Balancing the amount of N needed for optimum plant growth while minimizing the NO 3 that is transported to ground and surface waters remains a major challenge for everyone attempting to understand and improve agricultural nutrient use efficiency. Our objectives for this review are to examine how changes in agricultural management practices during the past century have affected N in midwestern soils and to identify the types of research and management practices needed to reduce the potential for nonpoint NO 3 leakage into water resources. Inherent soil characteristics and management practices contributing to nonpoint NO 3 loss from midwestern soils, the impact of NO 3 loading on surface water quality, improved N management strategies, and research needs are discussed. Artificial drainage systems can have a significant impact on water quality because they behave like shallow, direct conduits to surface waters. Nonpoint loss of NO 3 from fields to water resources, however, is not caused by any single factor. Rather, it is caused by a combination of factors, including tillage, drainage, crop selection, soil organic matter levels, hydrology, and temperature and precipitation patterns. Strategies for reducing NO 3 loss through drainage include improved timing of N application at appropriate rates, using soil tests and plant monitoring, diversifying crop rotations, using cover crops, reducing tillage, optimizing N application techniques, and using nitrification inhibitors. Nitrate can also be removed from water by establishing wetlands or biofilters. Research that is focused on understanding methods to minimize NO 3 contamination of water resources should also be used to educate the public about the complexity of the problem and the need for multiple management strategies to solve the problem across agricultural landscapes.

Nitrogen Management Strategies to Reduce Nitrate Leaching in Tile-Drained Midwestern Soils

ricultural fields, and increased use of manufactured N fertilizers Balancing the amount of N needed for optimum plant growth while Less than 50 yr ago, corn (Zea mays L) was generally minimizing the NO3 that is transported to ground and surface waters grown in rotation with cereal crops and forage legumes remains a major challenge for everyone attempting to understand such as alfalfa (Medicago sativa L), red clover (Trifoand improve agricultural nutrient use efficiency Our objectives for lium pratense L), and sweetclover (Melilotus spp) this review are to examine how changes in agricultural management practices during the past century have affected N in midwestern soils Through biological N fixation, the legumes generally and to identify the types of research and management practices needed increased the amount of residual N in the soil profile to reduce the potential for nonpoint NO3 leakage into water resources Cycling of this residual, biologically fixed N along with Inherent soil characteristics and management practices contributing N mineralized from SOM added with animal manure to nonpoint NO3 loss from midwestern soils, the impact of NO3 loading or deposited through rainfall was the primary process on surface water quality, improved N management strategies, and through which corn and other grain crops obtained N research needs are discussed Artificial drainage systems can have a Following World War II, increased availability of significant impact on water quality because they behave like shallow, commercial N fertilizer and decreased demand for fordirect conduits to surface waters Nonpoint loss of NO3 from fields age crops led to a significant reduction in crop rotations to water resources, however, is not caused by any single factor Rather, and a general substitution of purchased N for biological it is caused by a combination of factors, including tillage, drainage, N In Iowa, forage pasture represented more than 336% crop selection, soil organic matter levels, hydrology, and temperature and precipitation patterns Strategies for reducing NO3 loss through (3 389 160 ha) of the state’s total cropped area at the drainage include improved timing of N application at appropriate end of World War II (US Dep of Commerce, Bureau rates, using soil tests and plant monitoring, diversifying crop rotations, of the Census, 1945) By 1997, forage pasture area in using cover crops, reducing tillage, optimizing N application techIowa comprised only 128% (1 393 451 ha) of the state’s niques, and using nitrification inhibitors Nitrate can also be removed total cropped area (USDA Natl Agric Stat Serv, from water by establishing wetlands or biofilters Research that is 1997) Incorporation of legumes into a crop rotation focused on understanding methods to minimize NO3 contamination was no longer needed as commercial N inputs gradually of water resources should also be used to educate the public about replaced biological N fixation the complexity of the problem and the need for multiple management The increased availability of commercial N also facilistrategies to solve the problem across agricultural landscapes tated specialization and a national trend for separating crop and animal production enterprises Animal manure no longer served as an important crop nutrient resource, N is essential for growth and reproduction and meadow legumes were not required on farms that of all life forms, and except for legume crops and began to specialize in corn and soybean [Glycine max virgin soils with relatively high soil organic matter (L) Merr] production Although there is considerable (SOM), soil N must usually be supplemented to sustain variation among years and regions in N fertilizer usage, food, feed, and fiber production During the past 20 yr, the net result of this farming-system change was a napublic concern regarding N movement from agricultural tional average increase in commercial N fertilizer use nonpoint sources into broader water resources has inof 24 kg ha 1 yr 1 (Fig 1) between the mid-1960s and creased as problems such as hypoxia (Rabalais et al, the late 1990s The rise in commercial N fertilizer usage 1996) became more evident To understand current within Midwest states (Illinois, Iowa, Indiana, Michigan, questions about N management in the US northern Minnesota, Missouri, Nebraska, Ohio, and Wisconsin) Corn and Soybean Belt, it is necessary to examine the has slowed, with average use of commercial N fertilizer changes that have occurred in agriculture during the past increasing by 4 kg ha 1 (05 kg ha 1 yr 1 ) from 1991 to century These changes include the use of less diversified 1999 (Fig 2) However, the trend of steadily increased crop rotations, separation of crop production and animal usage of commercial N fertilizer may change if fossil fuel enterprises, changes in tillage intensity, drainage of agprices increase substantially and the fertilizer industry is Abbreviations: LCD, localized compaction and doming; LSNT, lateUSDA-ARS Natl Soil Tilth Lab, 2150 Pammel Dr, Ames, IA 50011 spring nitrate test; NIR, near infrared; PSNT, presidedress soil nitrate Received 27 Apr 2001 *Corresponding author (dinnes@nstlgov) test; SI–CD, subirrigation with controlled drainage; SOM, soil organic matter; WTM, water table management Published in Agron J 94:153–171 (2002)

