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© 2008 Plant Management Network.
Accepted for publication 7 March 2007. Published 30 July 2008.

Environmental Impacts of Enhanced-Efficiency Nitrogen Fertilizers

Peter P. Motavalli, Keith W. Goyne, and Ranjith P. Udawatta, Department of Soil, Environmental and Atmospheric Sciences, University of Missouri, Columbia 65211

Corresponding author: Peter P. Motavalli. motavallip@missouri.edu

Motavalli, P. P., Goyne, K. W., and Udawatta, R. P. 2008. Environmental impacts of enhanced-efficiency nitrogen fertilizers. Online. Crop Management doi:10.1094/CM-2008-0730-02-RV.

Abstract

The rapid increase in the anthropogenic production of N fertilizers has been a major factor accounting for the growth in agricultural food production. Despite the overall benefits the United States has experienced from use of reactive N, major environmental problems (e.g., soil and water acidification, contamination of surface and groundwater resources, increased ozone depletion and greenhouse gas levels, and loss of biodiversity) have developed due to the presence of excessive environmental N. The objective of this paper is to review research which is examining the use of enhanced-efficiency fertilizers, such as slow- (SRF) and controlled-release fertilizers (CRF), nitrification inhibitors (NI), and urease inhibitors (UI), as one management practice to enhance fertilizer N effectiveness, and possibly decrease environmental N losses through processes, such as nitrate leaching and runoff and gaseous losses of nitrous oxide and ammonia.

Environmental Problems Associated with Excessive N

Despite the unprecedented role synthetic N fertilizers have played in increasing agricultural crop and livestock production and meeting the nutritional requirements of a growing human population, strong evidence has emerged demonstrating the detrimental effects of the increasing amounts of reactive N in the environment (20,48). Among the deleterious effects of excessive environmental N are acidification of soils and water resources, eutrophication of coastal marine ecosystems, loss of biodiversity in terrestrial and aquatic ecosystems and invasion of N-loving weeds, increased greenhouse gas levels due to emissions of N₂O, depletion of stratospheric ozone, increased ozone-induced injury to crop, forest and other ecosystems, and increased atmospheric haze and production of airborne particulate matter (17). An example of the severity of excessive reactive N in the environment is that over 60% of coastal rivers and bays in the US have been moderately or severely degraded by nutrient pollution, especially by N (21).

N Loss Processes and N Fertilizer Recovery Efficiency

The primary mechanisms for N loss from agricultural fields are nitrate leaching, runoff and erosion, and gaseous losses from denitrification and ammonia volatilization (13). Global estimates of total N losses indicate that leaching, erosion and runoff constitute approximately 46% of all losses (Table 1). The relative magnitude of these N loss processes is affected by several factors, including variation in soil properties, climatic conditions, crop growth, and management practices (e.g., soil tillage method and the selection of the N source and its timing and method of application). Among the most important factors affecting N loss through soil processes, such as nitrate leaching and denitrification, are variations in soil water content and drainage either due to spatial differences in soil properties across agricultural fields or due to variation in precipitation during the growing season (34). For example, in the claypan region of the Midwestern United States, research by Bailey et al. (2) found relatively high rates of soil N₂O emissions that represented approximately 10% of the fertilizer N applied to a corn field. These high rates of observed soil N₂O loss may be attributed to the relatively poor drainage of claypan soils that can cause high soil water content early in the growing season shortly after N fertilizer application. Recent research by Udawatta et al. (46) in a paired watershed study in northeastern Missouri has shown agricultural watersheds in a common crop rotation in the Midwestern United States (i.e., corn and soybeans) are also vulnerable to N losses from both runoff and sediment loss, especially during large rainfall events, or after a sequence of closely-spaced small rainfall events.

Table 1. Estimated global N losses due to several N loss processes
[adapted from (41)].

 * 1 teragram (Tg) = 10¹² g = 1 million metric tons = 1.1 million US tons.

The overall result of the several possible N loss processes that may occur in agricultural fields is a decrease in the N fertilizer recovery efficiency (RE_N), or the proportion of applied N taken up by the fertilized crop. Cassman et al. (6) compared the RE_N of several cropping systems including corn-based cropping systems in the Midwestern United States and observed most of the systems had RE_N below 50%. In the corn-based cropping system, average (± standard deviation) RE_N was 37% ± 30. This low rate of RE_N presents a serious challenge to decreasing the environmental impact of N fertilizers used for crop production in the United States. However, some improvement in N fertilizer yield efficiency in corn systems has been observed over the last 28 years as evidenced by a 51% increase in the ratio of corn grain yield per unit of applied N fertilizer from 0.76 bu/lb N in 1980 to 1.15 bu/lb N in 2005 (11,12). This increased N fertilizer yield efficiency may be due to several factors, including improvements in plant genetics, tillage methods, and water and nutrient management practices (43).

