Nitrate-nitrogen export: magnitude and patterns from drainage districts to downstream river basins

Important Role of Overland Flows and Tile Field Pathways in Nutrient Transport

Nitrogen and phosphorus pollution is of great concern to aquatic life and human well-being. While most of these nutrients are applied to the landscape, little is known about the complex interplay among nutrient applications, transport attenuation processes, and coastal loads. Here, we enhance and apply the Spatially Explicit Nutrient Source Estimate and Flux model (SENSEflux) to simulate the total annual nitrogen and phosphorus loads from the US Great Lakes Basin to the coastline, identify nutrient delivery hotspots, and estimate the relative contributions of different sources and pathways at a high resolution (120 m). In addition to in-stream uptake, the main novelty of this model is that SENSEflux explicitly describes nutrient attenuation through four distinct pathways that are seldom described jointly in other models: runoff from tile-drained agricultural fields, overland runoff, groundwater flow, and septic plumes within groundwater. Our analysis shows that agricultural sources are dominant for both total nitrogen (TN) (58%) and total phosphorus (TP) (46%) deliveries to the Great Lakes. In addition, this study reveals that the surface pathways (sum of overland flow and tile field drainage) dominate nutrient delivery, transporting 66% of the TN and 76% of the TP loads to the US Great Lakes coastline. Importantly, this study provides the first basin-wide estimates of both nonseptic groundwater (TN: 26%; TP: 5%) and septic-plume groundwater (TN: 4%; TP: 2%) deliveries of nutrients to the lakes. This work provides valuable information for environmental managers to target efforts to reduce nutrient loads to the Great Lakes, which could be transferred to other regions worldwide that are facing similar nutrient management challenges.

Streamflow duration curve to explain nutrient export in Midwestern USA watersheds: Implication for water quality achievements

As part of federal programs to reduce nutrient pollution, states across the Midwest have developed nutrient reduction strategies, which focus on implementation of agricultural conservation practices (ACPs) or best management practices (BMPs). Despite several decades of federal investment in implementing ACPs/BMPs for reducing nutrient pollution, nutrient pollution is a continuing and growing challenge with profound implications for water quality and public health as well as ecological functions. Pollutant transport depends on water and sediment fluxes, which are governed by local hydrology. Therefore, knowing how flow conditions affect nutrients export is critical to develop effective nutrient reduction strategies. The objective of this study was to investigate the role of streamflow duration curve in controlling nutrient export in the western Lake Erie Basin and the Mississippi River Basin. To achieve this goal, we used long-term monitoring data collected by the National Center for Water Quality Research. We focused on the percentage of the annual pollutant load (nitrate-NO₃-N, dissolved reactive phosphorus-DRP, total phosphorus-TP, and total suspended solids-TSS) exported during five flow intervals that spanned the flow duration curve: High Flows (0-10th percentile), Moist Conditions (10-40th percentile), Mid-Range Flows (40-60th percentile), Dry Conditions (60-90th percentile), and Low Flows (90-100th percentile). The results show that the top 10% of flows (i.e., high flows) transported more than 50% of the annual nutrient loads in most of the studying watersheds. Meanwhile, the top 40% of flows transported 54-98% of the annual NO₃-N loads, 55-99% of the annual DRP loads, 79-99% of the annual TP loads, and 86-100% of the annual TSS loads across the studying watersheds. The percentage of the annual loads released during high flows increased as the percentage of the agricultural land use in the watershed increased, but it decreased as the watershed area increased across different watersheds. Finally, flow condition/nutrient export relationships were consistent over studying period. Therefore, reducing nutrient loads during high flow condition is the key for effective nutrient reduction.

