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# Review and Inte rpret at ion: Nitroge n M an ageme nt S trategies t o Reduce Nitrate Le a ching in T i leD rained Midwes te rn S oi l s

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# REVIEW AND INTERPRETATION

Nitrogen Management Strategies to Reduce Nitrate Leaching in Tile-Drained

Midwestern Soils

Dana L. Dinnes,* Douglas L. Karlen, Dan B. Jaynes, Thomas C. Kaspar, Jerry L. Hatfield, Thomas S. Colvin, and Cynthia A. Cambardella

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 NO 3 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 NO 3 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 NO 3 loss from midwestern soils, the impact of NO 3 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 for- direct conduits to surface waters. Nonpoint loss of NO 3 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 33.6% crop selection, soil organic matter levels, hydrology, and temperature
and precipitation patterns. Strategies for reducing NO 3 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 (U.S. 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 tech- Iowa comprised only 12.8% (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 NO 3 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 facili- strategies 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, 

# Nitrogen 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 na- public concern regarding N movement from agricultural tional average increase in commercial N fertilizer use nonpoint sources into broader water resources has in- of 2.4 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 U.S. 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 (0.5 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 ag- prices 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@nstl.gov). test; SI–CD, subirrigation with controlled drainage; SOM, soil organic matter; WTM, water table management. Published in Agron. J. 94:153–171 (2002).

153154 AGRONOMY JOURNAL, VOL. 94, JANUARY–FEBRUARY 2002

Fig. 1. Average U.S. commercial N fertilizer use for corn production

for the years 1964–1998. Fig. 2. Average commercial N fertilizer use in the Midwest for corn production for the years 1991–1997. unable to develop lower-cost energy sources or new strategies for fertilizer N production. An alternative to wet condition of these soils was modified by installation commercial N fertilizer inputs would be to reconsider of artificial drainage, tillage to prepare a seedbed for some of the traditional N management practices, espe- agricultural crops, and a change from perennial to sea- cially if they can be made more efficient and predictable sonal vegetation (Hewes and Frandson, 1952). With by applying 21st century technologies. these changes, the potential for mineralization of N from In addition to decreased diversity in crop rotation, the stored organic matter and N loading of surface wa- separation of crop and animal production enterprises, ters increased significantly. As stated by Randall (1997), and use of more commercial N fertilizers, an even more “Soils high in organic matter can mineralize a substantial dramatic change affecting N cycling within many mid- amount of nitrate-N which is susceptible to loss in sub- western soils was the installation of artificial subsurface surface tile drainage, especially when wet years follow drainage lines. Subsurface drainage lines began to be very dry years.” Collectively, the unintended side effect installed in the late 1800s in the Midwest, intensified of these changes in cultural practices and N management in the next century (Hewes and Frandson, 1952), and strategies has been an apparent reduction in N-cycling continues today. By 1987, more than 20.8 10 6 ha in efficiency compared with natural ecosystems or crop the states of Illinois, Indiana, Iowa, Ohio, Minnesota, production systems that have little to no reliance on Michigan, Missouri, and Wisconsin had been artificially commercial N inputs. drained (Zucker and Brown, 1998) in contrast to ap- Each agricultural region has its own specific environ- proximately 2 10 6 ha being irrigated in the same states mental challenges or imbalances associated with land (USDA Natl. Agric. Stat. Serv., 1999). Subsurface artifi- use decisions and crop production practices. In the Mid- cial drainage thus preceded and occurred in coincidence west, artificially drained areas, increased use of synthetic with other management changes in midwestern agricul- fertilizers, and decreased diversity in crop rotation are ture, most likely catalyzing other crop production and among the most notable causes of this region’s problem field management changes. with N contamination of water resources. The objectives

Drainage was deemed necessary and strongly pro- of this review are to (i) examine the scope of the NO 3 moted for the betterment of farmers, communities, and leaching problems associated with artificially drained the nation as a whole because natural surface drainage midwestern soils, (ii) discuss current strategies for reduc- was limited throughout much of the region. As a result, ing NO 3 losses from these soils, and (iii) identify future the landscape was significantly changed, and many areas research needs to develop new strategies for reducing that were previously classified as prairie potholes could NO 3 leaching losses. The desired impact of research on now be converted to row crop production. Artificial field- and watershed-scale NO 3 loss and opportunities drainage and an increased availability of N fertilizers for improving overall water quality by increasing N use are two of the most prominent practices that facilitated efficiency are discussed. a tremendous increase in the intensity of agricultural production throughout the midwestern USA. These 

### NITRATE LEACHING LOSSES FROM changes had enormous positive impact on the agricul- 

### AGRICULTURAL SYSTEMS tural economy and the expansion of agricultural exports. However, they drastically altered the farming practices We will focus the attention of this paper on the NO 3 that had evolved within the midwestern landscape and form of N because a number of leaching and drainage contributed to changes in soil and water quality that studies have consistently found that NO 3 is the dominant had an impact far beyond farm fields. form of N present in the soil water (Willrich, 1969; Baker

The ecosystem effects of artificial subsurface drainage et al., 1975; Kladivko et al., 1991; Jacinthe et al., 1999). in the Midwest were compounded because many of the Soil NO 3 can be derived from both organic and inorganic soils that had developed under a subhumid climate, in N. Whether the N source is animal manure or commerareas of low relief and poor surface drainage, had high cial N fertilizer, overapplication or ill-timed application of either source can provide too much plant-available (  5–6%) levels of organic matter. The predominatelyDINNES ET AL.: STRATEGIES TO REDUCE NITRATE LEACHING IN MIDWESTERN SOILS 155

N and increase the potential for NO 3 leaching (Hatfield surface waters (Hatfield et al., 1998; Hatfield et al., 1999). and Cambardella, 2001). Animal manure N sources pro-vide many physical, chemical, and biological benefits to Current practices of N fertilizer management are often very inefficient compared with natural systems, thus the soil environment beyond that of inorganic commer-cial N sources (Hatfield and Cambardella, 2001), and a increasing the potential for contamination of water resources (Sanchez and Blackmer, 1988; Kanwar et al., thorough discussion of these aspects and the N dynamicsof manures would be very extensive. Therefore, within 1993, 1996; Randall, 1997; Randall et al., 1997a; Cambardella et al., 1999). As a result, first-year recoveries of this review, we will not specifically discuss manure man-agement issues. Instead, we will focus on commercial fertilizer N by corn have been reported to be 35% (Bijeriego et al., 1979), 14 to 65% (Meisinger et al., 1985), 23 fertilizer and soil or crop management impacts on NO 3 leaching. to 45% (Kitur et al., 1984), 24 to 26% (Olson, 1980), 15 to 33% (Sanchez and Blackmer, 1988), and 45 to 59% Nitrate contributes to surface water degradation when it flows into subsurface drainage lines that dis- (Reddy and Reddy, 1993). Based on these N fertilizer recovery studies, one factor contributing to low fertilizer charge into streams and lakes or when it leaches below the active plant-root zone and into shallow ground water N recovery is that in any given year, corn plants can obtain up to 85% of their N from mineralized SOM. resources that connect to surface water bodies throughnatural processes such as baseflow. The intensification The potential impact of soil-derived N can be illustrated by estimating the amount of plant-available N of row crop production as a whole, not just the increaseduse of N fertilizers, has been identified as the primary associated with the SOM in many midwestern soils. Assuming there is approximately 4.5 10 6 kg of soil per cause for increased NO 3 contamination of surface waters during the past several decades (Keeney and DeLuca, hectare in the surface 30 cm and an average 10:1 C/N ratio for the SOM, there would be 4500 kg N ha

 1 for 1993). This is especially true for continuous corn produc- tion, which has repeatedly been identified as providing 1% SOM. Therefore, crops grown on a soil with 3% SOM, which would be about average for the subhumid the greatest amount of NO 3 to streams through subsur- face drainage (Kanwar et al., 1993; Weed and Kanwar, Midwest, would have the potential to obtain 13 500 kg

N ha

 1 just from the SOM. Obviously, only a small 1996; Randall et al., 1997a).

Field drainage systems can have a significant impact percentage of this soil-derived N would be available atany given point in time because it must first be mineralon water quality because they behave like shallow, di- rect pipelines or conduits to surface waters. They in- ized by microbes to a plant-available form such as NH 4 or NO 3 and the amount mineralized may not be enough crease the speed with which water moves off the land- scape by short-circuiting natural water flow into shallow to meet crop N needs during rapid growth periods. Schepers and Mosier (1991) stated that for a given ground water. Nutrients and pesticides are often trans- ported with subsurface drainage water directly into climatic region, a general estimate of N mineralization could be made based on SOM content. They estimated streams or lakes (Baker and Johnson, 1981; Buhler et al., 1993; Jayachandran et al., 1994; Kladivko et al., 1991; that assuming 2% of the total organic N in the surface 30 cm is mineralized annually, a soil with 1% SOM Randall et al., 1997a). Numerous studies have shown significant edge-of-field losses of NO 3 (Hanway and content could be expected to mineralize approximately 45 kg N ha

 1 yr  1 . It is important to remember that these Laflen, 1974; Gast et al., 1978; Miller, 1979; Benoit, 1973; Logan et al., 1980; Baker and Johnson, 1981; Bergstro  ̈ m, are general estimates because the amount of organic N made available through mineralization processes will 1987; Kanwar et al., 1988; Drury et al., 1996). One exam-ple is a study reported by Baker et al. (1975) where vary greatly over time due to factors such as temperature, precipitation, and tillage (Doran, 1980; Franzlueb- they found average NO 3 –N concentrations of 21 mg L

 1in subsurface drainage water leaving fields planted to bers et al., 1995; Wienhold and Halvorson, 1999). However, because of their high SOM levels, these estimates corn–soybean or corn–oat (Avena sativa L.) rotations. Similarly, for a 5130-ha watershed located on the Des illustrate that midwestern soils have a high potential for providing N throughout the entire growing season. Moines lobe in central Iowa, Jaynes et al. (1999) re- ported flow-weighted NO 3 –N concentrations in field With regard to N management, including both fertilizer and manure application, the SOM-N pool is exand county agricultural drainage lines that were often greater than the USEPA maximum contamination level tremely important for midwestern soils. Routine soil and crop management decisions not only affect the min- (MCL) for drinking water of 10 mg L

