Assessing nitrogen (N) mass balance and use efficiency is essential for tracking progress toward sustainable agriculture. However, the links between N surpluses or deficits, crop productivity, and environmental N losses remain poorly understood, particularly in rotational cropping systems. In this study, we combined natural abundance N isotope measurements across soil, water, and crop N pools with laboratory denitrification experiments to quantify N source-sink strengths in a tile-drained corn-soybean field in Illinois, USA. These isotope data were incorporated into a dual mass and isotope balance model to constrain the relationships among denitrification, soybean biological N fixation (BNF), and changes in soil organic N (SON), and to link partial and complete N balances. We found that N isotopes exhibited distinct distributions across the soil–water-crop continuum that were sensitive to crop rotation but unresponsive to split fertilizer application. Across a range of plausible scenarios, estimated denitrification losses were comparable in magnitude to nitrate leaching losses. Over the corn-soybean rotation, maintaining a stable SON stock requires soybean BNF rates sufficient to offset substantial N removal in soybean grain. Relative to corn residue, soybean residue appears to contribute little to SON formation, and its rapid decomposition can promote environmental N losses during periods of low crop N demand. Overall, this study demonstrates that integrating isotopic evidence with mass balance approaches can reveal the processes regulating N cycling in the soil–water-crop system and constrain agroecosystem responses to crop rotation and management practices.

Abstract Linking hydrograph dynamics with transport velocities and biogeochemical responses remains a central challenge in watershed hydrology, yet it is critical for understanding nitrogen reactive transport and its implications for water quality. In this study, we combined high‐frequency measurements of stable water isotopes and nitrate concentration with a StorAge Selection (SAS) function‐based flow‐transport model to simultaneously quantify storage dynamics, flow path activation, water mixing patterns, and nitrate export regimes in a tile‐drained, mesoscale lowland watershed. The high‐frequency tracer measurements revealed characteristic variability across event to seasonal timescales, reflecting complex hydrologic and biogeochemical responses to changing wetness conditions. Incorporation of these observations into the flow‐transport model identified root zone soils as a critical hydrologic filter that produces threshold‐like flow responses and delivers very young recharge to groundwater. The model also revealed a nested inverse storage effect in streamflow age, likely driven by the progressive activation of preferential flow networks facilitated by tile drainage. Event‐scale nitrate concentrations and export regimes were strongly correlated with stream water age. Decomposing stream nitrate concentrations into flow and water age endmembers further demonstrated substantial variability in nitrate in both young and old water, and showed that their mixing during flow generation underlies the observed range of nitrate export behavior. Building on these insights, we propose a conceptual model in which dynamic expansion and contraction of subsurface source zones govern coupled flow, age, and nitrate export responses. This framework provides new implications for constraining water transit times and nitrogen reactive transport in tile‐drained lowland agricultural watersheds.

Quantifying the carbon (C) uptake of Miscanthus x giganteus (M x g) in both aboveground and belowground structures (e.g., net primary productivity (NPP)) and differences among methodological approaches is crucial. Our objectives were to directly measure Mxg NPP and evaluate the effects of nitrogen application, location, and belowground biomass sampling methods. We hypothesize that increased nitrogen application increases the overall NPP of M x g and that quantifying rhizome biomass using excavations will produce the lowest variability between replicates. We collected biomass from mature M x g stands from three locations in Iowa with three nitrogen application rates and one site in Illinois. We destructively sampled at two time points, when rhizome mass is anticipated to be at a minimum (initial) and anticipated to be at its maximum (peak). Biomass was collected from 1 x 1 m quadrats in which one in-clump and one beside-clump cores were collected and then excavated to 30 cm depth to extract all rhizomes. We found that aboveground M x g NPP ranged from 15.4 Mg Da ha-1 year-1 to 36.4 Mg Da ha-1 year-1 and belowground M x g NPP ranged from 4.4 Mg Da ha-1 year-1 to 19.6 Mg Da ha-1 year-1. M x g NPP varied across sites, fertilization, and calculation assumptions. Aboveground NPP (yield) was on average 68.7% of the total NPP. Root-to-shoot ratios at peak biomass decreased with nitrogen application rate, from an average of 1.9 for 0 N plots to 0.89 for 224 N fertilized plots. There was more variation in core data than from excavations; however, when in-clump and beside-clump cores were averaged together, core and excavation averages were not different. Overall, these results show that the range of mature M x g NPP is driven by aboveground productivity, influenced by nitrogen application and site. Our results provide useful data to constrain agro-ecosystem models and provide crucial insights for future perennial belowground sampling.

