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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&ndash, 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&ndash, 0.020 mg/L, which did not differ significantly (p &gt, 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).

Growing cover crops is one of the most promising conservation practices with multiple benefits. However, the impacts of cover crops on the productivity of the maize-soybean [Zea mays L. - Glycine max (L.) Merr.] rotation system in the U.S. Midwest still have large uncertainties based on results obtained from field experiments, specifically across different soil properties, climate conditions, and land management practices. Process-based models fully validated with data from field experiments across these diverse conditions provide an effective tool to quantify cover crop impacts on cash crop productivity and optimize the management of cover crops accordingly. In this study, we aim to answer the following questions: (1) What are the impacts of cover crops on cash crop yield in the U.S. Midwest agroecosystems? (2) What are the mechanistic pathways of cover crop to affect cash crop yield (e.g. through influencing soil water, nitrogen and O2 dynamics)? (3) What management practices can be used to mitigate the negative impacts on cash crop yield caused by cover crops? To address these questions, we calibrated and validated a sophisticated process-based agroecosystem model, ecosys, using field experimental data from 2013 to 2018 across Illinois, and then used ecosys to assess the impacts of winter cover crops on maize and soybean yield under different management practices. Our study revealed the following findings: (1) planting non-legume cover crops can cause 3.9 ± 3% yield reduction for maize and no significant impacts on soybean yield, while planting legume cover crops has no significant impact on the yield of either maize or soybean; (2) the maize yield reduction caused by planting cover crops can be mainly explained by nitrogen deficiency induced by increased immobilization, water competition in dry areas, cooler soil surface and oxygen competition; (3) later termination of non-legume cover crops before maize can result in larger maize yield loss due to intensive competition for resources (e.g. water and nitrogen), and the impacts of non-legume cover crops on maize yield reduction can be minimized by optimizing cover crop termination time in the spring. Overall, in the U.S. Midwestern maize-soybean rotation system, we found that although non-legume cover crops cause yield reduction for maize through resource competition, this yield reduction can be minimized through management practices, such as controlling termination time of cover crop and proper fertilizer management.

Biological nitrogen fixation (BNF) is a fundamental process relied on in agriculture, yet few field studies have examined regulation through soil inorganic N feedbacks or considered seasonal effects. In a Michigan long-term field experiment we examined soil labile C and N pools and impact on BNF in two species, over multiple years. The ¹⁵N natural abundance method was used to quantify BNF, with nodulated and non-nodulated soybean [Glycine max (L.) Merr.] isolines and red clover (Trifolium pratense L.). Soil organic C status of plots was consistent with a gradient established through historical inputs: 0.8% (fertilizer management) to 0.9% (+ cover crop) to 1.2% (+ compost + cover crop). The fraction of nitrogen derived from atmosphere (fNdfa) in red clover and soybean over 3 yr was 61 to 58% with compost management, and 71 to 72% with fertilizer. This represented a downregulation of 19% (red clover) and 15% (soybean) in compost and cover crop managed plots, relative to inorganic fertilizer. In addition to management effects, we found that weather markedly influenced the total amount of N fixation over the 3 yr of the study. A mesic season supported vigorous soybean growth and 195 kg N ha–¹ BNF, compared to 95 and 90 kg N ha–¹ BNF in a dry and an excessively wet season. This study found that compost-based management increased pools of labile C and N that internally downregulated BNF, while enhancing soybean yield compared to conventional management.

Drainage water management (DWM) is a potential edge-of-field technique that is being studied as a method to improve soil water management in agricultural fields, which may reduce nitrate losses to surface waters during the non-growing season. Inline water level control structures were installed on two adjacent tile systems draining a 34 ha field located in the Upper Salt Fork of the Vermillion River Watershed in central Illinois to evaluate DWM from 2011 through 2013. The overall objective of this study was to determine the effectiveness of DWM in reducing nitrate losses from fields in a corn and soybean production system in east-central Illinois, as well as to investigate the fate of the retained water. A paired watershed approach was used to determine the impact of DWM on tile flow and nitrate load compared to a control treatment or free drainage (FD) tile system. The entire 34 ha field was under a corn (Zea mays L.) and soybean (Glycine max L.) rotation with continuous no-till. During 2011 and 2012, DWM was able to greatly reduce tile flow compared to the FD tile system. However, based on runoff and nitrate yields from the entire field, there was no measureable reduction in nitrate loss and shallow ground-water wells showed little area of influence in the field (∼2 ha). Water from the DWM tile system flowed laterally to the nearby FD tile system, increasing flow and nitrate loss from the FD system. In 2013, when both tiles were under DWM, water was retained and the water table level was increased in a larger area of the field (∼6 ha). However, at the end of the experiment when the control stoplogs were lowered the retained water was discharged through the tiles lines with little apparent reduction (10%) in overall water and nitrate loss for the year. Measurements of tile and well nitrate concentrations suggested that nitrate was not denitrified in the shallow groundwater of the field during the three-year study. Nitrate losses were directly proportional to tile flow each year of the study. Retrofitting DWM on an existing tile system was found to have a limited water quality benefit.