Management Strategies to Reduce N Loss

Management strategies to reduce soil N loss include improved timing of N fertilizer and manure applications, better use and development of soil, plant, and manure testing procedures to determine in-season N availability, better N fertilizer and manure recommendations, switching to use of variable-rate N fertilizer applications and other more effective N fertilizer application methods, increased adoption of nutrient management planning, application of nitrification or urease inhibitors, and use of N fertilizer sources that are suitable for local environmental conditions (9). Improvements in local weather forecasting and modeling may also assist in providing timely information to allow farmers and agricultural professionals to make better-informed decisions on N use (5).

Among the improved N management practices being examined, the use of enhanced-efficiency fertilizers, such as slow- (SRF) and controlled-release fertilizers (CRF), nitrification inhibitors (NI), and urease inhibitors (UI), are being extensively studied under a variety of environmental conditions and agronomic cropping systems to determine their effectiveness for increasing agricultural production and reducing environmental N losses. The possible advantages of these fertilizers are that they are generally simple and easy to use; different types are available with diverse characteristics so a particular enhanced-efficiency fertilizer can be selected to match the specific environmental conditions and cropping system; use of these fertilizers may reduce the risk of environmental N loss associated with application of N fertilizer under variable climatic conditions; and if these fertilizers are effective in increasing RE_N then recommended N fertilizer application rates could be decreased, thereby reducing potential environmental N losses. However, several challenges for adoption of these fertilizers for agronomic uses include their relatively higher cost compared to conventional fertilizers, the special handling practices and storage facilities they may require, and the lack of research-based recommendations on when they would be effective and how they should be managed under different environments and cropping systems. This review will discuss research, focusing on examples from Missouri, investigating these technologies and evaluate the potential impact of enhanced-efficiency fertilizers on reducing the environmental impacts of N fertilization.

Slow- and Controlled-Release Fertilizers

Slow- and controlled-release fertilizers have been defined as fertilizers that are formulated to either delay nutrient availability after application or result in a longer period of plant availability over time compared to conventional fertilizers, such as urea and ammonium nitrate (45). These forms of N fertilizers are designed with chemical and physical properties that regulate or slowly release N into soil solution to meet plant needs, reduce NH₄⁺ availability to nitrifying bacteria, and subsequently reduce NO₃^— leaching or gaseous loss as N₂O or NO (14,44). Examples of these fertilizer products include urea-formaldehyde based fertilizer, sulfur-coated urea, and polymer-coated/encapsulated products (45). Among the possible advantages of these fertilizers are that they may decrease the rate of N release at times when potential for N loss (i.e., leaching, runoff and gaseous loss) is high (e.g., wet conditions); they may improve the timing of fertilizer N release to match crop N requirements over the growing season thereby increasing RE_N; and they may decrease the potential for salt injury allowing the fertilizer to be placed closer to plants which may also increase RE_N.

Effects on nitrate leaching losses and runoff. Reduction of nitrate leaching losses has been observed in several studies using slow- and controlled-release fertilizers [e.g., (32,33,49)]. For example, Motavalli et al. (28) compared patterns of nitrate leaching with use of polymer-coated urea (PCU) and conventional urea in northeastern Missouri and observed higher nitrate leaching, especially at a depth of 18 inches, earlier in the 2004 growing season when conventional urea was applied compared to when polymer-coated urea was added (Fig. 1). Similarly, Wang and Alva (49) observed large reductions in N leaching in sandy soils amended with slow-release fertilizer (IBDU, isobutylidene diurea), and polyolefin resin-coated urea (Meister) as compared to applications of ammonium nitrate fertilizer. However, the benefits of using slow- and controlled-release N fertilizers to reduce potential nitrate leaching are dependent on variations in soil properties and rainfall. For example, Motavalli et al. (28) observed no significant differences in nitrate leaching between plots treated with PCU and conventional urea in 2005, a relatively drier year than 2004 (data not shown).

Fig. 1. Nitrate-N concentrations in water samples collected from suction lysimeters during 2004 in a field trial in northeastern Missouri at (A) the 6-nch depth and (B) the 18-inch depth in plots receiving different rates of conventional or polymer-coated urea (PCU). The values are averaged over drainage/irrigation treatments. DAN = days after N fertilizer applied. The vertical bars show Duncan’s Multiple Range Test (DMRT) to show which treatments are statistically different. NS = not significantly different.

