydrologic unit code [HUC]12) watersheds. Databases comprising soil attributes, agricultural land use, and light detection and ranging-derived elevation models were developed for two glaciated midwestern HUC12 watersheds: Iowa's Beaver Creek watershed has an older dissected landscape, and Lime Creek in Illinois is young and less dissected. Subsurface drainage is common in both watersheds. We identified locations for conservation practices, including in-field practices (grassed waterways), edge-of-field practices (nutrient-removal wetlands, saturated buffers), and drainage-water management, by applying terrain analyses, geographic criteria, and cross-classifications to field- and watershed-scale geographic data. Cover crops were randomly distributed to fields without geographic prioritization. A set of alternative planning scenarios was developed to represent a variety of extents of implementation among these practices. The scenarios were assessed for nutrient reduction potential using a spreadsheet approach to calculate the average nutrient-removal efficiency required among the practices included in each scenario to achieve a 40% NO-N reduction. Results were evaluated in the context of the Iowa Nutrient Reduction Strategy, which reviewed nutrient-removal efficiencies of practices and established the 40% NO-N reduction as Iowa's target for Gulf of Mexico hypoxia mitigation by agriculture. In both test watersheds, planning scenarios that could potentially achieve the targeted NO-N reduction but remove <5% of cropland from production were identified. Cover crops and nutrient removal wetlands were common to these scenarios. This approach provides an interim technology to assist local watershed planning and could provide planning scenarios to evaluate using watershed simulation models. A set of ArcGIS tools is being released to enable transfer of this mapping technology.

Nitrogen removal and greenhouse gas emissions from constructed wetlands receiving tile drainage water

Loss of nitrate from agricultural lands to surface waters is an important issue, especially in areas that are extensively tile drained. To reduce these losses, a wide range of in-field and edge-of-field practices have been proposed, including constructed wetlands. We re-evaluated constructed wetlands established in 1994 that were previously studied for their effectiveness in removing nitrate from tile drainage water. Along with this re-evaluation, we measured the production and flux of greenhouse gases (GHGs) (CO, NO, and CH). The tile inlets and outlets of two wetlands were monitored for flow and N during the 2012 and 2013 water years. In addition, seepage rates of water and nitrate under the berm and through the riparian buffer strip were measured. Greenhouse gas emissions from the wetlands were measured using floating chambers (inundated fluxes) or static chambers (terrestrial fluxes). During this 2-yr study, the wetlands removed 56% of the total inlet nitrate load, likely through denitrification in the wetland. Some additional removal of nitrate occurred in seepage water by the riparian buffer strip along each berm (6.1% of the total inlet load, for a total nitrate removal of 62%). The dominant GHG emitted from the wetlands was CO, which represented 75 and 96% of the total GHG emissions during the two water years. The flux of NO contributed between 3.7 and 13% of the total cumulative GHG flux. Emissions of NO were 3.2 and 1.3% of the total nitrate removed from wetlands A and B, respectively. These wetlands continue to remove nitrate at rates similar to those measured after construction, with relatively little GHG gas loss.

Agro-hydrologic landscapes in the Upper Mississippi and Ohio River basins

A critical part of increasing conservation effectiveness is targeting the "right practice" to the "right place" where it can intercept pollutant flowpaths. Conceptually, these flowpaths can be inferred from soil and slope characteristics, and in this study, we developed an agro-hydrologic classification to identify N and P loss pathways and priority conservation practices in small watersheds in the U.S. Midwest. We developed a GIS framework to classify 11,010 small watersheds in the Upper Mississippi and Ohio River basins based on soil permeability and slope characteristics of agricultural cropland areas in each watershed. The amount of cropland in any given watershed varied from <10 to >60 %. Cropland areas were classified into five main categories, with slope classes of <2, 2-5, and >5 %, and soil drainage classes of poorly and well drained. Watersheds in the Upper Mississippi River basin (UMRB) were dominated by cropland areas in low slopes and poorly drained soils, whereas less-intensively cropped watersheds in Wisconsin and Minnesota (in the UMRB) and throughout the Ohio River basin were overwhelmingly well drained. Hydrologic differences in cropped systems indicate that a one-size-fits-all approach to conservation selection will not work. Consulting the classification scheme proposed herein may be an appropriate first-step in identifying those conservation practices that might be most appropriate for small watersheds in the basin.