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

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

Reducing nitrate loads from corn and soybean, tile-drained, agricultural production systems in the Upper Mississippi River basin is a major challenge that has not been met. We evaluated a range of possible management practices from biophysical and social science perspectives that could reduce nitrate losses from tile-drained fields in the Upper Salt Fork and Embarras River watersheds of east-central Illinois. Long-term water quality monitoring on these watersheds showed that nitrate losses averaged 30.6 and 23.0 kg nitrate N ha -1 yr -1 (Embarras and Upper Salt Fork watersheds, respectively), with maximum nitrate concentrations between 14 and 18 mg N L -1 . With a series of on-farm studies, we conducted tile monitoring to evaluate several possible nitrate reduction conservation practices. Fertilizer timing and cover crops reduced nitrate losses (30% reduction in a year with large nitrate losses), whereas drainage water management on one tile system demonstrated the problems with possible retrofit designs (water flowed laterally from the drainage water management tile to the free drainage system nearby). Tile woodchip bioreactors had good nitrate removal in 2012 (80% nitrate reduction), and wetlands had previously been shown to remove nitrate (45% reductions) in the Embarras watershed. Interviews and surveys indicated strong environmental concern and stewardship ethics among landowners and farmers, but the many financial and operational constraints that they operate under limited their willingness to adopt conservation practices that targeted nitrate reduction. Under the policy and production systems currently in place, large-scale reductions in nitrate losses from watersheds such as these in east-central Illinois will be difficult.

Published in Agron. J. 105:1658–1664 (2013) doi:10.2134/agronj2013.0089 Available freely online through the author-supported open access option. Copyright © 2013 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. T value of using cover crops includes reduced soil erosion, gains in soil organic matter, capture of unused fertilizer N, decreased soil compaction, and suppression of diseases and weeds when compared with bare soil or winter fallow (Snapp et al., 2005; Tonitto et al., 2006; Cherr et al., 2006). Before industrial manufacturing of inorganic fertilizers, the most common reason for using a legume cover crop was to enhance soil fertility, especially N (Dinnes et al., 2002). Many studies have attributed the benefit of a legume cover crop before corn to an increase in soil N levels following legume incorporation (Hesterman et al., 1986; Blevins et al., 1990; Torbert et al., 1996; Vyn et al., 1999; Sanchez et al., 2001); however, other non-N rotational benefits have been identified as well (McVay et al., 1989; Raimbault and Vyn, 1991; Corak et al., 1991). In total, the direct contribution of N from legume N2 fixation coupled with factors such as improved soil physical, chemical, and biological properties may synergistically produce the overall “rotational effect” (Gaudin et al., 2013); however, legume cover crops are generally managed as a N source (Tonitto et al., 2006; Liebman et al., 2012). Red clover, when frost-seeded into winter wheat (Triticum aestivum L.), has been shown to fix and accumulate large amounts of N in the aboveground biomass and has been evaluated as a green manure prior to corn production (Hesterman et al., 1992; Tiffin and Hesterman, 1998; Schipanski and Drinkwater, 2011; Gaudin et al., 2013). Due to the low C/N ratio of red clover tissues, N turnover is generally faster than for nonleguminous cover crops (Varco et al., 1989; Wagger, 1989); however, environmental and management factors can markedly influence decomposition dynamics, and it is difficult to accurately predict the amount of N that will become available, or when it will become available, to a subsequent crop (Crews and Peoples, 2005; McSwiney et al., 2010; Ruffo and Bollero, 2003). Snapp et al. (2005) determined that uncertainty regarding N mineralization and availability was one of the chief barriers to adoption of legume cover crops among surveyed Michigan farmers. Determining the N credit value of a cover crop is critical for calculating the N fertilizer application rate that maximizes profit and protects the environment (Andraski and Bundy, 2002; Stute and Posner, 1995; Vyn et al., 1999). Knowledge is limited, however, concerning longer term, decadal interactions of producing cover crops within a given cropping system. Long-term field crop experiments greatly improve our understanding of historical management effects on soil properties, including slow processes such as soil organic matter accumulation (Sanchez et al., 2004; Robertson et al., 2008). During the first decade of a row crop experiment in southwest Michigan, the presence of cover crops in a corn–soybean [Glycine max (L.) Merr.]–wheat rotation supported soil C increases of 10 to 40 g C m–2 annually (Robertson et al., 2000). Similarly, diversification with cover crops in a 14-yr organic corn–soybean–wheat rotation experiment in Pennsylvania enhanced soil C by 42 g C m–2 annually (Drinkwater et al., 1998). Although these studies demonstrate gains in soil C and improved soil fertility, the timing of nutrient release, particularly with regard to soil N supply, has been shown to be one of the primary yield-limiting factors for cash crop production in organic farming systems (Cavigelli et al., 2008). Soil fertility amendments (i.e., fertilizers, composted dairy manure, and green manures) are generally applied to enhance the growth and yield of the cash crop. Although rarely studied, it can be inferred that practices improving soil nutrient and organic matter status will, in turn, enhance cover crop performance. We hypothesized that agricultural systems that have ABSTRACT Corn (Zea mays L.) production systems can benefit from introducing a leguminous winter cover crop into the rotation, especially with regard to increased N availability (i.e., legume N credit); however, it is not known if the full agronomic benefit is realized in the first year of cover crop introduction or if the benefit is cumulative with time. The objective of this study was to determine the apparent red clover (Trifolium pratense L.) N credit to corn in a conventional system where red clover was introduced for the first time compared with three agricultural systems that had a 14-yr history of using cover crops. The apparent red clover N credit was calculated by the difference in unfertilized corn N accumulation between cover and no-cover split-split plots. These data suggest that corn growers can realize the full benefits of a red clover cover crop in the first year of introduction.