Extreme low-flow conditions in a dual-chamber denitrification bioreactor contribute to pollution swapping with low landscape-scale impact

Denitrification bioreactors are an effective edge-of-field conservation practice for nitrate (NO₃) reduction from subsurface drainage. However, these systems may produce other pollutants and greenhouse gases during NO₃ removal. Here a dual-chamber woodchip bioreactor system experiencing extreme low-flow conditions was monitored for its spatiotemporal NO₃ and total organic carbon dynamics in the drainage water. Near complete removal of NO₃ was observed in both bioreactor chambers in the first two years of monitoring (2019-2020) and in the third year of monitoring in chamber A, with significant (p < 0.01) reduction of the NO₃-N each year in both chambers with 8.6-11.4 mg NO₃-N L^-1 removed on average. Based on the NO₃ removal observed, spatial monitoring of sulfate (SO₄), dissolved methane (CH₄), and dissolved nitrous oxide (N₂O) gases was added in the third year of monitoring (2021). In 2021, chambers A and B had median hydraulic residence times (HRTs) of 64 h and 39 h, respectively, due to varying elevations of the chambers, with drought conditions making the differences more pronounced. In 2021, significant production of dissolved CH₄ was observed at rates of 0.54 g CH₄-C m^-3 d^-1 and 0.07 g CH₄-C m^-3 d^-1 in chambers A and B, respectively. In chamber A, significant removal (p < 0.01) of SO₄ (0.23 g SO₄ m^-3 d^-1) and dissolved N₂O (0.21 mg N₂O-N m^-2 d^-1) were observed, whereas chamber B produced N₂O (0.36 mg N₂O-N m^-2 d^-1). Considering the carbon dioxide equivalents (CO₂e) on an annual basis, chamber A had loads (~12,000 kg CO₂e ha^-1 y^-1) greater than comparable poorly drained agricultural soils; however, the landscape-scale impact was small (<1 % change in CO₂e) when expressed over the drainage area treated by the bioreactor. Under low-flow conditions, pollution swapping in woodchip bioreactors can be reduced at HRTs <50 h and NO₃ concentrations >2 mg N L^-1.

Amending woodchip bioreactors with corncobs reduces nitrogen removal cost

Woodchip denitrification bioreactors are an effective agricultural practice to reduce nitrogen (N) export from subsurface drainage via the conversion of nitrate (NO₃^-) to nitrogen gas (N₂), but there are challenges associated with limited woodchip supplies and increasing prices. Previous lab studies indicate that corncobs could be a promising woodchip alternative from the perspectives of N removal rate and cost. This field study aimed to provide early performance and cost assessment of denitrification bioreactors with two woodchip-corncob treatments. The objectives were to i) compare N removal rates of bioreactors with different carbon and hydraulic retention time (HRT) treatments, ii) compare bioreactor N removal costs, and iii) conduct sensitivity analysis on full-scale bioreactors (FBR) N removal costs with varying corncob lifespans and prices. Nine replicated field pilot-scale bioreactors (PBRs) using three carbon treatments and three HRTs were assessed for N removal efficiency. The carbon treatments were woodchip-only (WC100), 25% (by vol.) corncobs + 75% woodchips media (CC25) in series, and 75% corncobs + 25% woodchips (CC75) in series set at HRTs of 2, 8, and 16 h. N concentrations were monitored at each PBR inlet and outlet, and the PBR N removal efficiencies were used to estimate FBR N removal rates and costs. At respective HRTs, the estimated N removal rates of CC75 were 1.6- to 10.1-fold higher than WC100, but CC25 exhibited 0.9-fold lower (at 8-hr HRT) to 2.8-fold higher than WC100. A 15-yr cost assessment indicated CC75 ($10.56 to $13.89 kg^-1 N) was the most cost-efficient treatment, followed by WC100 ($13.30 to $88.11 kg^-1 N) and CC25 ($22.41 to $60.13 kg^-1 N). This assessment showed CC75 as a promising alternative to WC100 in terms of N removal rate and cost, but CC25 did not provide sufficient N removal rate increase for it to be a cost-efficient option. Nonetheless, using corncobs as a bioreactor medium is a relatively new approach, and we encourage more field studies to explore the long-term opportunities and challenges.

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.