 1 , especially from April through July. Combining stream flow and NO 3 –N eralization process per se, but also the rates and timing of N release through interactions between the microbial concentration data for this intensively row-cropped ag-

ricultural watershed showed that total NO 3 –N losses communities and practices such as drainage or tillage and/or residue management. As a result, fertilizer N ranged from 4 to 66 kg ha

 1 yr  1 . The variation in NO 3 –N losses among years was directly linked to variation in that is not taken up by the crop to which it was applied can have many different fates. If rainfall is below normal annual precipitation (Hatfield et al., 1998). Those water-
shed-scale measurements also showed that 45% of the for midwestern conditions, NO 3 can accumulate within the soil profile. It can also be leached below the active average annual precipitation was drained from the soilprofile through the subsurface drainage lines. This em- plant root zone; lost through denitrification; or incorporated, cycled, and stabilized within many different phasizes the importance of these drainage lines as a primary pathway for herbicide and NO 3 movement to SOM-N pools. These factors significantly affect N man-156 AGRONOMY JOURNAL, VOL. 94, JANUARY–FEBRUARY 2002

agement in midwestern soils. For example, in Iowa to these processes increases as the time between N application and crop uptake increases. This is true for residalone, Swoboda (1990) reported that failure to account for residual N caused farmers to annually spend approxi- ual N as well as applied N (Magdoff, 1991; Karlen et al., 1998), especially in years that do not produce optimal mately $100 million on excess N fertilizer applications.This is not only an on-farm economic waste, but also yields (Power et al., 1998). Limiting the amount of inorganic N within the soil at the end of a crop’s growing an environmental waste with regard to both water qual-ity and the fossil energy needed to manufacture N fer- season and before the next crop has established an extensive root system is a key factor for reducing N losses. tilizer.

Nonpoint contamination is a major water quality con- Therefore, although timing, method of N application, and accounting for mineralizable soil N are important cern throughout the Midwest (Humenik et al., 1987), and use of N fertilizers on artificially drained agricul- for reducing potential NO 3 leaching, Power and Schepers (1989) concluded that the most important factor was tural land has been implicated as a major contributor to this problem (USEPA, 1992; U.S. Geological Survey, to apply the correct amount of N fertilizer. Typical N fertilizer management for corn production 1995; CAST, 1999). The 1992 national water quality inventory (USEPA, 1992) notes that in the rivers stud- in the subhumid Midwest currently consists of a single preplant application, usually in autumn before the year ied, 72% of the water quality problems were attributedto agriculture. Plant nutrients have been identified as that corn is grown. This practice was promoted by agricultural experts because the potential for soil compacsurface-water contaminants throughout the Midwest (Ba- ker, 1988; Thurman et al., 1992; USEPA, 1992; Goolsby tion following harvest is generally less, labor is oftenmore available, weather and soil conditions are generand Battaglin, 1993). Nitrate N concentrations in excess of 10 mg L

 1 in drinking water may pose risks to humans ally more favorable, and fertilizer prices are frequently lower than in the spring. However, fall application and livestock (USDA, 1991; Tyson et al., 1992) and have cost some communities millions of dollars for their places the applied N in the soil several months before the crop needs it, and thus increases the potential for removal or to provide alternate drinking water sources. For example, Des Moines, IA alone has spent in excess of leaching or other losses. Sanchez and Blackmer (1988) reported that 49 to 64% of the fall-applied fertilizer N $4.8 million for NO 3 removal from drinking waters be- tween 1991 and 1999 (G. Benjamin, unpublished data, was lost from the upper 1.5 m of the soil profile through pathways other than plant uptake. 2000).

Nitrogen loadings into the Mississippi River and its Changing the timing of a single preplant fertilizerapplication from fall to spring could significantly detributaries have also been identified as a potential cause for degradation of freshwater and marine ecosystems. crease N loss and increase fertilizer use efficiency. This was demonstrated for southern Minnesota (Randall et Elevated N concentrations have altered natural aquaticfloral and faunal population dynamics, exacerbated oc- al., 1992; Randall, 1997) where spring application increased N use efficiency by more than 20% compared currences of hypoxia and anoxia, and sped the process of eutrophication in the Gulf of Mexico (Alexander et with fall N application. In addition, annual NO 3 losses from tile drainage were reduced by an average of 36%. al., 1995; Rabalais et al., 1996). Turner and Rabalais (1991) reported that increased levels of NO 3 in the Mis- Despite the opportunities for increased nutrient use efficiency and decreased loss of N through drainage waters, sissippi River have paralleled increased use of commer-cial fertilizer throughout the river basin and increased many farmers continue fall fertilizer applications to minimize real and perceived risk. Spring rainfall patterns severity of hypoxic events in the Gulf of Mexico since the 1950s. Alexander et al. (1995) estimated that drain- can result in very wet soils and prevent or delay N fertilizer applications. This risk is very real because there age from the Upper and Central Mississippi Basins (in- cluding portions of Minnesota, Wisconsin, Iowa, Mis- are few options in most rainfed farming operations to compensate the farmer for yield losses and reduced souri, and Illinois) accounts for 39% of the N deliveredto the Gulf. This is the largest estimated source fraction income associated with an inadequate N supply. Therefore, to achieve significant farmer adoption of N manamong the Mississippi River’s various drainage basins. agement practices other than fall fertilization, concepts such as insurance policies against N deficiencies are 

### STRATEGIES FOR IMPROVED NITROGEN needed along with more flexible and efficient applica

### MANAGEMENT
tion methods. 

### Timing of Application and Nitrogen Rates Power et al. (2000) reported that midwestern fields frequently have a high degree of variability in soil NO 3 Many attempts to reduce NO 3 concentrations in shal- content from site to site within a single field. They stated low ground-water draining to tiles have focused on tim- that “. . . soils are seldom uniform throughout a field, ing and N application rate. Randall (1997) stated that so applying sufficient fertilizer N to assure high yields “. . . fertilizer N management, particularly rate and time for more productive areas of the field often results in of application, plays a dominant role in the loss of nitrate over-fertilization of the less productive areas. This may to surface waters.” The challenge is to manipulate N lead to greater nitrate leaching, particularly in those availability before, during, and after peak crop demand. areas of the field that are more susceptible to leaching.” Nitrogen molecules are susceptible to leaching, denitri- Kranz and Kanwar (1995) estimated that within a given fication, volatilization, and immobilization processes within the soil environment. The risk of N losses due field, 70% of the NO 3 leached typically comes fromDINNES ET AL.: STRATEGIES TO REDUCE NITRATE LEACHING IN MIDWESTERN SOILS 157

 30% of its area. To be effective, variable-rate technology must accurately identify within-field spatial and temporal variability and reliably interpret these patterns of variability (Sawyer, 1994). If this is accomplished, areliable variable-rate or site-specific N fertilizer application program could reduce application on areas requiring little to no additional N to optimize yield (Dinnes et al., 1998) and could limit application on areas sensitive to leaching and surface runoff (Larson and Robert, 1991). Methodologies to accurately assess within-fieldspatial and temporal variability for subsequent N application, however, have yet to be fully developed.

Most conventional methods of N fertilizer recommendation were developed on a state or regional scale, so it is questionable whether these methods can reasonably Fig. 3. General seasonal patterns for precipitation, N uptake rate by be used for variable-rate N management that attempts a corn crop, cropping system water use, and periods potentially to account for within-field spatial and temporal variabil- favorable for NO 3 leaching from midwestern corn production ity (Hergert et al., 1997). Several research studies have (adapted from Fig. 4 of Power et al., 1998).found large differences in crop yield and crop N response within individual fields (Ferguson et al., 1995; to slow NO 3 accumulation include use of nitrification Kitchen et al., 1995; Vetch et al., 1995), confirming the inhibitors (Kidwaro and Kephart, 1998) or temporary need for reliable methods to generate site-specific N immobilization by cycling N through cover crops (Doran recommendations (Hergert et al., 1997). and Smith, 1991; Staver and Brinsfield, 1998; Wagger

Pierce and Nowak (1999) discussed three basic man- et al., 1998). Minimizing NO 3 accumulation until the agement approaches currently being tested for variable- primary crop is actively growing can generally reduce rate N application. The first involves determining plant- leaching loss because, by that time, plant water use will available N levels from field grid sampling and interpre- generally exceed available precipitation (Hatfield et al., ting N rates based on current recommendation methods 1998; Power et al., 1998). (i.e., a N balance equation). The second approach bases Monitoring N mineralization to better match the reN rates on observed crop N responses using replicated quired amount of available N with crop needs is one strips with varying N rates across the landscape. The strategy for reducing NO 3 leaching potential. To accomplish this task, several versions of a presidedress soil third approach involves determining crop N status by nitrate test (PSNT) (Magdoff et al., 1984; Fox et al., monitoring (i.e., light reflectance or chlorophyll con- 1989; Magdoff et al., 1990) or modifications such as the tent). Usually this intervention-type approach uses a late-spring nitrate test (LSNT) (Blackmer et al., 1997) portion of the crop that is well fertilized as a standard have been developed. These tests generally involve samfor comparison. The best approach for a particular field, pling the soil approximately 6 wk after planting. The be it conventional or site-specific, will depend on the philosophy behind the tests is that by late spring, the net amount of spatial and temporal N variability (Pierce effects of mineralization, leaching, and other potential and Nowak, 1999). Pan et al. (1997) reported that tem- losses that may have occurred since the last crop was poral N variability could frequently exceed spatial vari- harvested can be accurately assessed. Soil test results ability. While highly predictable spatial variability may can then be used to determine the appropriate amount be amenable to multiple approaches of site-specific N of additional N fertilizer to apply. management, strong temporal variability is much more Plot-scale studies using PSNT or LSNT strategies to difficult to manage (Pierce and Nowak, 1999). In this determine fertilizer N rates have generally shown reduccase, Pierce and Nowak (1999) concluded that an inter- tions in measured or potential NO 3 leaching. In Iowa, vention strategy would be the most appropriate option. these procedures resulted in fertilizer N applications ranging from 50 to 168 kg N ha