Published in Agron. J. 105:493–502 (2013) doi:10.2134/agronj2012.0382 Copyright © 2013 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. C system productivity and sustainability are highly reliant on soil organic matter dynamics, including the turnover of labile C and N, and the renewal of stabilized pools (Wander, 2004; Weil and Magdoff , 2004). Th ese dynamics operate on short (seasonal) and long (years to decades) time scales, and understanding these dynamics is essential in moving toward more biologically-based cropping systems. Although soil organic matter is an extremely important indicator of overall soil quality, it can be insensitive to new management practices, as changes in total organic matter can take years to detect (Wander and Drinkwater, 2000; Wander, 2004). Th e limitations of total organic matter as an indicator have led many researchers to focus on the labile pool of organic matter. Th is pool is small (typically

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 (

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.

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 −1 in the Embarras and Kaskaskia Rivers, respectively. Annual fluxes of chloride were 72.5 and 61.2 kg ha −1 yr −1 in the Embarras and Kaskaskia watersheds, respectively. In both watersheds, fertilizer chloride was the dominant input (~49 kg ha −1 yr −1 ), with road salt likely the other major source (23.2 and 7.2 kg ha −1 yr −1 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.

A positive role for biodiversity is assumed for managed ecosystems. We conducted a 12-year study of this sustainability principle, through separate manipulation of management intensity and crop diversity. The site was located in southwest Michigan, representative of rain-fed production, with high climate variability and well-drained soils. Provisioning services of grain and protein yield were monitored, simultaneous with supporting services of soil fertility, C and N, and regulating services associated with water quality (N-use efficiency and nitrate-N leached in gravimetric lysimeters). Surprisingly, a strong role for management was shown, and almost nil for crop diversity. Organic management (ORG) sustained soil fertility, augmented soil C (36% increase), enhanced N retention (50% decrease in nitrate-N leaching) and improved N-use efficiency, compared to conventional, integrated (INT) management. Provisioning of grain – quantity, quality and temporal yield stability – was highest in INT continuous maize (monoculture and biculture) with an annual yield of 6.4 Mg ha−1, compared to ORG of 5.1 Mg ha−1. Biodiverse rotational systems (three and six species) produced 25% lower yield, but the grain was of high quality. A focus on ORG management rather than crop diversity is suggested as a means to sequester C, and produced grain in a semi-closed system.

Soils in conventional agroecosystems are purposely held in a nitrogen (N)-saturated state to maximize crop yields. Planting winter annual cover crops when fields are usually fallow has been proposed to ameliorate N losses from soils. In this study we introduced winter annual cover crops into an N rate study with plots fertilized at 0, 34, 67, 101, 134, 168, and 202 kg N/ha in maize (Zea mays L.) to determine how winter annual cover crops affect yields, N2O and NO3- fluxes, and N pools. At the six-leaf stage and during flowering, incorporation of cover crop into soil resulted in a 30% reduction in maize biomass. Three weeks after fertilization, KCl-extractable soil mineral N was 75-87% lower in covercropped soils than in no-cover soils, indicating that N had been immobilized in the covercropped soils. At physiological maturity, there was no difference between cover and no-cover treatments in crop yield, which was maximized at 9 Mg/ha in 2006 and 7 Mg/ha in 2007. Where N rates exceed crop requirements, cover crop incorporation may reduce N exports as NO3- and N2O. Tighter N cycling in conventional agroecosystems could be fostered by matching N rates to the amount of N removed with grain and using N immobilization to retain N and support yields. If N immobilization is viewed as a means for efficient fertilizer N use rather than a process that decreases crop productivity, growers might be more willing to adopt cover-cropping practices.

Nutrient enrichment is a frequently cited cause for biotic impairment of streams and rivers in the USA. Efforts are underway to develop nutrient standards in many states, but defensible nutrient standards require an empirical relationship between nitrogen (N) or phosphorus (P) concentrations and some criterion that relates nutrient levels to the attainment of designated uses. Algal biomass, measured as chlorophyll-a (chl-a), is a commonly proposed criterion, yet nutrient-chl-a relationships have not been well documented in Illinois at a state-wide scale. We used state-wide surveys of >100 stream and river sites to assess the applicability of chl-a as a criterion for establishing nutrient standards for Illinois. Among all sites, the median total P and total N concentrations were 0.185 and 5.6 mg L(-1), respectively, during high-discharge conditions. During low-discharge conditions, median total P concentration was 0.168 mg L(-1), with 25% of sites having a total P of > or =0.326 mg L(-1). Across the state, 90% of the sites had sestonic chl-a values of < or =35 microg L(-1), and watershed area was the best predictor of sestonic chl-a. During low discharge there was a significant correlation between sestonic chl-a and total P for those sites that had canopy cover < or =25% and total P of < or =0.2 mg L(-1). Results suggest sestonic chl-a may be an appropriate criterion for the larger rivers in Illinois but is inappropriate for small rivers and streams. Coarse substrate to support benthic chl-a occurred in

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.