Stacked conservation practices reduce nitrogen loss: A paired watershed study

Combinations of best management practices (BMPs) are needed to achieve nutrient reduction goals in the Mississippi/Atchafalaya River Basin (MARB), but field results are crucial to encourage stacked adoption of BMPs. A paired catchment-scale study (2015-18) was done to assess the impact of (i) BMPs, (ii) precipitation patterns, and (iii) seasonality on nitrogen (N) export. Flow-weighted samples were collected and analyzed for total ammonia nitrogen (TAN), nitrate (NO₃-N), and total nitrogen (TN). Catchments Low-BMP 11 and High-BMP 12 had 27.6% and 87.6% areal coverage of BMPs, respectively. No significant difference (p > 0.05) in TAN concentrations was found between Low-BMP 11 (0.023 mg L^-1) and High-BMP 12 (0.020 mg L^-1). However, NO₃-N and TN concentrations were significantly higher (p < 0.05) at Low-BMP 11 (NO₃-N: 26.0 mg L^-1, TN: 28.7 mg L^-1) than at High-BMP 12 (NO₃-N: 8.8 mg L^-1, TN: 9.2 mg L^-1). Two precipitation factors that affected N export patterns were observed. First, N flushing could continue for several years after a drought as elevated NO₃-N concentrations were observed in 2015 (i.e., two years after the 2011-2013 drought). Second, higher annual N export was observed when more precipitation occurred during the pre-planting or early-growing season versus later periods. For both catchments, the highest 50% of flows were responsible for majority of the NO₃-N export. We estimated that 33-37%, 61-62%, and 82-85% of the NO₃-N loads occurred in the 90th, 75th, and 50th flow percentiles, respectively. As demonstrated in High-BMP 12, stacked BMP application effectively lowered NO₃-N and TN loads by 60.3% and 59.1%, respectively, relative to Low-BMP 11. Although 27.6% BMP coverage area in Low-BMP 11 was considered low for this study, this coverage area is higher than many other parts of the MARB. This research highlights the importance of joint efforts between landowners in a watershed to meet downstream water quality goals.

Denitrifying bioreactor microbiome: Understanding pollution swapping and potential for improved performance

Denitrifying woodchip bioreactors are a best management practice to reduce nitrate-nitrogen (NO₃ -N) loading to surface waters from agricultural subsurface drainage. Their effectiveness has been proven in many studies, although variable results with respect to performance indicators have been observed. This paper serves the purpose of synthesizing the current state of the science in terms of the microbial community, its impact on the consistency of bioreactor performance, and its role in the production of potential harmful by-products including greenhouse gases, sulfate reduction, and methylmercury. Microbial processes other than denitrification have been observed in these bioreactor systems, including dissimilatory nitrate reduction to ammonia (DNRA) and anaerobic ammonium oxidation (anammox). Specific gene targets for denitrification, DNRA, anammox, and the production of harmful by-products are identified from bioreactor studies and other environmentally relevant systems for application in bioreactor studies. Lastly, cellulose depletion has been observed over time via increasing ligno-cellulose indices, therefore, the microbial metabolism of cellulose is an important function for bioreactor performance and management. Future work should draw from the knowledge of soil and wetland ecology to inform the study of bioreactor microbiomes.