Artificial drainage via subterranean drainage pipes called tiles transport nitrate from fields to streams, impairing downstream surface waters. Increased crop rotational diversity may conserve nitrogen, support greater system resiliency, and reduce tile nitrate loss. This 9-year study compares tile nitrate loss under a 3-year rotation of corn-soybean-winter wheat/double crop soybean plus cereal rye after corn versus a conventionally managed 2-year corn-soybean rotation. Tile flow and tile nitrate concentration were measured from 2015 through 2023 to determine annual flow weighted mean concentrations and loads of tile nitrate. Cereal rye following corn greatly reduced annual tile nitrate; however, more than 6 Mg ha−1 of above ground biomass may have decreased soybean yield. Winter wheat was efficient at using split-applied fertilizer nitrogen (N) as annual tile nitrate concentrations were reduced similar to cereal rye. Double crop soybean after winter wheat may be the most productive phase of the rotation; however, an early killing freeze greatly reduced soybean yield and increased tile nitrate loss. Interestingly, the subsequent corn crop may have benefitted from this unexpected N source. Overall, tile nitrate loss was approximately 50% less in the diverse 3-year rotational system, suggesting that a more diverse rotational system is efficient at retaining N, conserving nitrogen in the field, and reducing N loss to surface water. This study demonstrated proof of concept that a more diverse crop rotation can reduce tile nitrate loss from fields to streams, while maintaining crop productivity.

The combination of high nitrogen (N) inputs on tile-drained agricultural watersheds contributes to excessive nitrate (NO3 -) loss to surface- and groundwater systems. This study combined water age modeling based on StorAge Selection functions and NO3 - isotopic analysis to examine the underlying mechanisms driving NO3 - export in an intensively tile-drained mesoscale watershed typical of the U.S. Upper Midwest. The water age modeling revealed a pronounced inverse storage effect and strong young water preference under high-flow conditions, emphasizing evolving water mixing behavior driven by groundwater fluctuation and tile drain activation. Integrating NO3 - concentration-isotope-discharge relationships with water age dynamics disentangled the interactions between flow path variations and subsurface N cycling in shaping seasonally variable NO3 - export regimes at the watershed scale. Based on these results, a simple transit time-based and isotope-aided NO3 - transport model was developed to estimate the timescales of watershed-scale NO3 - reactive transport. Model results demonstrated variable NO3 - source availability and a wetness dependence for denitrification, indicating that interannual NO3 - chemostasis is driven by coupled and proportional responses of soil NO3 - production, denitrification, and flow path activation to varying antecedent wetness conditions. These findings suggest that intensively tile-drained Midwestern agricultural watersheds function as both N transporters and transformers and may respond to large-scale mitigation efforts within a relatively short timeframe. Collectively, the results of this study demonstrate the potential of integrated water age modeling and NO3 - isotopic analysis to advance the understanding of macroscale principles governing coupled watershed hydrologic and N biogeochemical functions.

Installation of subsurface drainage systems has profoundly altered the nitrogen cycle in agricultural regions across the globe, facilitating substantial loss of nitrate (NO3-) to surface water systems. Lack of understanding of the sources and processes controlling NO3- loss from tile-drained agroecosystems hinders the development of management strategies aimed at reducing this loss. The natural abundance nitrogen and oxygen isotopes of NO3- provide a valuable tool for differentiating nitrogen sources and tracking the biogeochemical transformations acting on NO3-. This study combined multi-years of tile drainage measurements with NO3- isotopic analysis to examine NO3- source and transport mechanisms in a tile-drained corn-soybean field. The tile drainage NO3- isotope data were supplemented by characterization of the nitrogen isotopic composition of potential NO3- sources (fertilizer, soil nitrogen, and crop biomass) in the field and the oxygen isotopic composition of NO3- produced by nitrification in soil incubations. The results show that NO3- isotopes in tile drainage were highly responsive to tile discharge variation and fertilizer input. After accounting for isotopic fractionations during nitrification and denitrification, the isotopic signature of tile drainage NO3- was temporally stable and similar to those of fertilizer and soybean residue during unfertilized periods. This temporal invariance in NO3- isotopic signature indicates a nitrogen legacy effect, possibly resulting from N recycling at the soil microsite scale and a large water storage for NO3- mixing. Collectively, these results demonstrate how combining field NO3- isotope data with knowledge of isotopic fractionations can reveal mechanisms controlling NO3- cycling and transport under complex field conditions.