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Review and Interpretation: Nitrogen Management Strategies to Reduce Nitrate Leaching in Tile-Drained Midwestern Soils

The extensive development of surface and subsurface drainage systems to facilitate agricultural production throughout North America has significantly altered the hydrology of landscapes compared to historical conditions. Drainage has transformed nutrient and hydrologic dynamics, structure, function, quantity, and configuration of stream and wetland ecosystems. In many agricultural regions, more than 80% of some catchment basins may be drained by surface ditches and subsurface drain pipes (tiles). Natural channels have been straightened and deepened for surface drainage ditches with significant effects on channel morphology, instream habitats for aquatic organisms, floodplain and riparian connectivity, sediment dynamics, and nutrient cycling. The connection of formerly isolated wetland basins to extensive networks of surface drainage and the construction of main channel ditches through millions of acres of formerly low-lying marsh or wet prairie, where no defined channel may have previously existed, have r...

Effects of Agricultural Drainage on Aquatic Ecosystems: A Review

Ths report addresses the following two questions: 
1) What are the loads (flux) of nutrients transported from the Mississippi-Atchafalaya River Basin to the Gulf of Mexico, and where do they come from within the basin? 
2) What is the relative importance of specific human activities, such as agriculture, point-source discharges, and atmospheric deposition in contributing to these loads? 
These questions were addressed by first estimating the flux of nutrients from the Mississippi-Atchafalaya River Basin and about 50 interior basins in the Mississippi River system using measured historical streamflow and water quality data. Annual nutrient inputs and outputs to each basin were estimated using data from the National Agricultural Statistics Service, National Atmospheric Deposition Program, and point-source data provided by the USEPA. Next, a nitrogen mass balance was developed using agricultural statistics, estimates of nutrient cycling in agricultural systems, and a geographic information system. Finally, multiple regression models were developed to estimate the relative contributions of the major input sources to the flux of nitrogen and phosphorus to the Gulf 
of Mexico.

Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin

Subsurface tile drainage from row-crop agricultural production systems has been identified as a major source of nitrate entering surface waters in the Mississippi River basin. Noncontrollable factors such as precipitation and mineralization of soil organic matter have a tremendous effect on drainage losses, nitrate concentrations, and nitrate loadings in subsurface drainage water. Cropping system and nutrient management inputs are controllable factors that have a varying influence on nitrate losses. Row crops leak substantially greater amounts of nitrate compared with perennial crops; however, satisfactory economic return with many perennials is an obstacle at present. Improving N management by applying the correct rate of N at the optimum time and giving proper credits to previous legume crops and animal manure applications will also lead to reduced nitrate losses. Nitrate losses have been shown to be minimally affected by tillage systems compared with N management practices. Scientists and policymakers must understand these factors as they develop educational materials and environmental guidelines for reducing nitrate losses to surface waters.

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Nitrate nitrogen in surface waters as influenced by climatic conditions and agricultural practices.

Nitrate-N concentration in tile water, loss from tile lines, and accumulation in soil profiles were determined following each of three annual applications of 20, 112, 224, and 448 kg N/ha to continuous corn (Zea mays L.) grown on a Webster clay loam (Typic Haplaquoll) in southern Minnesota. Plots were isolated to a depth of 1.8 m with plastic to allow an accurate assessment of the area drained. Water flow through the tile lines occurred annually for approximately 6 weeks in the period from mid-April through early July and constituted an equivalent from 20 to 46% of the precipitation during the flow periods and from 7 to 22% of the annual precipitation during the 3 years studied. There was relatively little increased NO₃-N accumulation in the soil profile or loss from tile lines at the recommended application rate of 112 kg N/ha compared to that for the check treatment. Nitrate-N losses through tile lines in 1975 (after 3 years treatment) were 19, 25, 59, and 120 kg/ha for the 20, 112, 224, and 448 kg N/ha applications, respectively, which had NO₃-N accumulations in the 0–3 m soil profiles of 54, 100, 426, and 770 kg NO₃-N/ha. Maximum NO₃-N accumulation in the soil profiles occurred at a depth of about 1 m with little evidence of movement below about 2.2 m.

G. W. Randall

Papers

Nitrate Accumulation in Soils and Loss in Tile Drainage Following Nitrogen Applications to Continuous Corn