Nutrient capture in an Iowa farm pond: Insights from high-frequency observations

Shallow constructed ponds are abundant landscape features in the midwestern United States, suggested as an edge of field best management practice (BMP) in voluntary nutrient reduction strategies. The efficacy of such features is highly uncertain, however, and previous studies have lacked sufficient temporal resolution to determine N and P removals during critical periods of transport. We utilized high-frequency in-situ measurements and flow-weighted grab sampling to determine water and nutrient budgets for a typical constructed "farm pond" in central Iowa situated within the Iowa Southern Drift Plain. Our monitoring approach yielded insight into in-stream nitrogen processing and the relative importance of transport-vs. supply-limited N delivery. Diel patterns in NO₃-N observed during early Spring, prior to canopy closure, revealed that in-stream primary production and NO₃-N assimilation can influence downstream N delivery in a stream with nitrate pollution (mean annual NO₃-N of nearly 5 mg/L). Analysis of discharge-concentration hysteresis for NO₃-N showed a shift from transport to supply limitation for NO₃-N delivery over the growing season, influenced by antecedent moisture, with wet antecedent conditions leading to supply limitation. Significant NO₃-N removal (64% of 19.8 kg/ha inputs) occurred within the 4.2 ha pond (230 ha watershed), but total N removal was much lower (36% removal of 22.3 kg/ha inputs). The lower total N removal highlights the importance of both particulate N and dissolved organic N and ammonia export to the N budgets of hypereutrophic small ponds. Total P removal in the pond was only 8% of 2.3 kg/ha inputs, likely due to internal loading of recent and legacy sedimentary P within the pond. High-flow events dominated N and P inputs, during which removal efficacy of the pond was significantly diminished. Poor process performance during critical moments may partially explain lower than expected water quality improvements post-BMP implementation. Accordingly, shifting hydroclimatic regimes (e.g., frequency of intense rainfall events) will impact the efficacy of small ponds and other edge of field BMPs for nutrient reduction.

A Multiscale Approach to Timescale Analysis: Isolating Diel Signals from Solute Concentration Time Series

Solute concentration time series reflect hydrological and biological drivers through various frequencies, phases, and amplitudes of change. Untangling these signals facilitates the understanding of dynamic ecosystem conditions and transient water quality issues. A case in point is the inference of biogeochemical processes from diel solute concentration variations. This analysis requires approaches capable of isolating subtle diel signals from background variability at other scales. Conventional time series analyses typically assume stationary or deterministic background variability; however, most rivers do not respect such niceties. We developed a time-series filtering method that uses empirical mode decomposition to decompose a measured solute concentration time series into intrinsic mode frequencies. Based on externally supplied mechanistic knowledge, we then filter these modes by periodicity, phase, and coherence with neighboring days. This method is tested on three synthetic series that incorporate environmental variability and sensor noise and on a year of 15 min sampled concentration time series from three hydrologically and ecologically distinct rivers in the eastern United States. The proposed method successfully isolated signals in the measured data sets that corresponded with variability in gross primary productivity. The strength the diel signal isolated through this method was smaller compared to the true signal in the synthetic series; however, uncertainty analysis showed that the process-model-based estimates derived from these signals were similar to other inference methods. This signal decomposition method retains information that can be used for further process modeling while making different assumptions about the data than Fourier and wavelet analyses.

Impact of stacked conservation practices on phosphorus and sediment export at the catchment scale

Best management practices (BMPs) are effective in reducing nutrient and sediment export, but further understanding of the benefits of the stacked BMPs is needed. This catchment-scale study was established to evaluate the impact of hydrology and BMPs on phosphorus (P) and sediment losses. Two adjacent catchments, one with a lower level of BMP adoption (Low-BMP #11) and one with a higher level (High-BMP #12), were compared for total P (TP) and total suspended solids (TSS) export. The BMPs include nutrient management plans, reduced tillage, grassed waterways, terraces, and perennial vegetation. The TP event-flow-weighted (EFW) concentration was significantly higher at Low-BMP #11 (0.293 mg L^-1) than at High-BMP #12 (0.069 mg L^-1). There was no significant difference in TP base-flow-weighted (BFW) concentrations between Low-BMP #11 (0.035 mg L^-1) and High-BMP #12 (0.037 mg L^-1). The TSS-EFW (148.0 vs. 18.6 mg L^-1) and TSS-BFW (13.3 vs. 6.9 mg L^-1) concentrations were also higher at Low-BMP #11 than at High-BMP #12. High-BMP #12 had lower TP (0.36 vs. 0.59 kg ha^-1 yr^-1) and TSS (253 vs. 1,961 kg ha^-1 yr^-1) loading than Low-BMP #11. The lower TP export at High-BMP #12 was likely attributed to the effectiveness of stacked erosion control BMPs and nutrient management plans. Overall, lower P and sediment loading was observed when a greater areal extent of stacked practices was implemented at the catchment level. This finding provides vital information to encourage wider BMP adoption at the watershed scale.

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.

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.

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.