 1 and significantly re- 

### Crediting Nitrogen Mineralization duced NO 3 loss to subsurface drainage tiles compared Nitrate, as an end product of mineralization and sub- with single preplant applications of only 112 kg N ha

 1 sequent nitrification of SOM, manure, crop residue, or (Kanwar et al., 1996). A study in Vermont (Durieux et previously applied fertilizer N that has cycled through al., 1995) also reduced leaching potential compared with soil organic N pools, can make significant contributions a yield-goal N management strategy. In this 3-yr study, toward meeting crop N requirements (Oberle and the PSNT–LSNT N management program resulted in Keeney, 1990; Kanwar et al., 1996). However, to ensure fertilization rates of 112, 123, and 123 kg N ha

 1 and

NO 3 is used efficiently, it is important to quantify the corresponding residual soil NO 3 levels in the upper 1.2 amount being produced and, where possible, minimize m of the soil profile of 87, 68, and 44 kg N ha

 1 in the its accumulation during noncropping periods. This strat- fall. In contrast, the yield-goal management program egy is important when crop uptake is minimal or nonex- required 168 kg N ha

 1 and resulted in residual soil NO 3 istent because much of the NO 3 produced during those levels in the upper 1.2 m of the soil of 138, 160, and 156 kg N ha

 1 , respectively. These plot-scale results suggest periods is susceptible to leaching (Fig. 3). Strategies158 AGRONOMY JOURNAL, VOL. 94, JANUARY–FEBRUARY 2002

that the PSNT–LSNT approach for N management has Hanna et al. (1999) measured anhydrous NH 3 distribution for several manifold designs during field applicathe potential for avoiding excess N application com- pared with yield-goal approaches (Magdoff, 1991; Du- tion at three flow rates (56, 112, and 168 kg N ha

 1 ). This experiment accounted for dynamic vibration of the rieux et al., 1995; Kanwar et al., 1996; Randall, 1997; Karlen et al., 1998). However, while designed to provide applicator’s distribution system, which can affect the anhydrous NH 3 vapor–liquid phase separation within an optimum N fertilizer rate at an appropriate time toimprove crop N use efficiency, the impacts on water the manifold. They found that a Vertical Dam manifold had less variability than a conventional manifold quality and risk to the farmer for adopting this approach have been poorly quantified at the whole-field scale and at the 56 kg N ha

 1 rate. At the two higher N rates, they detected little difference in variability between the have only begun to be evaluated at the watershed scale.

A regional study (Bundy et al., 1999) determined that two designs. When examining flow rates by manifold outlet location in relation to incoming flow, Hanna et the PSNT–LSNT program at the watershed scale failed to identify some field sites where a yield response to N al. (1999) found that the highest flow rates came from outlet ports located directly across from the incoming fertilizer was not achieved. Their follow-up investiga- tion found that such errors were reduced by increasing flow port. Next highest flow rates came from outlet ports near the incoming flow port. They theorized that the soil-sampling depth from 30 to 60 cm (Bundy et al., 1999). The end-of-season basal stalk NO 3 test (Blackmer this was due to flow reflected from the opposite side of

the manifold chamber. The least amount of flow came and Mallarino, 1997) has also been used to verify the accuracy of the PSNT–LSNT program. Stalk NO 3 con- from outlet ports located on both sides perpendicular to the incoming flow path. The conventional manifold centrations are categorized into four groups: low (  250 mg kg

 1 NO 3 ), marginal (250–700 mg kg

 1 NO 3 ), opti- exhibited these traits to a stronger degree than alternative designs, but all manifolds showed similar patterns. mal (700–2000 mg kg

 1 NO 3 ), and excess (  2000 mg kg

 1 NO 3 ). Although the information does not assist in Liquid N fertilizer applicators tend to have less variation in fertilizer distribution across all outlets along the correcting in-season deficiencies, it is useful for guiding future operations to improve N use efficiency. This test implements’ toolbars than conventional anhydrous NH 3 applicators. Recently, liquid N fertilizer applicators is helpful in assessing the performance of any N man-agement program for corn production (Magdoff, 1991; have become even more accurate with the use of hydraulic flow rate control devices. With careful installation Blackmer and Mallarino, 1997). and calibration, applied rates can easily be within 1 to 4% 

### Fertilizer Application Equipment of the target rate (Dinnes et al., unpublished data, 2000). Some recent innovations in the design and function When measuring the efficiency of any system, the of N fertilizer application equipment have focused on variation or error of each step is additive. Therefore, strategies to limit N losses from leaching, volatilization, the overall efficiency is only as good as the summed and runoff. One, a localized compaction and doming margins of error for each step. For site-specific N man- (LCD) applicator (Ressler et al., 1997), was developed agement, attempting to apply variable N rates will do to alter soil physical properties immediately above the little to improve overall N fertilizer use efficiency if the soil volume where knife-injected liquid N fertilizer was application equipment has a high degree of error. This placed. A compacted zone with a higher bulk density problem was confronted in studies examining the per- is created above the injection zone so that water infil- formance of conventional anhydrous NH 3 manifolds. tration through that area would be reduced compared Weber et al. (1993) found appreciable degrees of error with a conventional knife-injection applicator. When over entire fields, and other investigators documented compared with either conventional knife or broadcast large degrees of error across the individual outlets of methods of N application, the LCD design reduced the the equipments’ manifolds (Reichenberger, 1994; Fee, amount of NO 3 leaching following an intense rainfall 1997; Schrock et al., 1999). that occurred soon after fertilizer application (Ressler

Research efforts have concentrated on evaluating et al., 1998). new technology to reduce equipment application error. Another application technique developed to imBoyd et al. (2000) conducted an evaluation of anhydrous prove N management was the point injector, which un- NH 3 manifolds under field conditions at two target apder no-till conditions, was demonstrated to have the plication rates (84 and 168 kg N ha

 1 ). Several anhy- potential to reduce NH 3 volatilization and N immobili- drous NH 3 manifold entry methods were used for conzation at the surface without destroying surface residues ventional manifolds, and although each method had or adversely affecting corn yield (Baker et al., 1985). In differing coefficients of variation at both N rates, all Minnesota (Randall et al., 1997b), point injection of had similar trends, with greater coefficients of variation urea ammonium nitrate resulted in greater corn grain at 84 kg N ha

 1 (12–80%) than at 168 kg N ha

 1 (10– yield and total N uptake compared with surface-broad66%). Alternative manifold designs typically performed cast or banded urea ammonium nitrate in a ridge tillage better than conventional manifolds at both rates, with system. Additional agricultural drainage studies in Iowa less variability at the high rate. Rotaflow and small- from 1995 through 1997 (Iowa Dep. of Agric. and Land housing Vertical Dam manifolds produced the least Stewardship, 1997) showed that both LCD and point amount of variability across all outlets at both the low injection methods consistently reduced NO 3 concentraand high N rates. A John Blue FD-1200 manifold had a low variability at the higher rate. tions in tile drainage water compared with conventionalDINNES ET AL.: STRATEGIES TO REDUCE NITRATE LEACHING IN MIDWESTERN SOILS 159

knife-injection methods. In addition to having the lowest 1988). The variable results with nitrapyrin use are probably due to differences among factors affecting the miNO 3 concentrations in the drainage water, the LCD ap- plication method also resulted in the highest corn grain crobial process of nitrification (e.g., climate and soil type). Further research should focus on quantifying yield.

those factors to determine when and where the use of nitrapyrin will be most beneficial. 

### Nitrification Inhibitors

Nitrification inhibitors for N fertilizers have met with 

### Chlorophyll Monitoring varying success, generally depending on soil type and Other methods of monitoring N availability for corn weather pattern under which they were used. These have also been used to guide in-season fertilizer applicainhibitors function by limiting the activity and popula- tions. In some studies, chlorophyll measurements have tion of Nitrosomonas bacteria that convert NH 4 to NO 2
. been shown to correlate well with N concentrations in The primary use for nitrification inhibitors in the Upper the plant tissue and to have the ability to predict grain Midwest is to slow the conversion of fall-applied anhy- yield (Wood et al., 1992; Siambi et al., 1999). In Pennsyl- drous NH 3 fertilizer to the more leachable NO 3 form, vania, Piekielek and Fox (1992) showed that the chlorothus potentially reducing N fertilizer losses before peak phyll meter measures were as accurate as several soil N demand by subsequent corn crops. N availability tests (NO 3 concentration of the surface An 8-yr project in Ohio (Stehouwer and Johnson, 20 cm of soil at planting, ultraviolet absorbance at 200 1990) examined different application timings with and nm of a 0.01 M NaHCO 3 extract of the surface 20 cm without a nitrification inhibitor. The results showed that of soil at planting, and PSNT) in identifying N responat similar N rates, spring preplant application of urea sive and nonresponsive sites. However, the chlorophyll [(NH 2 ) 2 CO] or anhydrous NH 3 produced higher yields meter did not correlate well enough with soil N-supply- than fall applications. Addition of nitrapyrin [2-chloro- ing capacity (in-season mineralization) to accurately 6-(trichloromethyl)-pyridine] as a nitrification inhibitor make N sidedress recommendations. Chlorophyll meter with spring-applied N had no effect on grain yield but in combination with a sufficiency index was very success- did increase yields associated with fall N applications. ful as an in-season N management strategy for irrigated Similarly, in Minnesota, Randall et al. (1992) found that corn in Nebraska (Varvel et al., 1997a). The chlorophyll corn yield and N use efficiency were lowest with fall N meter identified when additional N fertilizer was re- applications without nitrapyrin, highest with spring N quired, but it did not estimate the amount needed. A applications, and intermediate with fall N plus nitra- sufficiency index was calculated [(treatment/well-fertil- pyrin. They reported N use efficiency of fall-applied N ized control) 100], and in-season N fertilizer applica- fertilizer was 16%, whereas with nitrapyrin, it improved tions were made when index values were below 95%. to 26%. Spring applications, with or without nitrapyrin, Their N adjustment was to hand-apply 30 kg N ha

