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.

Surface water nitrate (NO~-) pollution from agricultural production is well established, although few studies have linked field N budgets, NO~-loss in tile drained watersheds, and surface water NO~-loads. This study was conducted to determine field sources, transport, and river export of NO~-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~-concentrations and fluxes, and river NO~-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~ were found in four monitored drainage tiles (5-49 mg N L-l), and tile concentrations of NO~-were synchronous with Embarras River NO~-concentrations. High flow events contributed most of the yearly NO~-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~ export in tiles was much higher (44.2 kg N ha-~ yr-j) due to greater precipitation, and individual days were less important. On average, about 49% (average of 1688 Mg N yr-1 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. N ITRATE contamination of surface and groundwaters is of environmental concern throughout agricultural areas of the USA. High inputs of N fertilizer are required to support intensive row-crop agriculture, particularly for corn in the Midwest where fertilizer application rates are typically 100 to 200 kg N ha-I yr-1. It is difficult to maintain the fine balance of available N required to satisfy crop needs and at the same time minimize leaching losses, even though fertilization combined with soil mineralization can provide large amounts of inorganic N (Keeney and DeLuca, 1993). Under optimal growing season conditions and fertilizer N application rates, the crop grain yield contains typically only about 50% of the added fertilizer N (Oberle and Keeney, 1990). Throughout many areas of the Midwest and in particular in much of Illinois, agricultural fields are drained with subterranean tiles (perforated pipe...

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 N2 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 paflern 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.

tile drainage systems to surface waters. Three treatment wetlands (0.3 tems as nonpoint diffuse nutrient loading can be attrib to 0.8 ha in surface area, 1200 to S400 m 3 in volume) that intercepted uted to agriculture and associated land use practices in subsurface tile drainage ,,:ater were constructed in 1994 on Colo soils the Midwest (USEPA, 1989, 1990b). These practices (fine-silty, mixed, superactive, mesic Cumulic Endoaquoll) between have contributed to reduced ground water quality, re upland maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] duced surface water quality, and are also believed to cropland and the adjacent Embarras River. Water (tile flow, predpita be a contributing cause of hypoxia in the Gulf of Mexico tion, evapotranspiration, oudet Bow, and seepage) and nutrient (N and anoxia in estuarine and ocean ecosystems (Turner and P) budgets were determined from 1 Oct. 1994 through 30 Sept.

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.

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 ropping 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 <20% of the total), but pivotal to the rapid cycling of nutrients, soil aggregation, and C sequestration (Wander, 2004; Weil and Magdoff , 2004; Schmidt et al., 2011). Many measures of labile organic matter are sensitive and robust indicators of soil ecosystem change, but most are expensive to measure, and accordingly, are not off ered by standard commercial soil testing laboratories (Phillips, 2010). Th ere is a clear need for more inexpensive alternative measures of labile organic matter that enable farmers, extension educators, agronomists, and soil scientists to track and predict soil C and N dynamics in their fi elds. Predicting soil N availability in cropping systems has been an ongoing challenge due to the complexities and interacting forces of weather, soil biology and physical properties, residue quality, and management practices (Cabrera et al., 2005; Schomberg et al., 2009). In a research setting, soil N availability is oft en predicted with laboratory incubations of soil, that is, N mineralization potential (Stanford and Smith 1972). Th e time and costs associated with this analysis has limited the adoption into most standard commercial soil testing laboratories, although some specialized labs do off er N mineralization tests (e.g., Idowu et al., 2008). Instead of incubations, growers currently use two primary tools to predict soil N availability: the pre-sidress nitrate test (PSNT; Magdoff et al., 1984) and leaf chlorophyll content (Piekkielek and Fox, 1992; Scharf et al., 2006). Th e PSNT measures soil nitrate in corn at V4-V6 (Fox et al., 1989), while leaf chlorophyll content measures the greenness of early stage crop leaves relative to a highly-fertilized reference strip. Although both of these tests provide a prescriptive fertilizer recommendation, many growers do not use them for a variety of reasons (Markwell et al., 1995; Andraski and Bundy, 2002; Schmidt et al., 2009). Another approach has been to approximate N mineralization ...

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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.