Splitting fertilizer nitrogen (N) applications and using cover crops are management strategies to reduce nitrate in tile drainage water. We investigated split fertilizer N applications to corn (Zea mays L.) on crop yields and tile nitrate loss in both corn and soybean (Glycine max L.) in rotation from 2016 through 2019. We evaluated the inclusion of cover crops in a split-N treatment. Fertilizer N treatments included 100% in the fall; 50% in the fall + 25% at planting + 25% at side-dress; 100% as spring preplant; 75% as spring preplant (reduced N rate); 50% as spring preplant + 50% at side-dress; and 50% as spring preplant + 50% at side-dress with a cover crop. We did not find significant differences between split and single full rate N application treatments for corn yields or tile nitrate loss; however, the reduced N rate treatment significantly decreased corn yield by 10%. Cumulative tile nitrate losses (over four seasons) ranged from 115 kg ha-1 for all of the N in the fall to 65 kg ha-1 for 50% as spring preplant + 50% at side-dress with a cover crop, a decrease of 43%. Tile nitrate loss responded similarly to (corn) N treatments under both corn and soybean, with 64% of the loss under corn and 36% under soybean. Our results suggest that decreasing the fertilizer N rate may impact corn yield more than nitrate loss, while split fertilizer N application with a cover crop has potential to reduce tile nitrate loss without decreasing crop yield.

This repository contains the SAS model input data and the model results that can be used to reproduce the water age results for the three study tiles presented in Yu et al. Linking water age, nitrate export regime, and nitrate isotope biogeochemistry in a tile-drained agricultural field File 1: SAS_model_input_TileX.csv The input data for the SAS model calibration for the three tiles. File 2: behavioral_parameter_sets_Model#1_TileX.csv The behavioral parameter sets obtained from the calibration of SAS model 1 (i.e., time-invariant kQ). File 3: behavioral_parameter_sets_Model#2_TileX.csv The behavioral parameter sets obtained from the calibration of SAS model 2 (i.e., time-variant kQ). File 4: Cl_simulation_results_Model#1_TileX.csv Simulated chloride concentration based on the optimal parameter set of SAS model 1 (i.e., time-invariant kQ). File 5: Cl_simulation_results_Model#2_TileX.csv Simulated chloride concentration based on the optimal parameter set of SAS model 2 (i.e., time-variant kQ). File 6: median_water_age_Model#1_TileX.csv Median water age of tile discharge based on the optimal parameter set of SAS model 1 (i.e., time-invariant kQ). File 7: median_water_age_Model#2_TileX.csv Median water age of tile discharge based on the optimal parameter set of SAS model 2 (i.e., time-variant kQ). File 8: File_column_names.txt A text file that explains the column names for each file

Recent theoretical advances related to time-variant water age in hydrologic systems have opened the door to a new method that probes water mixing and selection behaviors using StorAge Selection (SAS) functions. In this study, SAS functions were applied to investigate storage, water mixing behaviors, and nitrate (NO3-) export regimes in a tile-drained corn-soybean rotation field in the Midwestern United States. The natural abundance stable nitrogen and oxygen isotopes of tile drainage NO3- were also measured to provide constraints on biogeochemical NO3- transformations. The SAS functions calibrated using chloride measurements at tile drain outlets revealed a strong young water preference during tile discharge generation. The use of a time-variant SAS function for tile discharge generated unique water age dynamics that reveals an inverse storage effect driven by activation of preferential flow paths and mechanically explains the observed variations in NO3- isotopes. Combining the water age estimates with NO3- isotope fingerprinting delineated NO3- export dynamics at the tile-drain scale, where a lack of strong contrast in NO3- concentration across the soil profile results in chemostatic NO3- export regimes. For the first time, NO3- isotopes were embedded into a water age-based transport model to model reactive NO3- transport under transient conditions. Results from this modeling study provided a proof-of-concept for the potential of coupled water age modeling and NO3- isotope analysis in elucidating complex mechanisms that control the coupled water and NO3- transport. Further integration of water age theory and NO3- isotope biogeochemistry is expected to significantly improve reactive NO3- transport modeling.