Agricultural watersheds in the upper Midwest are the major source of nutrients to the Mississippi River and Gulf of Mexico, but temporal patterns in nutrient export and the role of hydrology in controlling export remain unclear. Here we reporton NO3(-)-N, dissolved reactive phosphorus (DRP), and total P export from three watersheds in Illinois during the past 8-12 years. Our program of intensive, long-term monitoring allowed us to assess how nutrient export was distributed across the range of discharge that occurred at each site and to examine mechanistic differences between NO3(-)-N and DRP export from the watersheds. Last, we used simple simulations to evaluate how nutrient load reductions might affect NO3(-)-N and P export to the Mississippi River from the Illinois watersheds. Artificial drainage through under-field tiles was the primary mechanism for NO3(-)-N export from the watersheds. Tile drainage and overland flow contributed to DRP export, whereas export of particulate P was almost exclusively from overland flow. The analyses revealed that nearly all nutrient export occurred when discharge was > or = median discharge, and extreme discharges (> or = 90th percentile) were responsible for >50% of the NO3(-)-N export and >80% of the P export. Additionally, the export occurred annually during a period beginning in mid-January and continuing through June. These patterns characterized all sites, which spanned a 4-fold range in watershed area. The simulations showed that reducing in-stream nutrient loads by as much as 50% during periods of low discharge would not affect annual nutrient export from the watersheds.

Many studies have shown that maize (Zea mays L.) requires less fertilizer N for optimum yield when grown in rotation with soybean [Glycine max (L.) Merr] than when grown in monoculture, which is referred to as the `soybean N credit' in the maize growing areas of the United States. Because the specific source of this soybean N credit is unclear, our objective was to determine the role of nodules and N2 fixation as a contributing source of the soybean N credit. Our research approach was designed to separate the effect of symbiotic N2 fixation from other rotational effects, as the treatments included: maize grown after nodulated (N2 fixing) soybean and maize grown after non-nodulated (non N2 fixing) soybean. A separate experiment examined maize grown after maize. For each previous crop, maize was grown the following year with varying rates of fertilizer applied N. In both years, the yield differences between nodulated and non-nodulated soybean as the previous crop were much smaller than the apparent yield decrease associated with continuous maize. Although small in magnitude, maize following non-nodulated soybean accumulated less total N, was paler in leaf color, and yielded less than maize following nodulated soybean in the more favorable year of 1999, while most of these differences were not observed in 2000. These findings indicate that soybean nodules and N2 fixation, while having a certain role, are not the major determinants of the soybean N credit.

The occurrence of metabolites of many commonly used herbicides in streams has not been studied extensively in tile-drained watersheds. We collected water samples throughout the Upper Embarras River watershed [92% corn, Zea mays L., and soybean, Glycine max (L.) Merr.] in east-central Illinois from March 1999 through September 2000 to study the occurrence of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), metolachlor 12-chloro-N-(2-ethyl-6-methylphenyl)-N-(methoxy-1-methylethyl) acetamide], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide], acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl) acetamide], and their metabolites. River water samples were collected from three subwatersheds of varying tile density (2.8-5.3 km tile km(-2)) and from the outlet (United States Geological Survey [USGS] gage site). Near-record-low totals for stream flow occurred during the study, and nearly all flow was from tiles. Concentrations of atrazine at the USGS gage site peaked at 15 and 17 microg L(-1) in 1999 and 2000, respectively, and metolachlor at 2.7 and 3.2 microg L(-1); this was during the first significant flow event following herbicide applications. Metabolites of the chloroacetanilide herbicides were detected more often than the parent compounds (evaluated during May to July each year, when tiles were flowing), with metolachlor ethanesulfonic acid [2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxoethanesulfonic acid] detected most often (> 90% from all sites), and metolachlor oxanilic acid [2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxoacetic acid] second (40-100% of samples at the four sites). When summed, the median concentration of the three chloroacetanilide parent compounds (acetochlor, alachlor, and metolachlor) at the USGS gage site was 3.4 microg L(-1), whereas it was 4.3 microg L(-1) for the six metabolites. These data confirm the importance of studying chloroacetanilide metabolites, along with parent compounds, in tile-drained watersheds.

In east-central Illinois, fertilizer sales during the past 20 yr suggest that approximately half of the fertilizer nitrogen (N) applied to corn ( L.) occurs in the fall; however, fall fertilizer N sales were greatly reduced in 2009 as wet soil conditions restricted fall fieldwork, including fertilizer N applications. In 2010, we observed unusually low flow-weighted nitrate concentrations (approximately 40% below the long-term average) in two east-central Illinois rivers (5.7 mg N L in the Embarras River and 5.6 mg N L in the Lake Fork of the Kaskaskia River). Using long-term river nitrate data sets (1993-2012 for the Embarras and 1997-2012 for the Kaskaskia), we examined nitrate concentrations and developed regression models to estimate the association between fall fertilizer N application on riverine nitrate yields in these tile-drained watersheds. During these periods of record, annual riverine nitrate yields ranged from 8 to 57 kg N ha yr (30 kg N ha yr average) for the Embarras River and 2.6 to 59 kg N ha yr (32 kg N ha yr average) for the Kaskaskia. Multivariate linear regression relationships with the current and previous year's annual water yields, previous year's corn yield, and nine-county fall fertilizer sales accounted for 96% of the annual variation in nitrate yield in both watersheds. Running the regression models with fall fertilizer sales set to the 2009 amount suggests that the average reduction in nitrate yield (for the period of record) would be 17 and 20% for the Embarras and Kaskaskia Rivers, respectively. These data suggest that shifting fertilizer N application to the spring can be detected in watersheds as large as 481 km.