Metagenomic analysis of nitrogen-cycling genes in upper Mississippi river sediment with mussel assemblages

We investigated the impact of native freshwater mussel assemblages (order Unionoida) on the abundance and composition of nitrogen‐cycling genes in sediment of an upper Mississippi river habitat. We hypothesized that the genomic potential for ammonia and nitrite oxidation would be greater in the sediment with mussel assemblages, presumably due to mussel biodeposition products, namely ammonia and organic carbon. Regardless of the presence of mussels, upper Mississippi river sediment microbial communities had the largest genomic potential for nitrogen fixation followed by urea catabolism, nitrate metabolism, and nitrate assimilation, as evidenced by analysis of nitrogen cycling pathway abundances. However, genes encoding nitrate and nitrite redox reactions, narGHI and nxrAB, were the most abundant functional genes of the nitrogen cycling gene families. Using linear discriminant analysis (LDA), we found nitrification genes were the most important biomarkers for nitrogen cycling genomic potential when mussels were present, and this presented an opposing effect on the abundance of genes encoding nitric oxide reduction. The genes involved in nitrification that increased the most were amoA associated with comammox Nitrospira and nxr homologs associated with Nitrospira. On the other hand, the most distinctive biomarkers of microbial communities without mussels were norB and nrfA, as part of denitrification and dissimilatory nitrate reduction to ammonium pathways, respectively. Ultimately, this research demonstrates the impact of native mollusks on microbial nitrogen cycling in an aquatic agroecosystem.

Performance of an under-loaded denitrifying bioreactor with biochar amendment

Denitrifying bioreactors are recently-established agricultural best management practices with growing acceptance in the US Midwest but less studied in other agriculturally significant regions, such as the US Mid-Atlantic. A bioreactor was installed in the Virginia Coastal Plain to evaluate performance in this geographically novel region facing challenges managing nutrient pollution. The 25.3 m³ woodchip bed amended with 10% biochar (v/v) intercepted subsurface drainage from 6.5 ha cultivated in soy. Influent and effluent nitrate-nitrogen (NO₃-N) and total phosphorus (TP) concentrations and flowrate were monitored intensively during the second year of operation. Bed surface fluxes of greenhouse gases (GHGs) nitrous oxide (N₂O), methane (CH₄), and carbon dioxide (CO₂) were measured periodically with the closed dynamic chamber technique. The bioreactor did not have a statistically or environmentally significant effect on TP export. Cumulative NO₃-N removal efficiency (9.5%) and average removal rate (0.56 ± 0.25 g m^-3 d^-1) were low relative to Midwest tile bioreactors, but comparable to installations in the Maryland Coastal Plain. Underperformance was attributed mainly to low NO₃-N loading (mean 9.4 ± 4.4 kg ha^-1 yr^-1), although intermittent flow, periods of low HRT, and low pH (mean 5.3) also likely contributed. N removal rates were correlated with influent NO₃-N concentration and temperature, but decreased with hydraulic residence time, indicating that removal was often N-limited. GHG emissions were similar to other bioreactors and constructed wetlands and not considered environmentally concerning. This study suggests that expectations of NO₃-N removal efficiency developed from bioreactors receiving moderate to high NO₃-N loading with influent concentrations exceeding 10-20 mg L^-1 are unlikely to be met by systems where N-limitation becomes significant.

Nitrate uptake in an agricultural stream estimated from high-frequency, in-situ sensors