Tile drainage waters carry considerable loads of phosphorus (P) from agricultural fields to rivers and streams in the Midwestern U.S. An innovative and economical approach to monitor dissolved reactive P (DRP) flux in tile waters is needed to understand the extent of P loss in field-scale. In this study, a passive sampling technique was developed using iron oxide-coated polyacrylic/polystyrene anion exchange resins (hybrid resins) a P sink. Laboratory batch adsorption isotherm and kinetic experiments indicated that the hybrid resins had high P adsorption capacity (7.69–19.84 mg/g) and high kinetic performance. The passive sampling method with field-calibrated hybrid polyacrylic resin and hybrid polystyrene resins (sampling rate: 0.1351 and 0.0763 L/h, respectively) predicted the average DRP concentrations of 0.006–0.020 mg/L, which did not differ significantly (p > 0.05) from the auto-sampling data. A rapid increase in DRP concentration during storm events and subsequent flooding events was also predicted well. In conclusion, a passive detection method using iron oxide coated hybrid resins can be recommended for monitoring seasonally fluctuating DRP flux in agricultural waters as long as the hybrid resins are well-calibrated under specific field conditions (e.g., flow rate and concentration range).

Sulfur received extensive study as an input to terrestrial ecosystems from acidic deposition during the 1980s. With declining S deposition inputs across the eastern United States, there have been many studies evaluating ecosystem response, with the exception of agricultural watersheds. We used long-term (22 and 18 yr) sulfate concentration data from two rivers and recent (6 yr) data from a third river to better understand cycling and transport of S in agricultural, tile-drained watersheds. Sulfate concentrations and yields steadily declined in the Embarras (from ∼10 to 6 mg S L) and Kaskaskia rivers (from 7 to 3.5 mg S L) during the sampling period, with an overall -23.1 and -12.8 kg S ha yr balance for the two watersheds. There was evidence of deep groundwater inputs of sulfate in the Salt Fork watershed, with a much smaller input to the Embarras and none to the Kaskaskia. Tiles in the watersheds had low sulfate concentrations (<10 mg S L), similar to the Kaskaskia River, unless the field had received some form of S fertilizer. A multiple regression model of runoff (cm) and S deposition explained much of the variation in Embarras River sulfate ( = 0.86 and 0.80 for concentrations and yields; = 46). Although atmospheric deposition was much less than outputs (grain harvest + stream export of sulfate), riverine transport of sulfate reflected the decline in inputs. Watershed S balances suggest a small annual depletion of soil organic S pools, and S fertilization will likely be needed at some future date to maintain crop yields.

Chloride is a relatively unreactive plant nutrient that has long been used as a biogeochemical tracer but also can be a pollutant causing aquatic biology impacts when concentrations are high, typically from rock salt applications used for deicing roads. Chloride inputs to watersheds are most often from atmospheric deposition, road salt, or agricultural fertilizer, although studies on agricultural watersheds with large fertilizer inputs are few. We used long-term (21 and 17 yr) chloride water quality data in two rivers of east-central Illinois to better understand chloride biogeochemistry in two agricultural watersheds (Embarras and Kaskaskia), the former with a larger urban land use and both with extensive tile drainage. During our sampling period, the average chloride concentration was 23.7 and 20.9 mg L in the Embarras and Kaskaskia Rivers, respectively. Annual fluxes of chloride were 72.5 and 61.2 kg ha yr in the Embarras and Kaskaskia watersheds, respectively. In both watersheds, fertilizer chloride was the dominant input (∼49 kg ha yr), with road salt likely the other major source (23.2 and 7.2 kg ha yr for the Embarras and Kaskaskia watersheds, respectively). Combining our monitoring data with earlier published data on the Embarras River showed an increase in chloride concentrations as potash use increased in Illinois during the 1960s and 1970s with a lag of about 2 to 6 yr to changes in potash inputs based on a multiple-regression model. In these agricultural watersheds, riverine chloride responds relatively quickly to potash fertilization as a result of tile-drainage.