We examined the role of plants in the nutrient cycle of a 0.3 ha constructed wetland that received tile drainage water from agricultural fields. The objectives were to determine: 1) above- and below-ground production of wetland macrophytes; 2) production of algae; 3) accumulation and uptake rate of N and P by vegetation during the growing season; and, 4) role of wetland vegetation in the overall N and P budgets. Total biomass ranged seasonally from 12000 to 30000 kg ha−1 in the wetland, reaching a maximum in September, with roots accounting for 54 to 77% of the total. Above-ground macrophyte biomass ranged from 2000 to 5700 kg ha−1, and also reached a maximum in September. Algae were only present early in the growing season and had a maximum biomass of 233 kg ha−1 at the end of May. During the 1998 water year, tile input transported 715 kg ha−1 total N and 10 kg ha−1 total P into the wetland, whereas wetland output was 256 kg total N ha−1 (256 kg ha−1 in outlet flow and 120 kg ha−1 in seepage) an...

The Midwest has large riverine exports of nitrogen (N), with the largest flux per unit area to the Mississippi River system coming from Iowa and Illinois. We used historic and current data to estimate N inputs, outputs, and transformations for Illinois where human activity (principally agriculture and associated landscape drainage) have had a dominant impact. Presently, approximately 800,000 Mg of N is added each year as fertilizer and another 420,000 Mg is biologically fixed, primarily by soybean (Glycine max L. Merr.). These annual inputs are greater than exports in grain, which results in surplus N throughout the landscape. Rivers within the state export approximately 50% of this surplus N, mostly as nitrate, and the remainder appears to be denitrified or temporarily incorporated into the soil organic matter pool. The magnitude of N losses for 1880, 1910, 1950, and 1990 are compared. Initial cultivation of the prairies released large quantities of N (approximately 500,000 Mg N year(-1)), and resulted in riverine N transport during the late 19th century that appears to have been on the same order of magnitude as contemporary N losses. Riverine flux was estimated to have been at a minimum in about 1950, due to diminished net mineralization and low fertilizer inputs. Residual fertilizer N from corn (Zea mays L.), biological N fixed by soybean, short-circuiting of soil water through artificial drainage, and decreased cropping-system diversity appear to be the primary sources for current N export.

Nitrogen response trials throughout the United States Corn Belt show that economic optimum rates of N fertilization are usually less for maize (Zea mays L.) following soybean (Glycine max L.) than for maize following maize; however, the cause of this rotation effect is not fully understood. The objective of this study was to investigate the source of the apparent N contribution from soybean to maize (soybean N credit) by comparing soil N mineralization rates in field plots of unfertilized maize that had either nodulated soybean, non-nodulated soybean, or maize as the previous crop. Crop yields, plant N accumulation, soil inorganic N, and net soil mineralization were measured. Both grain yield (6.3 vs. 2.8 Mg ha −1 ) and above-ground N accumulation (97 vs. 71 kg ha −1 ) were greatly increased when maize followed nodulated soybean compared with maize following maize. A partial benefit to yield and N accumulation was also observed for maize following non-nodulated soybean. Cumulative net soil N mineralization following nodulated soybean, non-nodulated soybean, and maize was 112, 92 and 79 kg N ha −1 , respectively. Net mineralization of soil N appeared to be influenced by both quality (C:N ratio) and quantity of residue from the previous crop. In addition to an increase in plant available N from mineralization, the amount of soil inorganic N (especially in soil 5 cm from the row) was greater following nodulated soybean than non-nodulated soybean or maize. Based on these data, the soybean N credit appears to result from a combination of a decrease in net soil mineralization in continuous maize production and an increase in residual soil N from symbiotic fixation.

Much of the nonpoint N and P entering surface waters of the Midwest is from agriculture. We determined if constructed wetlands could be used to reduce nonpoint N and P exports from agricultural tile drainage systems to surface waters. Three treatment wetlands (0.3 to 0.8 ha in surface area, 1200 to 5400 m 3 in volume) that intercepted subsurface tile drainage water were constructed in 1994 on Colo soils (fine-silty, mixed, superactive, mesic Cumulic Endoaquoll) between upland maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] cropland and the adjacent Embarras River. Water (tile flow, precipitation, evapotranspiration, outlet flow, and seepage) and nutrient (N and P) budgets were determined from 1 Oct. 1994 through 30 Sept. 1997 for each wetland. Wetlands received 4639 kg total N during the 3-yr period (96% as NO 3 -N) and removed 1697 kg N, or 37% of inputs. Wetlands decreased NO 3 -N concentrations in inlet water (annual outlet volume weighted average concentrations of 4.6 to 14.5 mg N L -1 ) by 28% compared with the outlets. When the wetlands were coupled with the 15.3-m buffer strip between the wetlands and the river, an additional 9% of the tile NO 3 -N was apparently removed, increasing the N removal efficiency to 46%. Overall, total P removal was only 2% during the 3-yr period, with highly variable results in each wetland and year. Treatment wetlands can be an effective tool in reducing agricultural N loading to surface water and for attaining drinking water standards in the Midwest.