 1 had fertilizer N use efficiencies ranging from 42 to 48%. and water it within 24 h of when the chlorophyll meter The highest NO 3 losses associated with corn production measurements were taken and the sufficiency index fell occurred with fall N applications, with or without ni- below 95%. trapyrin. Currently, it is unlikely that chlorophyll meter data

The economics of nitrapyrin use have shown mixed alone will be sufficient to guide N fertilizer applications results. In a 7-yr study by Christensen and Huffman in subhumid regions. Factors other than N availability (1992), nitrapyrin in spring-applied N fertilizer signifi- affect leaf chlorophyll content and can confound chloro- cantly increased corn grain yield, with the increase more phyll meter data, thus limiting its applicability as a N than compensating for the added cost of the nitrapyrin. test (Piekielek and Fox, 1992). Piekielek and Fox (1992) In Minnesota, 7-yr averages of corn fertilized with 150 pointed out that leaf chlorophyll content could be af- kg N ha

 1 showed that yield was increased 5% by fall- fected by any of the following: plant leaf chlorosis due applied N with nitrapyrin, 5% by spring preplant N, to nutrient deficiencies other than N (i.e., K and Mg), and 10% by split-applied N (40% preplant and 60% disease, insect damage, cold temperatures, too high or sidedressed at V8 corn growth stage) compared with too low plant populations on soils with marginal N avail- fall-applied N without nitrapyrin (G.W. Randall, per- ability, and recent N fertilizer application. These authors sonal communication, 2000). Economic returns for the also stated that for soils with large organic N pools in 7-yr study were split-applied N ($239.40 ha

 1 yr  1 )  areas of highly variable weather conditions, any test that spring preplant N ($210.90 ha

 1 yr  1 )  fall-applied N measures plant N content at the six-leaf stage will not with nitrapyrin ($192.40 ha

 1 yr  1 )  fall-applied N always be indicative of soil N availability for the remain- without nitrapyrin ($166.70 ha

 1 yr  1 ). Although the der of the growing season nor always relate to final yield. nitrapyrin treatment did not result in an economic ad- Excess soil water content is another major factor in vantage for the spring and split N application treat- subhumid regions with high organic matter soils that can ments, the results indicate that for farmers who continue confound developing relationships between chlorophyll to fall-apply N, there is a positive economic return for meter data and plant N status. In the Upper Midwest, using nitrapyrin. However, a number of other studies poorly drained prairie potholes comprise an appreciable have found no significant yield effect nor economic ad- amount of the total cropland area. Even with surface vantage with the use of nitrapyrin (Hendrickson et al., inlets to subsurface tile drainage lines, these areas often have periods during spring and early summer when wa- 1978; Touchton et al., 1979; Blackmer and Sanchez,160 AGRONOMY JOURNAL, VOL. 94, JANUARY–FEBRUARY 2002

ter is ponded on the surface. Under such conditions, though the amount of reduction can be minimal, demany plant species often become chlorotic because of pending on climatic conditions (Randall et al., 1997a). aeration stress rather than insufficient plant-available With soybean, the leaching potential is reduced most N (Hocking et al., 1987). Lizaso and Ritchie (1997) when it is between growth stages V4 and R5, but leach- reported a three to seven times greater rate of senes- ing can be quite high during the early spring if large cence and loss of green leaf area in corn under ponded quantities of residual NO 3 remain following the corn conditions compared with a control at the 12-leaf stage. crop. Significant NO 3 leaching can also occur following Undoubtedly, denitrification and leaching can deplete soybean (Randall et al., 1992; Baker and Melvin, 1994; NO 3 when water is ponded on these areas, but most of Kanwar et al., 1996; Jaynes et al., 1999), especially if the pothole soils contain very high levels of organic significant mineralization occurs when soils are not fro- N, which can become available to the plant through zen during the late fall, winter, or early spring. mineralization later in the growing season (Cambardella Including perennial legume or nonlegume crops in et al., 1994). This situation has been observed in the rotations has also been shown to decrease NO 3 losses. preliminary results from our watershed-scale N manage- In Iowa, Baker and Melvin (1994) documented much ment research project (Walnut Creek N Initiative) lower NO 3 –N concentrations beneath alfalfa than for where the prairie pothole soils have been unresponsive corn or soybean. Also, in Minnesota, Randall et al. to additional N fertilizer (Dinnes et al., 1998) despite (1997a) measured NO 3 –N concentrations in drainage having lower corn leaf chlorophyll levels when mea- water from alfalfa fields and Conservation Reserve Pro- sured at V9 to V12 growth stages (Dinnes et al., unpub- gram (CRP) lands cropped to a mixture of alfalfa and lished data, 2000). It also suggests that one reason for the perennial grasses and found they were 37 and 35 times success of the chlorophyll meter method under irrigated lower than in drainage water from corn and soybean conditions is the absence of chlorosis from excess soil fields, respectively. They attributed the differences to water and the fact that the soils are generally well longer growing seasons and greater annual evapotransdrained and have relatively low SOM levels. piration in fields with perennials because both of these

Insufficient soil water content can also confound in- processes contribute to greater N uptake and less drainterpretation of chlorophyll meter measurements. age than in fields with only annual crops. Differences Leaves under water stress can have changes in the red and near-infrared (NIR) reflectances and because the in fertilizer management between annual and perennial chlorophyll meter computes the reading as the ratio of cropping systems also impact their relative NO 3 – red/NIR, deviations away from normal will affect this leaching potentials. Typically, perennial cropping sysratio in the absence of N stress. Red and NIR reflec- tems receive less tillage and N fertilizer than do annual tances change with age of the leaf, with the greater cropping systems. change in the NIR (Gausman et al., 1970). Observations Factors contributing to differences in NO 3 –leaching of leaf reflectance from corn grown under N and water potential for various crop rotations extend beyond fertil- stress showed that NIR was more affected by water izer practices. Interactions between hydrology and till- stress (Hatfield and Prueger, unpublished data, 2000). age are very important because any residual NO 3 that Changes in leaf reflectance with water stress have been accumulates in the soil profile, whether from N fertilizer known for several years, as shown by Thomas et al. or microbial processes, can be leached if it is not assimi- (1971), who found leaf water content could be estimated lated by microbes decomposing crop residue or taken through reflectance measurements. Chlorophyll meter up by another plant. When a crop such as alfalfa depletes readings can be affected by leaf water status, but the profile water content and the amount of precipitation change induced would predict lower N requirements is not sufficient to fully recharge the profile, the leaching because the meter readings would increase under long- potential will be minimal and very little water will be term water stress. moving into subsurface drainage lines. Differences in

The chlorophyll meter has been successfully used in residue and root decomposition relationships, as well conjunction with the end-of-season basal corn stalk NO 3 as soil–plant–water dynamics (i.e., soil water extraction test (Blackmer and Mallarino, 1997) to guide and im- capacity), among various plant species also influence prove future soil sampling for site-specific N fertilizer the leaching potential (Baker and Melvin, 1994; Randall recommendations for irrigated corn (Varvel et al., et al., 1997a; Malpassi et al., 2000). The rate of N cycling 1997b). By combining both monitoring techniques, field is important because although N-fixing legumes can reareas with different soil N pools and N mineralization lease large quantities of N to soils over time, organic N potential can be identified and sampled separately. The derived from plant and microbial residues is not as rapPSNT–LSNT program or any future soil tests developed idly available to plants as inorganic N provided by most to better predict N mineralization potentials could then be used to determine optimum fertilizer N rates for commercial fertilizers. Additionally, the gradual release those areas. This approach could assist in creating a of organic N is often better synchronized with subsereliable variable N rate protocol for areas where field- quent plant needs and microbial population dynamics scale variability in soil N content is very high (Cambar- than point-in-time applications of N fertilizers. The large della et al., 1994). flush of available N following an inorganic fertilizer N application can often supply more N than can be 

### Diversified Crop Rotations assimilated by plants and microbes. When this pool is nitrified, large amounts of NO 3 are susceptible to leachChanging from continuous corn to a corn–soybean

rotation has been shown to reduce NO 3 leaching al- ing and can potentially contaminate surface and groundDINNES ET AL.: STRATEGIES TO REDUCE NITRATE LEACHING IN MIDWESTERN SOILS 161

water resources. These processes are not limited to row 

### Tillage Impact on Nitrogen Cycling crop areas because research also suggests that if exces- Although tillage affects N cycling within soils, mid- sive N is applied to either annual or perennial forage western farmers generally do not intentionally use till- crops, water quality can be degraded by a resultant in- age to manage N. In more arid regions of the USA crease in soil NO 3 concentrations (Anderson et al., 1997). where fallow was traditionally used every second or third year, increased N availability following the fallow 

### Cover Crops period was often more important than the perceivedwater conservation associated with that soil manageCover crops have been shown to reduce the potential ment practice (Haas et al., 1974). Tillage alters the soil for NO 3 leaching from farm fields (Magdoff, 1991; environment by aerating the zone of disturbance and Staver and Brinsfield, 1998) by mimicking natural eco- increasing the availability of O 2 to soil microorganisms. systems such as prairies where some plant species are This favors different microbial species, populations, and growing whenever the ground is not frozen. They func- processes than a nontilled soil (Doran, 1987). The nettion by accumulating the inorganic soil N between mainresult of tillage is increased aerobic microbial activity, crop seasons and holding it in an organic form, thus leading to elevated oxidation of SOM and mineralizapreventing it from leaching (Magdoff, 1991; Staver and tion of soil N (Randall et al., 1997a). This N mineralizaBrinsfield, 1998). The N is subsequently released to the tion response, often associated with preplant tillage, is next crop as the cover crop residue decomposes. Cover also a benefit associated with using cultivation for weed crops also protect against soil erosion (Dabney, 1998; control during the growing season. Kaspar et al., 2001), increase SOM (Reicosky and For- Depending on tillage to release N for crop production cella, 1998), and suppress weed growth (Buhler et al., is generally not a wise soil management practice. From 1998; Lal et al., 1991). a soil quality perspective, it reduces the benefits of SOM