The US Department of Energy has mandated the production of 16 billion gallons (60.6 billion liters) of renewable biofuel from cellulosic feedstocks by 2022. The perennial grass, Miscanthus × giganteus, is a potential candidate for cellulosic biofuel production because of high productivity with minimal inputs. This study determined the effect of three different spring fertilizer treatments (0, 60, and 120 kg N ha−1 yr−1 as urea) on biomass production, soil organic matter (SOM), and inorganic N leaching in Illinois, Kentucky, Nebraska, New Jersey, and Virginia, along with N2O and CO2 emissions at the IL site. There were no significant yield responses to fertilizer treatments, except at the IL site in 2012 (yields in 2012, year 4, varied from 10 to 23.7 Mg ha−1 across all sites). Potentially mineralizable N increased across all fertilizer treatments and sites in the 0–10 cm soil depth. An increase in permanganate oxidizable carbon (POX‐C, labile C) in surface soils occurred at the IL and NJ sites, which were regularly tilled before planting. Decreases in POX‐C were observed in the 0 – 10 cm soil depth at the KY and NE sites where highly managed turfgrass was grown prior to planting. Growing M. × giganteus altered SOM composition in only 4 years of production by increasing the amount of potentially mineralizable N at every site, regardless of fertilization amount. Nitrogen applications increased N leaching and N2O emission without increasing biomass production. This suggests that for the initial period (4 years) of M. × giganteus production, N application has a detrimental environmental impact without any yield benefits and thus should not be recommended. Further research is needed to define a time when N application to M. × giganteus results in increased biomass production.

Current biofuel feedstock crops such as corn lead to large environmental losses of N through nitrate leaching and NO emissions; second-generation cellulosic crops have the potential to reduce these N losses. We measured N losses and cycling in establishing miscanthus (), switchgrass ( L. fertilized with 56 kg N ha yr), and mixed prairie, along with a corn ( L.)-corn-soybean [ (L.) Merr.] rotation (corn fertilized at 168-202 kg N ha). Nitrous oxide emissions, soil N mineralization, mid-profile nitrate leaching, and tile flow and nitrate concentrations were measured. Perennial crops quickly reduced nitrate leaching at a 50-cm soil depth as well as concentrations and loads from the tile systems (year 1 tile nitrate concentrations of 10-15 mg N L declined significantly by year 4 in all perennial crops to <0.6 mg N L, with losses of <0.8 kg N ha yr). Nitrous oxide emissions were 2.2 to 7.7 kg N ha yr in the corn-corn-soybean rotation but were <1.0 kg N ha yr by year 4 in the perennial crops. Overall N balances (atmospheric deposition + fertilization + soybean N fixation - harvest, leaching losses, and NO emissions) were positive for corn and soybean (22 kg N ha yr) as well as switchgrass (9.7 kg N ha yr) but were -18 and -29 kg N ha yr for prairie and miscanthus, respectively. Our results demonstrate rapid tightening of the N cycle as perennial biofuel crops established on a rich Mollisol soil.

Perennial biofuel crops such as Miscanthus and switchgrass are thought to increase soil organic matter and therefore may increase soil nitrogen (N) mineralization rates. Our objective was to evaluate a range of N-mineralization indices for soils with established biofuel crops and compare these results with soils in a traditional corn and soybean rotation. We sampled surface soil (0–10 cm deep) from switchgrass (6 years after establishment) and Miscanthus plots (5 years) in a high-organic-matter Mollisol. The longest potential N mineralization index, a 24-day incubation, was significantly greater in Miscanthus soils compared to switchgrass and corn–soybean. In addition, 7-day anaerobic N and potassium chloride–extractable ammonium N were both greater in Miscanthus soils compared to switchgrass and corn–soybean. Our results do support our hypothesis that N-mineralization rates are greater in soils under biofuel production.