Constructed wetlands positioned in the landscape between row crop agriculture and surface waters can be used to intercept tile drainage and serve as agricultural waste water detention basins. A potential exit pathway in constructed wetlands for detained water and possibly NO3 -N is via seepage through and under an earthen berm. The objective of this study was to determine if seepage was an important pathway for NO3 -N transport from two constructed wetlands receiving tile drainage from adjacent agricultural land (wetland A, surface area of 0.6 ha; wetland D, 0.78 ha). A mean apparent hydraulic conductivity (K) was calculated (10.8 cm h 1 , range 8.2‐14.3 cm h 1 ) using empirical water budgets. Using Darcy’s law, which included the apparent hydraulic conductivity, effective seepage area and daily hydraulic gradient measurements, daily seepage volumes for both wetlands were calculated for the 1997 water year. Total seepage volumes for wetlands A and D were 26 and 22 million liters, respectively, for the 1997 water year, which represented 47 and 27% of the total inlet flow. The amount of NO3 -N exiting wetlands A and D in seepage water was estimated to be 61 and 25 kg N, respectively, and represented 10 and 4% of the total inlet NO3 -N load. Seepage connected the wetland with the riparian buffer strip and transported NO3 -N to populations of denitrifiers deeper in the sediment profile and outside the wetland perimeter, thereby enhancing overall NO3 -N removal efficiencies. © 2000 Elsevier Science B.V. All rights reserved.

Agricultural nonpoint sources are important contributors of N and P to surface waters. We determined N and P net anthropogenic inputs for Illinois, examining changes during the last 50 yr and linkages to surface water export of N and P. Inputs (fertilizer, atmospheric deposition, and N 2 fixation) were compared to exports (grain export, after accounting for animal and human consumption, plus animal product export) from 1945 through 1998 using state-reported data on fertilizer sales, crop production, and human and animal populations. Large inputs of N were found beginning about 1965, coinciding with increased N fertilizer applications (about 800 000 Mg N yr -1 ). The N input (about 400 000 Mg N yr -1 ) was 8.6 million Mg N for the 1979 to 1996 crop years, with a corresponding riverine flux of 4.4 million Mg N (51% of net anthropogenic inputs discharged by rivers). Using literature estimates of field and in-stream denitrification, we could account for nearly all of the missing N in a mass balance. For P, a different pattern was found for state net anthropogenic inputs with a large input from 1965 to 1990, and on average no net inputs since 1990. For rivers, we estimated that 16% of the total N load and 47% of the total P load was from sewage effluent. We estimate that Illinois contributed 15 and 10% of the annual total N and P loads of the Mississippi River, respectively, and that any reduction strategy in Illinois must address agricultural sources.

Offsite transport of N fertilizers and pesticides through subterranean drainage pipes (tiles) has been linked to surface water contamination in the U.S. Corn Belt. This study was conducted from water years 1995 to 1997 to evaluate N export from two tile systems (Tiles A and B) in adjacent fields [in seed corn-soybean rotation (Zea mays L.-Glycine max (L.) Merr.)] in response to timing and form of N application. In addition, during the 1997 water year, concentrations of two herbicides were determined on grab samples to relate tile herbicide losses with field application rates. During the 1995 and 1996 water years, Tile A exported approximately 20% more N per unit area than Tile B; however, during the 1997 water year, Tile A exported nearly 70% more N. This result was partly caused by the leaching of 142 kg of NH 4 + -N following a winter application of (NH 4 ) 2 SO 4 fertilizer in the drainage area of Tile A. The fertilizer was applied on top of 10 cm of snow, and within 4 d, a series of afternoon melting events began. We hypothesize that increased tile flow rates were caused by rapid infiltration of melt water through partially frozen soil (Drummer silty clay loam, fine-silty, mixed mesic Typic Haplaquolls) that allowed the NH 4 + ion to bypass the soil matrix (cone reached 278 mg NH 4 + -N L -1 ). The incidence of elevated concentrations of N fertilizer and herbicides in tiles during high flow events following agrichemical application indicated rapid water transport via preferential flow paths, thereby limiting the contact of these solutes with the soil.

Quantitative estimates of denitrification are needed in designing artificial wetlands to optimize nitrate (NO 3 - ) removal. Acetylene blockage and 15 N-tracer methods were employed to quantify denitrification in constructed wetlands receiving agricultural tile drainage, using plastic tubes to enclose in situ mesocosms. Estimates were also made through NO 3 - disappearance from mesocosm water columns. The 15 N and C 2 H 2 methods yielded comparable rates. At 4 to 25°C, and with 9 to 20 mg NO 3 -N L -1 initially in the mesocosm water columns, denitrification rates by the C 2 H 2 technique ranged from 2.0 to 11.8 mg N m -2 h -1 . In the June-August 15 N experiment, when wetland NO 3 was below detection, a time series of denitrification rates followed a bell-shaped curve after a pulse input of NO 3 - (∼15 mg N L -1 , 70 atom% 15 N). The maximal denitrification rate (9.3 mg N m -2 h -1 ) was observed 5.4 d after the pulse. After 33 d, 58% of the 15NO3 - had been evolved as N 2 , only ∼0.1% as N 2 O; 6 to 10% was recovered in plant shoots and as organic N in the upper 5 cm of sediment. From 32 to 36% of the 15 NO 3 - spike was not recovered, and presumably seeped into the sediments. The NO disappearance rates in the water column ranged from 12 to 63 mg N m -2 h -1 at 11 to 27°C. Because water infiltration carries NO 3 through the anaerobic sediment/water interface for denitrification, a subsurface-flow wetland may denitrify more NO 3 - than a surface-flow wetland.