Desirable attributes for midwestern cover crops in- such as cation exchange capacity, soil structure, and clude the ability to establish rapidly under less-than- water retention capacity merely for the release of plantideal conditions, grow vigorously despite cool tempera- available N. It also exposes the soil surface to wind and tures and decreasing daylength, and not inhibit the water forces, thus increasing the potential for increased growth of subsequent row crops. The biggest problem erosion (Reicosky et al., 1995). Furthermore, depending facing cover crops in this region is the short and gener- on seasonal weather patterns, temperature, and rainfall, ally cool growing season between harvest and planting tillage during autumn or early spring can cause N minerof the subsequent row crop. In studies reviewed by Mei- alization too early and increase the potential for NO 3 singer et al. (1991), cover crops reduced both the mass leaching before subsequent crops have an opportunity of N leached and NO 3 concentration of leachate 20 to to assimilate the N provided by these processes. 80% compared with no cover crop control. They also Effects of tillage on N management have been demondetermined that grasses and brassicas were two to three strated in studies comparing no-till with conventional times more effective than legumes in reducing NO 3 tillage at several midwestern locations. In a long-term leaching. Rye ( Secale cereale L.) has been used success- Minnesota study (Randall and Iragavarapu, 1995), refully as a cover crop in the northern Corn and Soybean
sidual soil NO 3 contents in the 0- to 1.5-m soil profile Belt. But because rye overwinters, it must be killed or were significantly higher with conventional tillage than it can reduce the yield of subsequent corn crops by using no-till for 5 out of 11 yr and were not significantly too much water in the spring or immobilizing soil N different for the other 6 yr. Average flow-weighted (Munawar et al., 1990; Karlen and Doran, 1991; Tollen-

NO 3 –N concentrations were 13.4 and 12.0 mg L

 1 for aar et al., 1993; Johnson et al., 1998). Other evidence conventional and no-till corn production treatments, resuggests that the corn yield depression caused by the spectively. Furthermore, while the no-till treatment had rye cover crop may be due to allelochemicals (Tollenaar 12% greater subsurface drainage flow than the convenet al., 1993). Further research may identify rye geno- tional treatment, NO 3 losses were marginally greater types that do not release these compounds. Oat has (about 5%) with conventional tillage. Although insignifbeen demonstrated to be an effective cover crop for icant, these results suggest a minimal trend toward this region because seed is easy to obtain and inexpen- greater NO 3 losses with conventional tillage in this study. sive, fall growth is more vigorous than rye, and it winter-
The authors concluded that NO 3 losses through tile kills, thus eliminating the need for herbicide or tillage drainage depend more on growing-season precipitation in spring (Johnson et al., 1998). than on tillage. Recently, Randall and Mulla (2001) Other plant species, including legumes, cereals,

concluded that NO 3 losses from agricultural fields are grasses, and brassicas, have been evaluated and used as minimally affected by differences in tillage systems comcover crops, but these species have generally not been pared with N management practices. successful as true cover crops in the northern Corn and In Iowa, Kanwar et al. (1993) monitored NO 3 leaching Soybean Belt. One exception was a study done in Wis- beneath both continuous corn and corn–soybean rotaconsin where hairy vetch ( Vicia villosa Roth) and red tions managed using moldboard plowing, chisel plowing, clover grown in an oat–corn rotation showed a favorable ridge tillage, and no-tillage practices. The 3-yr average economic comparison with continuous corn fertilized

NO 3 –N concentration in drainage water from continu- with 180 kg N ha

 1 commercial N fertilizer (Stute and Posner, 1995). ous corn plots receiving moldboard tillage was signifi-162 AGRONOMY JOURNAL, VOL. 94, JANUARY–FEBRUARY 2002

cantly greater (35.8 mg L

 1 ) than for either no-till or Disturbance of plant residue through tillage can significantly increase the decomposition rates (Douglas et ridge tillage treatments (22.2 and 21.8 mg L

 1 , respectively). Similar trends were observed for the corn–soy- al., 1980; Doran, 1987; Holland and Coleman, 1987; Aulakh et al., 1991). Additionally, root residues may rebean rotation where the 3-yr average NO 3 –N concentra- tions in drainage water were 19.0, 13.4, and 13.9 mg spond differently to tillage or disturbance than shoot residues. For example, Martin (1989) observed that de- L

 1 for moldboard plow, no-tillage, and ridge tillage treatments, respectively. However, due to a higher vol- composition of root residues was more rapid and more complete when they were left undisturbed in the soil ume of water moving through the soil with no-till and chisel plow, the 3-yr average NO 3 –N losses in subsurface than when air-dried roots were mixed with moist or airdried soil. drain flow were greater with no-till and chisel plow systems (61.2 and 64.3 kg ha

 1 , respectively) than with Even in the absence of disturbance, decomposition rates of root and shoot residue may differ. In a labora- moldboard plow (45.8 kg ha

 1 ) under continuous corn.Nitrate N load losses by tillage method for the corn– tory simulated no-till experiment, Gale and Cambardella (2000) found that 75% of the new C inputs into soil soybean rotation produced trends that differed from those of continuous corn. Under the corn–soybean rota- after 1 yr of decomposition were root derived and 25% were shoot derived. They concluded that accrual of soil tion, greatest NO 3 –N losses were observed with chisel plow (32.1 kg ha

 1 ), followed by moldboard plow (27.5 organic C associated with no-till is primarily due to the greater retention of root-derived C in the soil. kg ha

 1 ), no-till (23.9 kg ha

 1 ), and ridge tillage (23.7 kg ha

 1 ). These results led Weed and Kanwar (1996) to conclude that despite having an influence on N miner- 

### NITROGEN REMOVAL STRATEGIES alization, tillage had less impact than crop rotation. If agricultural management of N by all in-field means (e.g., crop rotations, cover crops, fertilizer application 

### Plant Residue Management best management practices, and tillage) cannot satisfac- Management of plant residues is directly related to torily reduce NO 3 concentrations, alternative strategies tillage and affects N cycling in soils because plant tissue may be needed to remove NO 3 from subsurface drain- is a primary source and sink for C and N. Normally, age, shallow ground water, and/or surface water. Nu- when plant residues with C/N ratios greater than ap- merous methods for removing NO 3 from water have proximately 20:1 are added to the soil, available N is been identified. These include ion exchange, biological immobilized during the first few weeks of decomposi- denitrification and assimilation, chemical denitrifica- tion (Sinha et al., 1977; Doran and Smith, 1991; Somda tion, reverse osmosis, electrodialysis, and catalytic deni- et al., 1991; Green and Blackmer, 1995). Green et al. trification (Kapoor and Viraraghavan, 1997). Of these, (1995) observed that incorporation of corn stover into only biological denitrification and assimilation seem soil resulted in rapid immobilization of all available suitable and adaptable for removing NO 3 from subsur- inorganic N during the decomposition period. This oc- face drainage in agricultural fields and watersheds. Mul- curred because the microbial population decomposing tiple basic strategies that rely on biological denitrifica- the plant residue had increased its biomass in response tion and/or assimilation as either primary or secondary to the C source. If such immobilization occurs when mechanisms for reducing NO 3 concentrations in shallow plants need N for growth and development, the avail- subsurface waters have been and continue to be re- ability of NO 3 for the plants may be reduced. When this searched. Several are discussed below. involves the primary crop, the immobilization process can have a negative economic impact by ultimately lim- 

### Buffersiting yield. Eventually, as residue decomposition proResearch on effects of riparian buffers on shallow ceeds, the C/N ratio will begin to approach that of SOM ground-water quality has been conducted in many areas ( ≈ 10:1 or 12:1), microbial biomass will decrease, and N
across the USA and overseas. A consistent conclusion from plant residues that was incorporated in the micro- of research projects on riparian buffers is that the buffers bial biomass will once again be released into the soil.
decrease the NO 3 concentration of the shallow ground However, if temperature, water content, or other factors water flowing through them, in many cases to a dramatic slow the residue decomposition process, N may not be degree, ranging from 48 to 100% (Peterjohn and Correll, released from the plant residue or microbial biomass 1983; Lowrance et al., 1984; Jacobs and Gilliam, 1985; until the primary crop has matured and stopped assimi- Haycock and Pinay, 1993; Jordan et al., 1993; Hubbard lating N. This was hypothesized by Varvel and Peterson and Lowrance, 1997; Verchot et al., 1997; Snyder et al., (1990) as a factor affecting continuous corn production 1998; Addy et al., 1999; Rickerl et al., 2000; Spruill, when they estimated that 80% of the applied fertilizer 2000). However, there are differences among these studN was still immobilized in crop residues, SOM, and ies as to the reported primary mechanisms responsible microbial biomass at the end of the growing season.

for these reductions. This emphasizes the importance of understanding all of One potential contributing factor to reduced NO 3 the factors affecting plant residue decomposition and concentrations in shallow ground water is dilution. Prehow they might be manipulated to reduce the potential cipitation that infiltrates in the buffer area can contain for NO 3 leaching without decreasing availability of N

low NO 3 concentrations compared with the shallow to the primary crop or adversely affecting the soil C and N pools. ground water originating from the adjacent cropland.DINNES ET AL.: STRATEGIES TO REDUCE NITRATE LEACHING IN MIDWESTERN SOILS 163