Real-time, continuous, in situ water quality sensors were deployed on a fourth-order Iowa (U.S.) stream draining an agricultural watershed to evaluate key in-stream processes affecting concentrations of nitrate during a 24-day late summer (Aug-Sep) period. Overall, nitrate-nitrogen (NO₃-N) concentrations declined 0.11 mg L^-1 km^-1, or about 1.9% km^-1 and 35% in total across 18 km. We also calculated stream metabolic rates using in situ dissolved oxygen data and determined stream biotic N demand to be 108-117 mg m^-2 day^-1. From this, we estimate that 11% of the NO₃-N concentration decline measured between two in-situ sensors separated by 2 km was a result of biotic NO₃-N demand, while groundwater NO₃-N data and estimates of groundwater flow contributions indicate that dilution was responsible for 53%. Because the concentration decline extends linearly across the entire 18 km of stream length, these processes seem consistent throughout the basin downstream of the most upstream sensor site. The nitrate-dissolved oxygen relationship between the two sites separated by 2 km, calculations of biotic NO₃-N demand, and diurnal variations in NO₃-N concentration all indicate that denitrification by anaerobes is removing less NO₃-N than that assimilated by aquatic organisms unable to fix nitrogen for their life processes, and thus the large majority of the NO₃-N entering this stream is not retained or removed, but rather transported downstream.

Estimation of tile drainage contribution to streamflow and nutrient loads at the watershed scale based on continuously monitored data

Nitrogen losses from artificially drained watersheds degrade water quality at local and regional scales. In this study, we used an end-member mixing analysis (EMMA) together with high temporal resolution water quality and streamflow data collected in the 122 km² Otter Creek watershed located in northeast Iowa. We estimated the contribution of three end-members (groundwater, tile drainage, and quick flow) to streamflow and nitrogen loads and tested several combinations of possible nitrate concentrations for the end-members. Results indicated that subsurface tile drainage is responsible for at least 50% of the watershed nitrogen load between April 15 and November 1, 2015. Tiles delivered up to 80% of the stream N load while providing only 15-43% of the streamflow, whereas quick flows only marginally contributed to N loading. Data collected offer guidance about areas of the watershed that should be targeted for nitrogen export mitigation strategies.

Effect of freshwater mussels on the vertical distribution of anaerobic ammonia oxidizers and other nitrogen-transforming microorganisms in upper Mississippi river sediment

Targeted qPCR and non-targeted amplicon sequencing of 16S rRNA genes within sediment layers identified the anaerobic ammonium oxidation (anammox) niche and characterized microbial community changes attributable to freshwater mussels. Anammox bacteria were normally distributed (Shapiro-Wilk normality test, W-statistic =0.954, p = 0.773) between 1 and 15 cm depth and were increased by a factor of 2.2 (p < 0.001) at 3 cm below the water-sediment interface when mussels were present. Amplicon sequencing of sediment at depths relevant to mussel burrowing (3 and 5 cm) showed that mussel presence reduced observed species richness (p = 0.005), Chao1 diversity (p = 0.005), and Shannon diversity (p < 0.001), with more pronounced decreases at 5 cm depth. A non-metric, multidimensional scaling model showed that intersample microbial species diversity varied as a function of mussel presence, indicating that sediment below mussels harbored distinct microbial communities. Mussel presence corresponded with a 4-fold decrease in a majority of operational taxonomic units (OTUs) classified in the phyla Gemmatimonadetes, Actinobacteria, Acidobacteria, Plantomycetes, Chloroflexi, Firmicutes, Crenarcheota, and Verrucomicrobia. 38 OTUs in the phylum Nitrospirae were differentially abundant (p < 0.001) with mussels, resulting in an overall increase from 25% to 35%. Nitrogen (N)-cycle OTUs significantly impacted by mussels belonged to anammmox genus Candidatus Brocadia, ammonium oxidizing bacteria family Nitrosomonadaceae, ammonium oxidizing archaea genus Candidatus Nitrososphaera, nitrite oxidizing bacteria in genus Nitrospira, and nitrate- and nitrite-dependent anaerobic methane oxidizing organisms in the archaeal family “ANME-2d” and bacterial phylum “NC10”, respectively. Nitrosomonadaceae (0.9-fold (p < 0.001)) increased with mussels, while NC10 (2.1-fold (p < 0.001)), ANME-2d (1.8-fold (p < 0.001)), and Candidatus Nitrososphaera (1.5-fold (p < 0.001)) decreased with mussels. Co-occurrence of 2-fold increases in Candidatus Brocadia and Nitrospira in shallow sediments suggests that mussels may enhance microbial niches at the interface of oxic–anoxic conditions, presumably through biodeposition and burrowing. Furthermore, it is likely that the niches of Candidatus Nitrososphaera and nitrite- and nitrate-dependent anaerobic methane oxidizers were suppressed by mussel biodeposition and sediment aeration, as these phylotypes require low ammonium concentrations and anoxic conditions, respectively. As far as we know, this is the first study to characterize freshwater mussel impacts on microbial diversity and the vertical distribution of N-cycle microorganisms in upper Mississippi river sediment. These findings advance our understanding of ecosystem services provided by mussels and their impact on aquatic biogeochemical N-cycling.