The extensive land use conversion expected to occur to meet demands for bioenergy feedstock production will likely have widespread impacts on agroecosystem biodiversity and ecosystem services, including carbon sequestration. Although arthropod detritivores are known to contribute to litter decomposition and thus energy flow and nutrient cycling in many plant communities, their importance in bioenergy feedstock communities has not yet been assessed. We undertook an experimental study quantifying rates of litter mass loss and nutrient cycling in the presence and absence of these organisms in three bioenergy feedstock crops-miscanthus (Miscanthus x giganteus), switchgrass (Panicum virgatum), and a planted prairie community. Overall arthropod abundance and litter decomposition rates were similar in all three communities. Despite effective reduction of arthropods in experimental plots via insecticide application, litter decomposition rates, inorganic nitrogen leaching, and carbon-nitrogen ratios did not differ significantly between control (with arthropods) and treatment (without arthropods) plots in any of the three community types. Our findings suggest that changes in arthropod faunal composition associated with widespread adoption of bioenergy feedstock crops may not be associated with profoundly altered arthropod-mediated litter decomposition and nutrient release.

Silicon (Si) concentrations in biofuel feedstock crops have a critical role in combustion processes. The purpose of this study was to quantify Si concentrations in plant biomass samples and to evaluate the contributing factors for Si concentrations. We determined total Si concentrations in Miscanthus x giganteus (M. x giganteus) collected from various research trial plots in the eastern U.S. and in Miscanthus spp. and Panicum virgatum, 'Cave-in-Rock' (switchgrass) from an additional eight trial plots established across Illinois. Whole aboveground plant biomass at each site were air-dried and ground. Total Si concentrations in plant samples were determined by dry-ashing plant tissue in a muffle furnace, followed by alkaline fusion and then colorimetric analysis. Average Si concentrations in statewide M. x giganteus plant samples ranged from 0.72% to 1.6% and samples from within Illinois ranged from 0.55% to 2.4%. The overall median value of concentrations in M. x giganteus samples among all sites was 1.08%. The median value in switchgrass samples (1.5%) was 1.4 times higher than that for M. x giganteus. Among six other Miscanthus spp. samples from the Urbana trial plot in Illinois, Si concentrations were about 1/3 that of M. x giganteus. Variation in Si concentrations tended to be associated with temperature and precipitation of the location where the biofuel crops are being grown. We did not find any relationship between soil type and plant Si concentrations. Long-term evaluations of soil mineral concentrations and additional environmental factors are required to better understand the contributing factors for Si concentrations. (C) 2011 Elsevier Ltd. All rights reserved.

Biomass crops are being promoted as environmentally favorable alternatives to fossil fuels or ethanol production from maize (Zea mays L.), particularly across the Corn Belt of the United States. However, there are few if any empirical studies on inorganic N leaching losses from perennial grasses that are harvested on an annual basis, nor has there been empirical evaluation of the hydrologic consequences of perennial cropping systems. Here we report on the results of 4 yr of field measurements of soil moisture and inorganic N leaching from a conventional maize-soybean [Glycine max (L.) Merr.] system and two unfertilized perennial grasses harvested in winter for biomass: Miscanthus x giganteus and switchgrass (Panicum virgatum cv. Cave-in-Rock). All crops were grown on fertile Mollisols in east-central Illinois. Inorganic N leaching was measured with ion exchange resin lysimeters placed 50 cm below the soil surface. Maize--soybean nitrate leaching averaged 40.4 kg N ha(-1) yr(-1), whereas switchgrass and Miscanthus had values of 1.4 and 3.0 kg N ha(-1) yr(-1), respectively. Soil moisture monitoring (to a depth of 90 cm) indicated that both perennial grasses dried the soil out earlier in the growing season compared with maize-soybean. Later in the growing season, soil moisture under switchgrass tended to be greater than maize-soybean or Miscanthus, whereas the soil under Miscanthus was consistently drier than under maize--soybean. Water budget calculations indicated that evapotranspiration from Miscanthus was about 104 mm yr(-1) greater than under maize-soybean, which could reduce annual drainage water flows by 32% in central Illinois. Drainage water is a primary source of surface water flows in the region, and the impact ofextensive Miscanthus production on surface water supplies and aquatic ecosystems deserves further investigation.