Most studies have reported insignificant effects of dilu- Although riparian buffers have repeatedly been proven very effective at removing NO 3 from shallow tion on the observed NO 3 concentration reductions, but there have been a couple of exceptions. In a study of ground water, any field tile drainage line that passes through a buffer and empties directly into a surface various riparian forest management techniques on NO 3 reduction in the Georgia coastal plain, Hubbard and water stream will bypass all NO 3 remediation benefits of a riparian buffer. In order to utilize riparian buffer Lowrance (1997) found that dilution did play a role in reducing NO 3 concentration of the shallow ground water technology in tile-drained areas, alterations would be required. One option would be to terminate tile lines from a clear-cutting management treatment. Spruill (2000) found a 95% reduction in NO 3 concentration of before entering a buffer zone. The tile flow would then seep through the buffer to the stream. Though effective, young ground water in riparian buffer areas and esti- mated that 30 to 35% of the reduction was due to dilu- this strategy would severely limit the drainage capacityof the tile. A more appropriate option may be to distion effects. The remaining 65 to 70% reduction of NO 3concentration was attributed to other processes. charge the tile effluent to a wetland retention area adjacent to the riparian buffer to treat the tile drainage Vegetative assimilation is one potential biological fate

of NO 3 in shallow ground water as it passes through a before it enters the stream. riparian buffer. Forest trees, and the grass strips that are frequently placed between the trees and cropland, 

### Wetlands do assimilate NO 3 from the shallow ground water. It is Natural and constructed wetlands have been used the degree to which each vegetative group assimilates successfully as biological treatment systems for NO 3 re- NO 3 that varies among the research studies. Lowrance moval (Gersberg et al., 1983; Crumpton et al., 1995; (1992) measured a seven- to ninefold decrease in the Romero et al., 1999). Eriksson and Weisner (1997) con- shallow ground water NO 3 and NO 3 /Cl ratio within the firmed that epiphytic biofilms on submerged vegetation first 10 m of riparian forest buffer. He attributed this could remove NO 3 by denitrification. In a laboratory change in NO 3 concentration primarily to forest vegeta- wetland microcosm experiment, Ingersoll and Baker tion assimilation because the denitrification potentials (1998) determined NO 3 removal efficiencies at two temof the soils at the typical depths of the water table were peratures (28 and 35  C), varying hydraulic-loading rates too limited to account for the dramatic losses of NO 3
. (5–20 cm d

 1 ) and C additions (1–6 g wk

 1 dried plant Hubbard and Lowrance (1997) also determined that residue) with NO 3 –contaminated water (30 mg L

 1 vegetative assimilation was a significant sink of shallow NO 3 –N). They measured NO 3 removal efficiencies from subsurface-flow NO 3 . They reported that a grass buffer
8% to  95%, which decreased with increasing hydrau- assimilated some of the NO 3 but that a downslope ripar- lic-loading rates and increased with increasing C addi- ian forest was more efficient at assimilating NO 3 from tion rates. Nitrate removal efficiencies increased as the shallow ground water. Other studies (Jacobs and Gil- C/N ratio increased to 5:1, and at C/N ratios  5:1, NO 3 liam, 1985; Verchot et al., 1997; Addy et al., 1999) re- loss was nearly complete. One conclusion was that the ported that vegetative assimilation did not play a signifi- denitrification rate constant directly depended on the cant role in reducing NO 3 concentration of shallow C addition rate. Therefore, the authors suggested that ground water. In these reports, denitrification was iden-
a wetland could become more efficient at NO 3 removal tified as the main loss mechanism. if its plant growth were to be increased or if the plants Riparian buffers adjacent to cropland and placed in were cut and the residue was left in place. wet soils can provide several of the environmental fac- Crumpton et al. (1995) studied the efficacy of wettors that are essential to drive the processes of denitrifi- lands for removing NO 3 by assimilation and denitrificacation. Riparian buffers can supply the C required by tion. They estimated that NO 3 draining from approxithe denitrifying bacteria, helping to fuel a biologically mately 100 ha of land producing corn could potentially active zone within the soil profile. Denitrification rates be removed by a 1-ha wetland. However, given an aphave been found to be much greater within riparian proximate 100:1 ratio of cropland to wetland area sugbuffers placed in soils with high water tables and organic gested by Crumpton et al. (1995), such a wetland may C than in well-drained soils that are more aerobic (Ja- not provide sufficient residence time to remove an apcobs and Gilliam, 1985; Ambus and Lowrance, 1991; preciable amount of NO 3 from drainage water during Gold and Groffman, 1995; Addy et al., 1999). high rainfall events, which are typical for the Midwest

An additional factor that influences denitrification is the residence time of the shallow ground water as it during spring and early summer. Xue et al. (1999) came to similar conclusions in their study of the denitrification passes through the soil. Areas where flow of shallow ground water is relatively slow are the most effective capacity of three wetlands in Illinois. Mean monthly

NO 3 –loading rates to the wetlands were highly variable at removing NO 3 because of increased residence time in anaerobic and biologically active zones (Lowrance et (4–451 kg N) due to a wide range in amounts of monthly precipitation. The ratios of denitrification capacity and al., 2000). Snyder et al. (1998) observed the smallest degree of NO 3 reduction in a riparian buffer where the mean NO 3 load ranged from 19 to 59% for the threewetlands, with an average of 33%. The authors estiterrain was steep and ground water flow was rapid. They found the greatest NO 3 reductions in wet soils where mated that for months of low inputs, almost all of the

NO 3 could be denitrified, but for months of high inputs, the topographic gradient was very low and ground water movement was low. only a small percentage would be removed. A conclu-164 AGRONOMY JOURNAL, VOL. 94, JANUARY–FEBRUARY 2002

sion from their studies was, “The ultimate wetland deni- change the level of the drainage line outlet, or both. Such drainage control studies have used one or more trifying efficiency depends on both wetland capacity and the water residence time in each wetland.” of three basic strategies to reduce NO 3 contamination of surface waters. One strategy is to increase the anaerobic volume of the soil profile to enhance denitrification. A 

### Bioreactors second is to decrease the amount of drainage water Substantial research has been done on designing bior- exiting the artificial drainage system. The third is to eactors for denitrification (McCleaf and Schroeder, decrease the depth of the soil profile through which 1995; Reising and Schroeder, 1996; Shanableh et al., water infiltrates to reduce the leaching potential of soil 1997). Most designs require a supplemental C source NO 3 . All three of these strategies can be accomplished such as sucrose (Sison et al., 1995), ethanol [C 2 H 5 OH] by creating a shallower depth to the water table than or acetic acid [C 2 H 4 O 2 ] (Constantin and Fick, 1997), or exists with conventional, uncontrolled artificial drain- methane [CH 4 ] (Thalasso et al., 1997) to be effective. age designs. They also require a high level of management for in- In Indiana, Kladivko et al. (1999) conducted a subsur- field or edge-of-field treatment of subsurface drainage face drain-spacing study where they installed drain lines water. Solid C sources have also been tested and would at spacings of 5, 10, and 20 m, all at an average depth appear to be more amenable to field application. Volok- of 0.75 m. They found a consistent trend of greater loads ita et al. (1996) used shredded newspaper as a C source of NO 3 removed with increasingly narrow drain line in laboratory columns to obtain N removal rates ranging spacing during the 3 yr of study. Skaggs and Chescheir from 0.056 to 0.875 mg N g

 1 newspaper d

 1 . Blowes (1999) ran a simulation model for drain depths of 0.75 et al. (1994) used a fixed-bed bioreactor filled with a and 1.5 m and varied spacing for corn production on a compost mixture of sand, tree bark, wood chips, and sandy loam soil near Plymouth, NC. Their predictions leaves to treat drainage water from a farm field. Over showed that NO 3 losses could be reduced by a factor a year, a 200-L bioreactor was able to remove nearly of more than 2.5 by placing drainage lines relatively all NO 3 from a 10 to 60 L d

 1 discharge of field drainage shallow and close together. These predicted results are water containing 3 to 6 mg L

 1 NO 3 –N. Neither study contrary to spacing effects reported by Kladivko et al. differentiated between assimilation and denitrification (1999), which may likely be due to the differing climates as the NO 3 loss mechanism. and soils of Indiana and North Carolina.

Robertson and Cherry (1995) demonstrated the NO 3 Jacinthe et al. (1999) used soil columns with added removal potential of a bioreactor constructed in situ. NO 3 –N fertilizer (2.11 g column

 1 KNO 3 –N ) to simulate They filled a 0.6-m-wide trench that extended 0.75 m two water table management (WTM) techniques. The below a shallow water table with sand containing 20% first WTM treatment (WTM1) had a static water table (v/v) coarse sawdust and measured the concentration maintained at 0.5 m below the soil surface for 92 d, after of NO 3 in ground water before and after flowing through which the water table was raised to 0.1 m below the soil the mixture. Very high NO 3 –N concentrations (57–62 surface for the next 18 d. The other WTM treatment mg L

 1 ) were reduced to much lower concentrations (WTM2) simulated a dynamic water table where the (2–25 mg L

 1 ) in ground water passing laterally through water table was held at 0.5 m below the soil surface for the bioreactor. They attributed the removal of NO 3 to 7 d and raised to 0.1 m for the next 4 d. The water table heterotrophic denitrification, with the sawdust serving was then lowered gradually to the 0.7-m depth, held as a labile C source, and estimated that this denitrifica- there for the following 4 d, and then raised back to 0.5 tion wall would have an effective lifetime of 20 to 200 m and maintained at that level for the next 43 d. Before yr. However, they offered only indirect evidence that termination of the experiment, the researchers raised denitrification was the primary removal mechanism, cit- the water table back to the 0.1-m depth below the soil ing only reduced O 2 and SO 4 concentrations in water surface, holding it at that level for another 18 d. The passing through the wall. In a similar study with a con- WTM1 treatment removed 9 to 14% of the added structed denitrification wall, Schipper and Vojvodic-Vu- NO 3 –N during the 130-d simulation. The WTM2 treat- kovic (1998) found NO 3 –N concentrations to be reduced ment removed 24 to 43% of the added NO 3 –N, and the from between 5 and 16 mg L