In general, few studies have addressed the effects of adding slow- and controlled-release fertilizers on runoff in agronomic crops. Research on turfgrass indicates that slow- and controlled-release fertilizers may be effective in reducing N in runoff during the first year of turfgrass establishment, but not in subsequent years (10). Nitrogen concentrations in floodwater used in rice production were lower when sulfur-coated urea was applied compared to urea and ammonium sulfate fertilizer applications (7). Further research is needed to examine the effects of using slow- or controlled-release fertilizers on runoff N losses in agronomic crops, such as corn and wheat, since reduction in agricultural N inputs into water resources is a major priority to lessen the impact of N fertilization on water quality. With any future research, the effects of the type, placement and rate and application timing of slow- or controlled-release N fertilizer on runoff may need to be examined.

Effects on gaseous losses of nitrous oxide. One of the major greenhouse gases emitted by the agricultural sector is nitrous oxide (N₂O) and ongoing research is examining whether use of slow- and controlled-release N fertilizers reduce emissions of this greenhouse gas (42). For example, recent research completed by Merchan-Paniagua (25) in northeast Missouri showed that PCU compared to urea can reduce efflux of N₂O from claypan soils under relatively wet climatic conditions caused by the absence of adequate tile drainage (Fig. 2). Motavalli et al. (29) also observed early season differences in soil N₂O flux among different N fertilizer sources across several landscape positions in a field in northeast Missouri (Table 2). Other researchers have also shown decreased N₂O emissions with use of slow- or controlled-release N fertilizers compared to conventional fertilizers, such as urea, under diverse crops and soil types (26,40,47). However, reduction of soil N₂O emissions due to use of slow- and controlled-release fertilizers has not always been consistent from year-to-year. For example, Merchan-Paniagua (25) observed a decrease in N₂O emissions with use of PCU compared to urea in 2004 but not in 2005. These results are indicative of the complex soil, climatic and management factors that affect soil N₂O emissions, and the high variability in emissions of this gas that occurs within fields and over time.

Fig. 2. Estimated cumulative soil N2O efflux with different N fertilizer treatments at the 250-lb/acre N rate in a field trial in northeastern Missouri in 2004 for (A) non-irrigated, non-drained, and (B) non-irrigated, drained plots. DAA is days after application of N fertilizer. Designation (PCU) denotes polymer-coated urea fertilizers. Vertical bars show LSD at α ≤ 0.05. NS denotes not significantly different. Note: 1 g of N2O-N per m² is equal to 10 kg of N2O-N per ha.

Table 2. Effects of N fertilizer source and landscape position on soil nitrous oxide flux for two selected dates after N fertilizer application (DAA) of N fertilizer treatments in northeast Missouri during 2005.

 ^x Fisher’s (protected) least significant difference at P < 0.05.

 ^y 1 mg N₂O-N/m²/day = 0.01 kg N₂O-N/ha/day.

 ^z NS = not statistically significant.

Nitrification Inhibitors

Nitrification inhibitors (NI) are chemicals that slow down, delay, or restrict the nitrification process, thereby decreasing the risk that large losses of nitrate will occur before the fertilizer N is utilized by plants. These chemicals inhibit the metabolism of Nitrosomonas bacteria involved in the nitrification process. Therefore, they can potentially reduce the amount of nitrate that can be leached out of the rooting zone or transformed into N₂O gas through denitrification. Among the common nitrification inhibitors are nitrapyrin (sold under the trade name N-Serve, Dow Chemical Co., Midland, MI) and dicyandiamide (sold under the trade name Guardian, Conklin Co., Shakopee, MN). These inhibitors can delay nitrification for between 4 to 10 weeks (31) depending on soil and environmental conditions and the inhibitor type (8,50). The highest probability for increased yield with use of NI is when it is applied to sandy soils which have a high risk for N leaching losses and in poorly-drained, fine-textured soils in which there is a high risk of gaseous N losses through denitrification (22).

Effects on nitrate leaching losses. An example of the possible reduction in nitrate leaching losses that can occur with application of NI is a three-year study in Canada on a loamy sand soil grown to irrigated corn (3). This study compared the effects of sidedressing the NI, dicyandiamide (DCD), with urea-ammonium nitrate (UAN) on nitrate leaching, corn grain yield, and plant N uptake. Concentrations of nitrate in soil solution were reduced by addition of DCD, and grain yield and N uptake were higher when the NI was co-applied with UAN. Improved N uptake and reduced leaching in DCD-treated plots resulted in 49% lower cumulative N loss compared to when UAN solution was applied by itself (Table 3).