Alternative futures of dissolved inorganic nitrogen export from the Mississippi River Basin: influence of crop management, atmospheric deposition, and population growth

Nitrogen (N) export from the Mississippi River Basin contributes to seasonal hypoxia in the Gulf of Mexico (GOM). We explored monthly dissolved inorganic N (DIN) export to the GOM for a historical year (2002) and two future scenarios (year 2022) by linking macroeonomic energy, agriculture market, air quality, and agriculture land management models to a DIN export model. Future scenarios considered policies aimed at encouraging bioenergy crop production and reducing atmospheric N-emissions, as well as the effect of population growth and the states' infrastructure plans on sewage fluxes. Model-derived DIN export decreased by about 9% (from 279 to 254 kg N km-2 year-1) between 2002 and 2022 due to a 28% increase in area planted with corn, 24% improvement in crop N-recovery efficiency (NRE, to 0.52), 22% reduction in atmospheric N deposition, and 23% increase in sewage inputs. Changes in atmospheric and sewage inputs had a relatively small effect on DIN export and the effect of bioenergy crop production depended on nutrient management practices. Without improved NRE, increased production of corn would have increased DIN export by about 14% (to 289 kg N km-2 year-1) between 2002 and 2022. Model results suggest that meeting future crop demand while reducing the areal extent of hypoxia could require aggressive actions, such improving basin-level crop NRE to 0.62 or upgrading N-removal capabilities in waste water treatment plants beyond current plans. Tile-drained cropland could contribute up to half of DIN export; thus, practices that reduce N losses from tile drains could also have substantial benefit.

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.

Effect of variable annual precipitation and nutrient input on nitrogen and phosphorus transport from two Midwestern agricultural watersheds

Precipitation patterns and nutrient inputs affect transport of nitrate (NO3-N) and phosphorus (TP) from Midwest watersheds. Nutrient concentrations and yields from two subsurface-drained watersheds, the Little Cobb River (LCR) in southern Minnesota and the South Fork Iowa River (SFIR) in northern Iowa, were evaluated during 1996-2007 to document relative differences in timings and amounts of nutrients transported. Both watersheds are located in the prairie pothole region, but the SFIR exhibits a longer growing season and more livestock production. The SFIR yielded significantly more NO3-N than the LCR watershed (31.2 versus 21.3kgNO3-Nha(-1)y(-1)). The SFIR watershed also yielded more TP than the LCR watershed (1.13 versus 0.51kgTPha(-1)yr(-1)), despite greater TP concentrations in the LCR. About 65% of NO3-N and 50% of TP loads were transported during April-June, and <20% of the annual loads were transported later in the growing season from July-September. Monthly NO3-N and TP loads peaked in April from the LCR but peaked in June from the SFIR; this difference was attributed to greater snowmelt runoff in the LCR. The annual NO3-N yield increased with increasing annual runoff at a similar rate in both watersheds, but the LCR watershed yielded less annual NO3-N than the SFIR for a similar annual runoff. These two watersheds are within 150 km of one another and have similar dominant agricultural systems, but differences in climate and cropping inputs affected amounts and timing of nutrient transport.

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.