 1 to  2 mg L

 1 in shallow researchers detected a faster rate of NO 3 removal when ground water passing through a wall. They attributed the water table was perched near the soil surface. Their the NO 3 removal to denitrification and reported that conclusion was that NO 3 removal can be stimulated and denitrifying enzyme activity reached a plateau of 906 enhanced by raising a water table into the upper soil ng g  1 h

 1 after 6 mo of operation. Placing C source profile layers, but the need for doing so during time denitrification walls around or near tile lines may help periods conducive for denitrification—coinciding with reduce NO 3 –N concentrations of shallow ground water a summer annual crop’s growing season—could limit before it enters the tiles and discharges into surface application of WTM practices due to possible damage waters. to the crop. An Iowa research project examined NO 3 transport in 

### Drainage Control Strategies shallow ground water under two different WTMschemes (Kalita and Kanwar, 1993). One scheme mainOther investigations have focused on methods to ma- tained water table depths at 0.2, 0.3, 0.6, 0.9, and 1.1 m nipulate water table depth by altering the conventional designs of artificial drainage lines, using structures to for time periods of 53 to 96 d after planting of corn forDINNES ET AL.: STRATEGIES TO REDUCE NITRATE LEACHING IN MIDWESTERN SOILS 165

1989 and 1990 and 45 to 97 d after planting in 1991. increased management time may make drainage control impractical on landscapes of steeper slope (Skaggs and Samples of shallow ground water were taken from pi- ezometers installed at depths of 1.2, 1.8, and 2.4 m for Chescheir, 1999). each water table depth treatment. In general, there was a trend of increasing NO 3 –N concentrations in ground 

### FUTURE RESEARCH NEEDS water with increasing depth to the water table. The Several methods for improving N management and 1.2-m piezometer yielded average NO 3 –N concentrareducing NO 3 contamination of water resources have tions varying from 7 to 2.5, 14.7 to 8.2, and 20.3 to 17 previously been proposed by others (Kanwar et al., 1996; mg L

 1 under shallow, medium, and deep water table Randall, 1997). Among them are recommendations for depths, respectively. The second WTM scheme had wa- (i) better use of soil tests to properly credit N sources ter tables maintained at 0.3, 0.6, and 0.9 m from 50 to other than commercial N fertilizers, (ii) abandoning tile 53 d after planting to the time of harvest during the 3- and surface drainage systems, (iii) installing constructed yr study. Piezometers were again located at the 1.2-, wetlands or denitrifying ponds, (iv) implementing crop 1.8-, and 2.4-m depths. The researchers found a trend rotations that include perennial and cover crops, (v) of decreasing NO 3 concentrations in ground water with improving the timing of N fertilizer application, and (vi) time during the growing season at all three sampling applying the proper rate of N fertilizer. The remainder depths, with the lowest average NO 3 concentrations obof this review will focus on our perspectives of these served under the 0.3-m water table depth. Corn grain suggestions and on some potential new directions for yields were negatively affected by increasingly higher research and development. water table levels at both sites for each year of the project. The researchers concluded that ground water 

### Improved Monitoring of Soil Nitrogen for Split NO 3 concentrations could be reduced by maintaining 

### Nitrogen Application Programs shallow water table depths and that the 0.3-m water table depth would provide the most beneficial water The logistics and time required for soil sampling and quality results. However, maintaining a water table at analysis in relation to the window of opportunity for such a shallow depth would restrict the ability to pro- fertilizer application has prevented widespread adopduce high corn grain yields. tion of the PSNT–LSNT N management approach. A

Fisher et al. (1999) compared a subirrigation with test is needed that can be performed earlier in the season controlled drainage (SI–CD) treatment with a subsur- and that relies on real-time meteorological and soil data face drainage treatment with no drainage control in a to predict plant-available soil N status in late spring. field-plot scale corn–soybean production system. The This would allow for more timely soil sampling and
SI–CD treatment had a water table maintained at 0.4 m. would increase the window of opportunity for N applicaThe authors found that mean soil NO 3 concentrations tion. One approach to developing a model to predict were not affected by the SI–CD treatment at the 0- to soil N mineralization from soil and meteorological data 15- and 15- to 30-cm depths, but at the 30- to 75-cm is to use soil respiration measurements as a surrogate depth, the SI–CD treatment reduced the 2-yr mean soil for measurements of soil N mineralization (Parkin et

NO 3 concentration by 46% compared with the subsur- al., 1996). Because soil N mineralization is the direct face drainage treatment. Compared with the subsurface result of microbial activity, measurements of soil respidrainage treatment, the SI–CD treatment increased av- ration without actively growing plants present are an erage corn N uptake by 13% and yield by 19% and integrated measure of soil microbial activity and are increased average soybean N uptake by 62% and yield likely correlated with soil N mineralization. The advanby 64%. The researchers concluded that proper imple- tage of using soil respiration measures as a surrogate mentation of a SI–CD management system could stabi- for soil N mineralization measurements is that soil respilize crop yield and N use efficiency and significantly ration can be measured almost continuously, responds
reduce soil NO 3 concentrations deeper in the soil profile quickly to changes in meteorological conditions, and compared with subsurface drainage management, re- can be used to determine cumulative activity of microorsulting in an overall reduction in NO 3 leaching. ganisms over very short or very long time periods. This

Although drainage control methods have shown posi- approach should be more amenable to determining the tive results in reducing NO 3 loading to surface waters effect of meteorological conditions on soil N mineralizafrom tile drainage, these technologies currently have tion than direct measurements of changes in soil NO 3 substantial limitations. Drainage control is typically lim- concentrations, which must be taken at discrete points ited to landscapes with 1% slope or less due to the in time. Coupling respiration measurements with meteocosts of drainage control structures (Evans et al., 1992; rological data and estimates of soil water movement Shirmohammadi et al., 1992; Skaggs and Chescheir, should allow the prediction of soil NO 3 accumulation. 1999). At a 1% slope, there would be a 1-m depth to A model that predicts N mineralization based on temwater table difference from the height of the drainage perature (growing degree days) has been developed control structure to an upslope point 100 m along the (Honeycutt et al., 1988). However, application of the drainage path. This may necessitate the need for multi- model requires a calibration procedure for the soil of ple drainage control structures, even within a moder- interest. This calibration involves lengthy incubations of ately sized field of 32 ha, to maintain a uniform water soil samples (30 to 60 d) to determine N mineralization potential, and thus precludes widespread adoption and table depth. Even if structure costs were reduced, the166 AGRONOMY JOURNAL, VOL. 94, JANUARY–FEBRUARY 2002

use. However, N mineralization potential is often not tion practices in the northern Corn and Soybean Belt.Cultural and plant-breeding research is needed to imfully realized due to variations in climatic conditions.Soil respiration measurements may be a means of in prove the compatibility of cover crops with current cropping systems. For example, rye cover crops grow well situ calibration. Improving our understanding of N im-mobilization and nitrification processes in soils and N in the northern Corn and Soybean Belt, but rye genotypes that do not reduce corn yields need to be found assimilation processes in plants would also facilitate de- velopment of predictive models based on interactions or developed. A better understanding of the timing of cover crop N release from shoot and root residues as between local soils and weather patterns, and thus re-duce or eliminate the need for PSNT–LSNT sampling well as their interactions with tillage and surface conditions is needed. Agronomic research focusing on imand soil analyses. proved establishment of cover crops, reduced management costs, and decreased negative impacts on 

### Variable-Rate Nitrogen Application Models subsequent row crops is also needed. Achievements in 

### and Methodologies these areas may be especially important as N manage- Pierce and Nowak (1999) state that, “There appear ment tools for both conventional and organic produc- to be no standards regarding the underlying agronomic tion systems. principles that should be guiding the development and application of precision agriculture.” At this time, there 

### Developing Perennial Cash-Cropping Systems appears to have been little advancement in regard to this problem, which indicates that much research is Based on the native prairie model, incorporation of more perennial crops into midwestern crop rotations needed in this area if precision farming techniques are to become adopted and successfully practiced by farm- could greatly reduce the amount of NO 3 that leaches into subsurface drainage lines. However, farmers and ers. The nature and predictability of spatial and tempo- ral variability within farm fields, at least at the local landowners will not adopt these crops until they havebeen proven to maintain or improve net economic relevel, need to be identified to guide farmers as to whichprecision farming techniques may improve their profit- turns. Research is desperately lacking in this area despite evidence that native prairies, forage crops, and ability and protect the environment.

Improvements in long-range weather prediction mod- Conservation Reserve Program land can substantially improve the quality of subsurface drainage water. els and incorporation of soil drainage and organic mat- ter decomposition modules for various soil types are Public funding and use of government resources are vital to this area of research because currently there is needed to create more reliable guidelines for variable- rate N management. Remote-sensing technologies that very little economic incentive for private industry orcommodity groups to support such efforts. Developreplace chlorophyll meters may play an integral role in establishing variable-rate strategies, but further re- ment of alternative cropping strategies, however, would be of very great benefit to the environment, farmers, search is needed to refine the technologies and their application for agricultural processes. To be effective, and rural communities. Adoption of alternative and/orperennial crops could reduce farmer reliance on continremote sensing must be able to correctly differentiatecrop areas that are N deficient due to low soil N from uous corn and corn–soybean rotations, offer an opportunity for improved environmental quality, bolster prices all other conditions that may lead to chlorosis in plants,such as water-saturated soils, Mg and K deficiencies, or of current commodities by reducing their overproduction, and reduce government spending on farm support disease and insect infestations. Currently, it remains to be seen whether sensing across multiple wavelengths and emergency relief programs.

Aside from the current social and environmental con(hyperspectral remote sensing) can separate these fac- tors and correctly identify those areas where soils are N cerns of genetic modification, this recent technology could serve to vastly accelerate the development of both deficient. If these areas cannot be accurately identified, simply using remote sensing to determine variable rates more suitable cover crops and perennial cash crops.