Agricultural runoff can be a source of P, a limiting factor for freshwater eutrophication. To develop a simple method to estimate P export from the cropland, we studied 1.2-μm filtered dissolved phosphorus (DP) output from four tiles draining areas ranging from 8 to 25 ha, and from a river draining a 48173 ha watershed in east-central Illinois during 1993 to 1996. The land was under maize (Zea mays L.)-soybean (Glycine max L.) rotation. The tiles were estimated to contribute more than 86% of the river flow and 65 to 69% of the river DP export during 1995 to 1996. The DP load from tiles followed consecutive pseudo first-order kinetin in terms of tile flow (DP load depended on the amount of DP remaining in the soil matrix). The kinetic curves indicated a soluble-inorganic-P pool that was quickly depleted and replenished. In contrast, for DP export from the river at the watershed scale we observed pseudo zero-order kinetics based on river flow (DP export was independent of how much DP remained in the watershed). The contribution from numerous tiles and surface runoff to the river may have stabilized DP export at the watershed scale and therefore could explain the different kinetic orders. For the study watershed, a one parameter equation could estimate watershed-wide DP export: k' x (surface water discharge from the watershed) x (watershed area), with k' being 3.94 x 10 -6 mg P L -1 ha -1 . Our approach should be tested in watersheds with different geographic and agricultural characteristics.

Nitrogen (N) in surface waters has been linked to agricultural crop production, and more specifically, to NO3− exported by tile drainage. The objective of this study was to evaluate agricultural N pools and fluxes in a seed corn/soybean (Zea maize L./Glycine max L.) watershed (40 ha) to relate soil inorganic N pools with annual losses of NO3− in drainage tiles. During a 2-year period beginning in October 1993, soil samples in the top 50 cm located near the tile systems (predominantly Drummer silty clay loam, fine–silty, mixed mesic Typic Haplaquolls) were analyzed for microbial biomass C and N, inorganic N, and N mineralization rates. Water flow and NO3− concentrations were continuously measured in the three drainage tiles. Soil microbial biomass N ranged from 83 to 156 kg N ha−1, and appeared more closely related to soil moisture than soil inorganic N pools. Soil inorganic N ranged from a low of 13 kg N ha−1 during the soybean growing season to a high of 115 kg N ha−1 after N fertilization. Following good growing seasons in 1993 and 1994, high crop uptake of N resulted in relatively small soil inorganic N pools of 40 and 24 kg N ha−1, respectively, after crop harvest. In 1995, however, when poor growing conditions decreased crop N accumulation, 98 kg N ha−1 remained in the watershed after harvest. Based on an average effective drainage area of 30 ha, 38 and 64 kg N ha−1 leached out of the watershed through tile drainage for a total of 3.1 Mg N for the 1995 and 1996 water years, respectively. Tile N export from the watershed was greatest during high flow events when there concurrently existed large pools of soil inorganic N in the form of NO3−. Differences in annual N export for each tile were the result of a combination of factors including; timing and area of N fertilization, amount and distribution of precipitation, crop uptake of soil derived N, and inorganic N pools remaining after harvest.

Surface water nitrate (NO 3 - ) pollution from agricultural production is well established, although few studies have linked field N budgets, NO 3 - loss in tile drained watersheds, and surface water NO 3 - loads. This study was conducted to determine field sources, transport, and river export of NO 3 - from an agricultural watershed. The Embarras River watershed at Camargo (48 173 ha) in east-central Illinois was investigated. The watershed is a tile-drained area of fertile Mollisols (typical soil is Drummer silty clay loam, a fine-silty, mixed mesic Typic Haplaquoll) with primary cropping of maize (Zea mays L.) and soybean (Glycine max L.). Agricultural field N sources and sinks, tile drainage NO 3 - concentrations and fluxes, and river NO 3 - export were estimated for the entire watershed. Large pools of inorganic N were present following each harvest of maize and soybean (average of 3670 Mg N yr -1 over a 6-yr period). The source of most of the inorganic N was divided between N fertilizer and soil mineralized N. High concentrations of NO 3 were found in four monitored drainage tiles (5-49 mg N L '), and tile concentrations of NO 3 - were synchronous with Embarras River NO 3 - concentrations. High flow events contributed most of the yearly NO 3 - loss (24.7 kg N ha -1 yr -1 ) from tile drained fields in the 1995 water year (1 Oct, 1994 through 30 Sept. 1995) where high rainfall events occurred in a low overall precipitation year (in one tile 21% of the annual load was exported in 1 d). During the 1996 water year, NO 3 - export in tiles was much higher (44.2 kg N ha -1 yr -1 ) due to greater precipitation, and individual days were less important. On average, about 49% (average of 1688 Mg N yr ' over a 6-yr period) of the field inorganic N pool was estimated to be leached through drain tiles and seepage and was exported by the Embarras River, although depending on weather and field N balances this ranged from 25 to 85% of the field N balance over the 6-yr period. It seems likely that agricultural disturbance (high mineralization inputs of N) and N fertilization combined with tile drainage contributed significantly to NO, export in the Embarras River.