Woodchip Denitrification Bioreactors: Impact of Temperature and Hydraulic Retention Time on Nitrate Removal

Woodchip denitrification bioreactors, a relatively new technology for edge-of-field treatment of subsurface agricultural drainage water, have shown potential for nitrate removal. However, few studies have evaluated the performance of these reactors under varied controlled conditions including initial woodchip age and a range of hydraulic retention times (HRTs) and temperatures similar to the field. This study investigated (i) the release of total organic C (TOC) during reactor start up for fresh and weathered woodchips, (ii) nitrate (NO-N) removal at HRTs ranging from 2 to 24 h, (iii) nitrate removal at influent NO-N concentrations of 10, 30, and 50 mg L, and (iv) NO-N removal at 10, 15, and 20°C. Greater TOC was released during bioreactor operation with fresh woodchips, whereas organic C release was low when the columns were packed with naturally weathered woodchips. Nitrate-N concentration reductions increased from 8 to 55% as HRT increased. Nitrate removal on a mass basis (g NO-N m d) did not follow the same trend, with relatively consistent mass removal measured as HRT increased from 1.7 to 21.2 h. Comparison of mean NO-N load reduction for various influent NO-N concentrations showed lower reduction at an influent concentration of 10 mg L and higher NO-N reductions at influent concentrations of 30 and 50 mg L. Nitrate-N removal showed a stepped increase with temperature. Temperature coefficient () factors calculated from NO-N removal rates ranged from 2.2 to 2.9.

Use Alkalinity Monitoring to Optimize Bioreactor Performance

In recent years, the agricultural community has reduced flow of nitrogen from farmed landscapes to stream networks through the use of woodchip denitrification bioreactors. Although deployment of this practice is becoming more common to treat high-nitrate water from agricultural drainage pipes, information about bioreactor management strategies is sparse. This study focuses on the use of water monitoring, and especially the use of alkalinity monitoring, in five Iowa woodchip bioreactors to provide insights into and to help manage bioreactor chemistry in ways that will produce desirable outcomes. Results reported here for the five bioreactors show average annual nitrate load reductions between 50 and 80%, which is acceptable according to established practice standards. Alkalinity data, however, imply that nitrous oxide formation may have regularly occurred in at least three of the bioreactors that are considered to be closed systems. Nitrous oxide measurements of influent and effluent water provide evidence that alkalinity may be an important indicator of bioreactor performance. Bioreactor chemistry can be managed by manipulation of water throughput in ways that produce adequate nitrate removal while preventing undesirable side effects. We conclude that (i) water should be retained for longer periods of time in bioreactors where nitrous oxide formation is indicated, (ii) measuring only nitrate and sulfate concentrations is insufficient for proper bioreactor operation, and (iii) alkalinity monitoring should be implemented into protocols for bioreactor management.

How efficiently do corn- and soybean-based cropping systems use water? A systems modeling analysis

Agricultural systems are being challenged to decrease water use and increase production while climate becomes more variable and the world's population grows. Low water use efficiency is traditionally characterized by high water use relative to low grain production and usually occurs under dry conditions. However, when a cropping system fails to take advantage of available water during wet conditions, this is also an inefficiency and is often detrimental to the environment. Here, we provide a systems-level definition of water use efficiency (sWUE) that addresses both production and environmental quality goals through incorporating all major system water losses (evapotranspiration, drainage, and runoff). We extensively calibrated and tested the Agricultural Production Systems sIMulator (APSIM) using 6 years of continuous crop and soil measurements in corn- and soybean-based cropping systems in central Iowa, USA. We then used the model to determine water use, loss, and grain production in each system and calculated sWUE in years that experienced drought, flood, or historically average precipitation. Systems water use efficiency was found to be greatest during years with average precipitation. Simulation analysis using 28 years of historical precipitation data, plus the same dataset with ± 15% variation in daily precipitation, showed that in this region, 430 mm of seasonal (planting to harvesting) rainfall resulted in the optimum sWUE for corn, and 317 mm for soybean. Above these precipitation levels, the corn and soybean yields did not increase further, but the water loss from the system via runoff and drainage increased substantially, leading to a high likelihood of soil, nutrient, and pesticide movement from the field to waterways. As the Midwestern United States is predicted to experience more frequent drought and flood, inefficiency of cropping systems water use will also increase. This work provides a framework to concurrently evaluate production and environmental performance of cropping systems.