Table 3. Average fall nitrate leaching losses over three years due to sidedress applications of nitrification inhibitor dicyandiamide (DCD) with UAN solution in a loamy sand grown to irrigated corn in Canada (3).

The effects of late fall (October) application of NI (as nitrapyrin) with anhydrous ammonia on nitrate leaching has been compared with that of late fall application without NI, and with leaching losses from spring pre-plant and split applications of anhydrous ammonia in a tile-drained clay loam soil with a corn-soybean rotation in Minnesota (36). This study found that use of NI with fall-applied anhydrous ammonia reduced nitrate leaching losses by 18% compared to when the anhydrous ammonia was fall-applied without added NI. Differences in the periods of maximum potential nitrate leaching due to climate variation (e.g., higher rainfall in the spring or fall) among agricultural regions may have a large influence which N application strategies, such as use of NI or split N applications, are effective (1).

Effects on gaseous losses of nitrous oxide. A recent review of research results in the Midwest US evaluating the agronomic and environmental effectiveness of the NI, nitrapyrin, concluded that in comparison to N fertilization without added nitrapyrin, the NI treatment, on average, increased crop yield by 7%, soil N retention increased by 28%, while N leaching decreased by 16% and greenhouse gas emissions were reduced by 51% (51).

The effectiveness of NI in reducing N₂O emissions is shown in research by McTaggart et al. (26) in Scotland which observed that applications of DCD persisted in reducing N₂O emissions for up to two months by 58 to 78% when applied with urea and 41 to 65% when applied with ammonium sulfate N fertilizers in a poorly drained clay loam soil in perennial ryegrass. In the same study, DCD reduced N₂O emissions from applied urea, but no decrease in gas emissions where observed when the DCD was applied with ammonium nitrate to spring barley (Fig. 3). In general, the effectiveness of NI in reducing N₂O losses is greater under climatic and soil conditions and management practices that favor denitrification (e.g., poorly drained soils, no-till systems).

Fig. 3. Cumulative nitrous oxide emissions over 56 days after spring applications of urea and ammonium nitrate (AN) with and without DCD nitrification inhibitor to spring barley in Scotland. Columns with the same letter are not statistically different (26).

Urease Inhibitors

Urease inhibitors (UI) reduce the rate at which urea is hydrolyzed and converted to ammonium by inhibiting the activity of urease, a common enzyme in soil (45). By delaying this hydrolysis, volatile losses of ammonia which occur primarily at the soil surface can be reduced. Ammonia volatilization losses due to surface application of urea range from 5 to 20% and can be as high as 50% under extreme conditions (16,22). Conditions favoring ammonia volatilization from urea or urea-based fertilizers include surface-application of the fertilizer, relatively high surface soil pH, high amounts of surface residue, warm and windy weather, high relative humidity, and adequate soil water content (4,15). Additional possible benefits of use of urease inhibitors to delay urea hydrolysis include possible reductions in nitrate leaching losses when conditions for nitrate leaching are high (35), lower N₂O losses, and decreased damage to germinating seeds and seedlings caused by released ammonia and nitrite when urea is band placed close to seed (24).

Among the most common commercially-available urease inhibitors is N-(n-butyl) thiophosphoric triamide (NBPT) (sold as Agrotain, Agrotain International, St. Louis, MO). The effectiveness of this product has been widely researched (19), including a recent three-year study in Missouri with no-till corn and wheat crops treated with broadcast surface-applied urea or urea+UI (as NBPT). The Missouri study found that the urea+UI treatment increased crop yield response by an average of 4 bu/acre in wheat and 4 bu/acre in corn (39). In general, urease inhibitors will have the greatest benefit in reducing environmental N losses through ammonia volatilization when urea cannot be physically incorporated into the soil, when the urea is not moved into the soil by infiltrating water either through rainfall or irrigation, or when the soil has high urease activity due to lack of cultivation or the accumulation of surface residue (18).

Effects on ammonia volatilization. Relatively little research has been conducted to assess the effectiveness of UI under diverse management practices and environmental conditions in the field to reduce ammonia volatilization losses (14). Some examples of recent research include a study in Missouri which found that use of NBPT with surface applications of urea or urea ammonium nitrate (UAN) solution reduced ammonia volatilization losses by 60% and 30%, respectively (Peter Scharf, unpublished data). In Canada, total ammonia volatilization from application of urea, applied under the warmer temperatures in July compared to those in May, ranged from 20 to 50% of applied N and was highest in a fine sandy loam textured soil compared to a clay loam soil (37). Table 4 shows that application of NBPT with the urea was highly effective in reducing ammonia volatilization losses for the clay loam soil and for the sandy loam soil under the cooler conditions in May, but not as effective for the sandy loam soil when the urea and NBPT were applied in July. Other researchers have observed that NBPT was more effective in reducing ammonia volatilization in coarser-textured soils (16). In contrast, UI are not always effective at improving N uptake and reducing ammonia volatilization in slightly acidic, high organic matter soils (38).