Genetic modification could enhance nutrient contents of N application is no guarantee against overapplication and, thus, an increased potential for NO 3 leaching and of perennial crops or add pharmaceutical traits, makingthem more valuable commodities. Also, genetic modificontamination of water resources.

If these challenges are met by science and technology cation could improve other traits of perennial and covercrops that would make it easier for farmers to incorpoin developing reliable variable-rate application method- ologies, they should play a significant role in improving rate them into their operations.water quality, protecting the environment, and improving crop production efficiencies. Whether farmer eco- 

### Nitrate Removal Strategies nomics are improved will depend on how much it will If field-based N management fails to provide ade- cost the farmer to adopt and learn the new method- quate protection against NO 3 contamination of subsur- ologies. face drainage water, alternative strategies will have to be developed for treating the water before it enters 

### Cover Crop Options and Management Strategies surface waters. Unless subsurface drainage lines are blocked or cut, riparian buffers will not provide the Presently, there are no cover crop species or geno- types that integrate well with corn and soybean produc- protection needed because these drains often bypassDINNES ET AL.: STRATEGIES TO REDUCE NITRATE LEACHING IN MIDWESTERN SOILS 167

5–6 Dec. 1995. USEPA Publ. 855R97001. USEPA Natl. Cent. for buffer areas and discharge directly into streams. Re- Environ. Publ. and Inf., Washington, DC. search will be needed to address a variety of limitations Ambus, P., and R. Lowrance. 1991. Comparison of denitrification in for wetlands, buffers, and bioreactors, including resi- two riparian soils. Soil Sci. Soc. Am. J. 55:994–997. dence time for remediation and assimilation, available Anderson, I.C., D.R. Buxton, D.L. Karlen, and C.A. Cambardella. 1997. Cropping system effects on nitrogen removal, soil nitrogen, C sources, land area limitations, and costs of implemen- aggregate stability, and subsequent corn yield. Agron. J. 89:881– tation. Also, because the predominant types of limita- 886. tions will differ geographically, a suite of options will Aulakh, M.S., J.W. Doran, D.T. Walters, A.R. Mosier, and D.D. need to be developed rather than a single solution. Francis. 1991. Crop residue type and placement effects on denitrification and mineralization. Soil Sci. Soc. Am. J. 55:1020–1025. Research on drainage control or WTM strategies has
Baker, D.B. 1988. Sediment, nutrient, and pesticide transport in seproduced some favorable water quality and crop yield lected lower Great Lakes tributaries. EPA–905/4-88-001. USEPA, results. The most promising appears to be systems that Chicago, IL. allow for dynamic control of the water table during the Baker, J.L. K.L. Campbell, H.P. Johnson, and J.J. Hanway. 1975. growing season to change the drainage level in response Nitrate, phosphorus, and sulfate in subsurface drainage water. J.
Environ. Qual. 4:406–412. to precipitation events. However, these technologies are Baker, J.L., T.S. Colvin, S.J. Marley, and M. Dawelbeit. 1985. Im- economically limited at this time to lands with little to proved fertilizer management with a point-injector applicator. In no slope. Future research in this area needs to examine Proc. 1985 Winter Meetings of the Am. Soc. of Agric. Eng., Chi- designs that are low cost and will allow for easy retrofit- cago, IL. 17–20 Dec. 1985. Am. Soc. of Agric. Eng., St. Joseph, MI. Baker, J.L., and H.P. Johnson. 1981. Nitrate-nitrogen in tile drainage ting of existing field tile drainage lines.
as affected by fertilization. J. Environ. Qual. 10:519–522. Baker, J.L., and S.W. Melvin. 1994. Chemical management, status, 

### SUMMARY AND CONCLUSIONS and findings. p. 27–60. In Agricultural drainage well research and demonstration project—annual report and project summary. Iowa The most desirable outcome associated with research Dep. of Agric. and Land Stewardship, Des Moines, IA. and technology development to minimize NO 3 contami- Benoit, G.R. 1973. Effect of agricultural management of wet sloping nation of water resources throughout the Midwest is a soil on nitrate and phosphorus in surface and subsurface water.

Water Resour. Res. 9:1296–1303. better public recognition that several different manage- Bergstro  ̈ m, L. 1987. Nitrate leaching and drainage from annual and ment practices will be required to ensure that surface perennial crops in tile-drained plots and lysimeters. J. Environ. and ground water resources achieve a quality acceptable Qual. 16:11–18. to society as a whole. The problem is not solely caused Bijeriego, M., R.D. Hauck, and R.A. Olson. 1979. Uptake, translocation and utilization of 15 N-depleted fertilizer in irrigated corn. Soil by N fertilizer management nor any other single factor

Sci. Soc. Am. J. 43:528–533. but is a combination of the soil management practices Blackmer, A.M., and A.P. Mallarino. 1997. Cornstalk testing to evalu- and inherent physical, chemical, and biological charac- ate nitrogen management. PM-1584. Iowa State Coop. Ext., Iowa teristics of the soil. State Univ., Ames. Several current and potential N management prac- Blackmer, A.M., and C.A. Sanchez. 1988. Response of corn to nitrogen-15-labeled anhydrous ammonia with and without nitrapyrin in tices are being evaluated and used to reduce the NO 3 Iowa. Agron. J. 80:95–102. loading of subsurface drainage water, ground water, and Blackmer, A.M., R.D. Voss, and A.P. Mallarino. 1997. Nitrogen fertil- surface water resources. Based on current experiences, izer recommendations for corn in Iowa. PM-1714. Iowa State Coop. a combination of two or more management practices Ext., Iowa State Univ., Ames. Blowes, D.W., W.D. Robertson, C.J. Ptacek, and C. Merkley. 1994. may work in harmony to reduce NO 3 loss to field drainRemoval of agricultural nitrate from tile-drainage effluent water age, and thus make implementation of the practices using in-line bioreactors. J. Contam. Hydrol. 15:207–221. more cost effective. One example would be to establish Boyd, P.M., H.M. Hanna, J.L. Baker, M. White, and T.S. Colvin. 2000. a perennial cash crop that could also function as a tile Field evaluation of anhydrous ammonia distribution manifolds. ASAE Paper 001140. 2000 ASAE Int. Meet., Milwaukee, WI. Am. drainage line biofilter by growing the crop directly
Soc. of Agric. Eng., St. Joseph, MI. above subsurface drainage lines. The perennial crop Buhler, D.D., K. Kohler, and M.S. Foster. 1998. Spring-seeded may be able to remove NO 3 from the water as it flows smother plants for weed control in corn and soybean. J. Soil Water toward the drainage lines. An income-producing bio- Conserv. 53:272–275. filter such as this would reduce the farmer’s costs for Buhler, D.D., G.W. Randall, W.C. Koskinen, and D.L. Wyse. 1993. Atrazine and alachlor losses from subsurface tile drainage of a clay achieving compliance and accelerate adoption of man- loam soil. J. Environ. Qual. 22:583–588. agement practices that more effectively mitigate NO 3 Bundy, L.G., D.T. Walters, and A.E. Olness. 1999. Evaluation of soil contamination of water resources. nitrate tests for predicting corn nitrogen response in the north central region. North Central Regional Res. Publ. 342. Wisconsin Agric. Exp. Stn., Madison. ACKNOWLEDGMENTS Cambardella, C.A., T.B. Moorman, D.B. Jaynes, T.B. Parkin, and D.L. The authors thank Dr. Gyles Randall, Jim Schepers, and Karlen. 1999. Water quality in Walnut Creek watershed: Nitrate Dean Martens for their helpful comments and advice for nitrogen in soils, subsurface drainage water and shallow groundwater. J. Environ. Qual. 28:25–34. this review.
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# LETTER FROM THE EDITOR

As of 1 Jan. 2002, I succeeded Dr. Ken Barbarick as Editor scripts assigned to them. Manuscript Tracker will be a major change in how we review and handle manuscripts, and we of Agronomy Journal (AJ). During the 6 yr that Dr. Barbarick was Editor, AJ experienced many significant changes, possibly anxiously anticipate this exciting, new development. We readily see how valuable this software will be to our authors more so than at any time in the history of the journal. Dr. Barbarick’s excellent leadership during this unprecedented and editors. We do, however, anticipate a startup phase with Manuscript Tracker, and we ask for your indulgence during period of change is noteworthy. These changes were necessary to address current demands and to prepare for anticipated this time. As I begin as Editor of AJ, I welcome your questions and needs.

While AJ has experienced many changes in recent years, comments about how we can better serve our authors andreaders. You may communicate with me by telephone, inmore changes are on the horizon. Sometime this year, we expect to begin using Manuscript Tracker. This web-based ternet, or letter, whichever is most convenient for you. We encourage authors to submit manuscripts electronically. software will allow authors, editors, and headquarters staff toaccess and input information appropriate to their responsibilit- International authors are well served with electronic submissions by the savings they obtain in time and mailing costs. ies. With the use of Manuscript Tracker, manuscripts will be submitted and moved electronically through the steps of the Authors who wish to submit manuscripts electronically should

continue to send them to Dr. Robert Lascano at r-lascano@ review process. This will greatly speed the transfer of manu- scripts from one person to another. Ultimately, everything tamu.edu.As of January 2002, paper-copy manuscripts should be subelse staying the same, the use of Manuscript Tracker will reduce the time it takes for manuscripts to go from submission mitted to me at the address below. to publication. Obviously, Manuscript Tracker does not have any control over manuscripts during the time when authors Dr. Calvin H. Pearson are making revisions on their papers. Typically, the longest Editor, Agronomy Journal time spent in the review and publication process is when au- Colorado State University thors are revising their manuscripts. Agricultural Experiment Station

Manuscript Tracker will give authors new freedom in that Western Colorado Research Center–Fruita they can follow the review of their manuscripts using the 1910 L Road, Fruita, CO 801521 internet and they can check on the status of their manuscripts Phone: 970-858-3629 whenever they want. Editors will also be able to conveniently Fax: 970-858-0461 Email: calvin.pearson@colostate.edu enter and maintain up-to-date review information for manu-