in the Mississippi River Basin Mark B. David,*,† Courtney G. Flint,‡ Gregory F. McIsaac,† Lowell E. Gentry,† Mallory K. Dolan,‡ and George F. Czapar †Department of Natural Resources and Environmental Sciences, University of Illinois , W503 Turner Hall, 1102 S. Goodwin Avenue, Urbana, Illinois 61801, United States ‡Department of Sociology, Social Work & Anthropology, Utah State University, 0730 Old Main Hill, Logan, Utah 84322-0730, United States Illinois State Water SurveyPrairie Research Institute, University of Illinois, 2204 Griffith Dr., Champaign, Illinois 61820-7495, United States

Although N, especially mixtures of NO 3 and NH 4 , can increase the productivity of maize (Zea mays L.), it is unclear when this N needs to be available for maximum benefit. This study was conducted to determine the importance of preanthesis N form and the need for available postanthesis N on the productivity of maize. Four single-cross hybrids (B73×LH51, LH74×LH51, LH74×LH82, and LHE136×LH82) were grown in field-hydroponics and supplied N as either all NO 3 or an equal mixture of NO 3 and NH 4 (mixed N nutrition) until anthesis, whereupon N was withdrawn from one-half of the plants (.)

Simple nitrogen (N) input/output balance calculations in agricultural systems are used to evaluate performance of nutrient management; however, they generally rely on extensive assumptions that do not consider leaching, denitrification, or annual depletion of soil N. We constructed a relatively complete N mass balance for the Big Ditch watershed, an extensively tile-drained agricultural watershed in east-central Illinois. We conducted direct measurements of a wide range of N pools and fluxes for a 2-yr period, including soil N mineralization, soybean N(2) fixation, tile and river N loads, and ground water and in-stream denitrification. Fertilizer N inputs were from a survey of the watershed and yield data from county estimates that were combined with estimated protein contents to obtain grain N. By using maize fertilizer recovery and soybean N(2) fixation to estimate total grain N derived from soil, we calculated the explicit change in soil N storage each year. Overall, fertilizer N and soybean N(2) fixation dominated inputs, and total grain export dominated outputs. Precipitation during 2001 was below average (78 cm), whereas precipitation in 2002 exceeded the 30-yr average of 97 cm; monthly rainfall was above average in April, May, and June of 2002, which flooded fields and produced large tile and riverine N loads. In 2001, watershed inputs were greater than outputs, suggesting that carryover of N to the subsequent year may occur. In 2002, total inputs were less than outputs due to large leaching losses and likely substantial field denitrification. The explicit change in soil storage (67 kg N ha(-1)) offsets this balance shortfall. Although 2002 was climatically unusual, with current production trends of greater maize grain yields with less fertilizer N, soil N depletion is likely to occur in maize/soybean rotations, especially in years with above-average precipitation or extremely wet spring periods.

Agriculture is a major nonpoint source of phosphorus (P) in the Midwest, but how surface runoff and tile drainage interact to affect temporal concentrations and fluxes of both dissolved and particulate P remains unclear. Our objective was to determine the dominant form of P in streams (dissolved or particulate) and identify the mode of transport of this P from fields to streams in tile-drained agricultural watersheds. We measured dissolved reactive P (DRP) and total P (TP) concentrations and loads in stream and tile water in the upper reaches of three watersheds in east-central Illinois (Embarras River, Lake Fork of the Kaskaskia River, and Big Ditch of the Sangamon River). For all 16 water year by watershed combinations examined, annual flow-weighted mean TP concentrations were >0.1 mg L(-1), and seven water year by watershed combinations exceeded 0.2 mg L(-1). Concentrations of DRP and particulate P (PP) increased with stream discharge; however, particulate P was the dominant form during overland runoff events, which greatly affected annual TP loads. Concentrations of DRP and PP in tiles increased with discharge, indicating tiles were a source of P to streams. Across watersheds, the greatest DRP concentrations (as high as 1.25 mg L(-1)) were associated with a precipitation event that followed widespread application of P fertilizer on frozen soils. Although eliminating this practice would reduce the potential for overland runoff of P, soil erosion and tile drainage would continue to be important transport pathways of P to streams in east-central Illinois.

A better understanding of the controls on algae and dissolved O2 in agricultural streams of Illinois is needed to aid in development of nutrient standards. We investigated the relationships between dissolved nutrients, algal abundance, and dissolved O2 in five streams in east-central Illinois from March through November 2004. The streams drained watersheds from 25 to 777 km2 that were dominated by row crop agriculture. Three sites had open canopies and two were bordered by a narrow forest of deciduous trees. Algal abundance was measured as chlorophyll-a (chl-a) concentration in the water column (sestonic) and on the streambed (periphytic). Mean NO3-N concentrations ranged from 5.5 to 8.8 mg N L(-1) and did not relate to algal abundance. Sestonic chl-a values ranged from nearly zero to15 mg m(-3) with no differences between open and shaded streams and only a weak correlation with dissolved reactive P (mean concentrations were 44-479 microg L(-1)). The results suggest that sestonic chl-a is a poor criterion for assessing nutrient-related problems in these streams. Greatest periphytic chl-a occurred during low flow from August through October, but periphyton occurred consistently in only two of the five streams. The abundance of filamentous algae explained 64% of the variation in diel O2 saturation, but was not correlated with nutrients. Currently it appears that hydrology and light, rather than nutrients, control algal abundance in these streams, and in the agricultural landscape of east-central Illinois, it may not be possible to reduce nutrient concentrations sufficiently to limit filamentous algal blooms.