Table 4. Reduction in ammonia volatilization losses in Canada with
use of the urease inhibitor, NBPT, in two soils with urea applied to
the soil surface on two dates in Canada (37).

 * Measured ammonia volatilization losses ranged between 20 and
50% of total applied N.

Effects on nitrate leaching. Some studies have examined whether application of NBPT decreases nitrate leaching losses due to the delay in urea hydrolysis induced by this UI. In a laboratory study using a fine sand from central Florida, Prakash et al. (35) observed reduced N leaching losses with use of NBPT-coated urea as compared to urea alone. In contrast, in a lysimeter study with sandy loam and clay loam soils to which urea and urea + NBPT were applied, N leaching losses were higher with the UI-treated urea compared to urea alone (16). The researchers in this study attributed the greater N leaching losses with the UI treatment to greater N mineralization caused by the UI treatment, especially in the coarser-textured soil.

Management Practices

Use of enhanced-efficiency N fertilizers is one of several possible management practices that can be effective in reducing environmental N losses and increasing N-use efficiency. Understanding the factors that influence N loss and the technical characteristics of the different enhanced-efficiency fertilizers is critical for determining their optimal use. In addition, quantification of the reductions in environmental N loss that can occur with use of these enhanced-efficiency N fertilizers under diverse cropping systems and environmental conditions would assist in the development of government-sponsored practice incentive programs designed to promote use of management practices that conserve and protect natural resources.

A major limitation to use of many of these fertilizer products is their higher cost, limited availability and possible special handling procedures for transport, storage and application. Therefore, research which provides an economic evaluation of the costs and benefits of using the enhanced-efficiency fertilizers is important to assist growers in making informed decisions whether use of a particular fertilizer product is cost-effective for their specific site conditions.

Recent research in Missouri is exploring several management practices which may lower costs of using enhanced-efficiency fertilizers (29). One approach is to mix controlled- or slow-release fertilizers with conventional urea or lower application rates of the enhanced-efficiency fertilizer because of its possible higher N-use efficiency. Another approach is to use a variable source N application strategy in which conventional urea fertilizer is applied to areas of a field which have a low risk of N loss and the enhanced-efficiency N fertilizer to the high risk areas of a field using a multi-bin spreader. Research conducted from 2005 through 2007 in a claypan soil in northeastern Missouri has shown a consistent 24 to 29 bu/acre corn yield increase with use of PCU compared to urea in low-lying areas of a field but not on the sideslopes or summit positions of the field (29). Mapping the yield and economic differences between the PCU- and urea-treated plots has shown a distinct region of the field in which the PCU has a greater economic return (Fig. 4). Ongoing research is exploring methods to delineate the regions of fields in which there is a greater probability for increased agronomic response and reduced environmental N loss with use of an enhanced-efficiency N fertilizer.

Fig. 4. Map of grain yield differences in 2007 between the PCU- and urea-treated plots across a field with claypan soil in northeastern Missouri grown to corn. Second map shows the variation in net economic return for applying PCU versus urea. Positive values indicate higher PCU yields or economic returns compared to those of urea (30).

Conclusions

•  Fertilizer N is essential in providing food, feed, fiber, and fuel for the world's human population, yet relatively low N recovery efficiency in agriculture has contributed to several major environmental problems.

•  Among the many suggested management practices to mitigate environmental N contamination is use of enhanced-efficiency fertilizers (e.g., slow- and controlled-release fertilizers, nitrification inhibitors, and urease inhibitors).

•  Extensive research has examined the impacts of enhanced-efficiency fertilizers on processes, such as nitrate leaching, N₂O emissions, and ammonia volatilization, but more research is needed on the impacts of these fertilizers on N losses due to surface runoff and erosion.

•  Major hurdles to use of these fertilizers for decreased environmental impacts are:

- Improved understanding of the environmental conditions and management practices that will optimize use of these fertilizers for increased RE_N;

- Development of methods to identify when and where these fertilizers will be agronomically and economically effective (e.g., variable source N management);

- Quantification of the environmental benefits of using these fertilizers to justify potential funding of government-sponsored practice incentive programs.

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