Little is known about how long-term biofuel production might alter soil nitrogen (N) gas emissions. We conducted a laboratory incubation of surface soils (0–16cm) from perennial biofuel trial plots (established 2002) at sites in Northern (Dekalb, Mollisols), Central (Urbana, Mollisols), and Southern (Dixon Springs, Alfisols) Illinois, USA. Soils from unfertilized plots of Miscanthus (Miscanthus×giganteus) and switchgrass (Panicum virgatum) were compared to fertilized corn–soybean plots during early spring and again in mid-summer. Fresh soils were packed into jars at a bulk density of 1.2gcm−3 and adjusted to a water-filled pore space of 85%. We added about 10mgNO3-Nkg−1 dry soil in each sample, incubated for 24h, and collected gas samples at 0, 1, 2, and 4h to measure production of N2O and N2 using a C2H2 inhibition technique, which allowed calculation of the N2O mole fraction (N2O:(N2O+N2)). The mean N2O mole fraction (MF) was significantly higher for the cropped plot (0.83 and 0.99) than that for Miscanthus (0.48 and 0.31) and switchgrass (0.45 and 0.22) plots at the Southern site in spring and summer, respectively. There were no significant differences in N2O MF among treatment plots for the Central and Northern sites. Exchangeable soil nitrate concentrations best explained the N2O MFs for all treatments in both seasons, and production of perennial biofuel feedstock crops did not exhibit an apparent influence on N2O MFs. It appeared that soil type combined with fertilizer additions were the major factors controlling the MF of N2O in our fields, and was much more important than the crop grown or any new soil C added.

Nitrate loss from drainage tiles across the cornbelt of the upper midwestern US is a result of intensive agriculture with limited crop diversity, extensive periods of fallow soil, and the need for high fertilizer applications to corn, all located on a hydrologically modified landscape. Two methods proposed to reduce tile nitrate export are managed or controlled drainage to limit tile flow and bioreactors to enhance denitrification. Nitrogen budgets and tile flow monitoring were conducted over two- to three-year periods between 2006 and 2009. We estimated N budgets in a seed corn-soybean rotation farming system near DeLand, east-central Illinois, USA, with free (FD) and controlled drainage (CD) patterned tile systems. In addition, wood chip filled trenches (bioreactors) were installed below the CD structures, one lined with plastic and one unlined. We measured daily tile flow and nitrate-N (NO3-N) concentrations and calculated cumulative N loss from the tile water at both FD and CD areas for a period of three cropping years. We also monitored the tile flow and nitrate concentration in inlet and outlet of the bioreactor associated with a CD system and evaluated the efficiency of the bioreactor for two cropping years. Most components of the N balance were unaffected by CD (yields and therefore N harvested, surface soil denitrification), and there was a negative N balance in the soybean cropping year (−165 and −163 kg N ha−1 at FD and CD areas, respectively), whereas seed corn cropping in the following year resulted in positive N balances (29 and 34 kg N ha−1 at FD and CD areas, respectively). For two years, the overall N balances were −136 and −129 kg N ha−1 at FD and CD areas, respectively, consistent with other recent corn belt studies showing a small net depletion of soil organic N. Controlled drainage greatly reduced tile N export, with a three-year average loss of 57.2 kg N ha−1 yr−1 from FD compared to 17 kg N ha−1 yr−1 for CD. There was high uncertainty in denitrification measurements and thus the fate of missing N in the CD system remained unknown. Nitrate reduction efficiency of the bioreactor varied greatly, with periods where nearly 100% of the nitrate was denitrified. The overall efficiency of the bioreactor associated with the CD system in reducing the tile N load was 33%. When nitrate was non-limiting, the nitrate removal rate of the bioreactor was 6.4 g N m−3 d−1. Little N2O emission was found from the bioreactor bed and is not thought to be a problem with these systems. Both the tile bioreactor and controlled drainage greatly reduced tile nitrate export in this leaky seed corn and soybean agricultural field.