Tilth

Abstract: 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)