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Invited paper: Nitrogen Use Efficiency in Cereal Systems

Achim Dobermann

Department of Agronomy and Horticulture, University of Nebraska-Lincoln,
P.O. Box 830915, Lincoln, NE 68583-0915, USA; adobermann2@unl.edu

Abstract

At a global scale, cereal yields and fertilizer N consumption have increased in a near-linear fashion during the past 40 years and are highly correlated with one another. However, large differences exist in historical trends of N fertilizer usage and nitrogen use efficiency (NUE) among regions, countries, and crops. Future global nitrogen needs will depend on: (i) changes in cropped cereal area and the associated yield increases required to meet increasing cereal demand from population and income growth, and (ii) changes in NUE at the farm level. The anticipated 38% increase in global cereal demand by 2025 can be met by a 30% increase in N use on cereals, provided that the steady decline in cereal harvest area is halted and the yield response to applied N can be increased by 20%. Interventions to increase NUE and reduce N losses to the environment must be accomplished at the farm- or field-scale through a combination of improved technologies and carefully crafted local policies that contribute to the adoption of improved N management practices. Examples from several countries show that increases in NUE at rates of 1% yr-1 or more can be achieved if adequate investments are made in research and extension.

Key Words

Nitrogen, nitrogen use efficiency, cereals

Reactive nitrogen and the need to increase fertilizer nitrogen use efficiency

Nitrogenous fertilizers have contributed much to the remarkable increase in food production that has occurred during the past 50 years (Smil 2001). Globally, however, N fertilizers also account for 33% of the total annual creation of reactive nitrogen (Nr) or 63% of all anthropogenic sources of Nr (Table 1). Reactive nitrogen is defined as all biologically, photochemically, and/or radiatively active forms of N -- a diverse pool of nitrogenous compounds that includes organic compounds (e.g. urea, amines, proteins, amides), mineral N forms, such as NO3- and NH4+ as well as gases that are chemically active in the troposphere (NOx, NH3, N2O) and contribute to air pollution and the greenhouse effect (Galloway et al. 1995). Asia alone accounts for more than 50% of the global N fertilizer consumption as well as 37% for the global Nr creation. Smil (1999) estimated that only about half of all anthropogenic N inputs to cropland are taken up by harvested crops and their residues, with the remainder contributing significantly to Nr enrichment of the atmosphere, ground and surface waters.

It is widely believed that accumulation of excessive amounts of Nr in terrestrial and aquatic ecosystems as well as in the troposphere leads to significant costs to society that occur through direct and indirect negative effects on environmental quality, ecosystem services, biodiversity, and human health (Pretty et al. 2000; Schweigert and van der Ploeg 2000; Townsend et al. 2003). Such estimates are not very precise, however, and it is not clear whether they place an appropriate value on the large positive impact of N fertilizer on ensuring food security and adequate human nutrition. Environmental benefits also accrue from fertilizer use by avoiding expansion of agriculture into natural ecosystems and marginal areas that cannot sustain crop production and provide critical habitat for protecting biodiversity (Cassman et al. 2003). Regardless of what the true societal costs of accumulation of Nr in cultivated and natural ecosystems are, it is clear that Nr creation associated with human activities must slow down. Mitigation options include:

(i) Reduction of Nr emissions from fossil fuel combustion,

(ii) Transformation of Nr to non-reactive N forms (e.g., denitrification to N2 or sequestration of N in soil organic matter),

(iii) Changes in human diet and associated changes in food, feed, and fertilizer demand, and

(iv) Improvements in fertilizer nitrogen use efficiency (NUE) in agricultural systems: less N fertilizer per unit food produced.

Table 1. Creation of reactive N from anthropogenic and natural sources in the mid 1990s (Boyer et al. 2004).

 

Anthropogenic (million t/yr)

Natural (million t/yr)

Total

Region

Fertilizer

BNF

Import

Depos.

Total

BNF

Lightng.

Total

  

Africa

2.1

1.8

0.5

2.9

7.3

25.9

1.4

27.3

34.6

Asia

44.2

13.7

2.3

3.8

64.0

21.4

1.2

22.6

86.6

Europe + FSU

12.9

3.9

1.0

2.9

20.7

14.8

0.1

14.9

35.6

Latin America

5.1

5.0

-0.9

1.8

11.0

26.5

1.4

27.9

38.9

N. America

12.6

6.0

-2.9

2.7

18.4

11.9

0.2

12.1

30.5

Oceania

0.7

1.1

-0.3

0.3

1.8

6.5

0.2

6.7

8.5

Total

77.6

31.5

-0.3

14.4

123.2

107.0

4.5

111.5

234.7

Many of these mitigation strategies are of long-term nature and they are closely linked to policy decisions that need to be made. However, improving NUE in agriculture has been a concern for decades and numerous new technologies have been developed in recent years to achieve this. Therefore, fertilizers and their management will be at forefront of measures to improve the global N balance over both the short- and long-term. This paper addresses three issues: (i) global status of NUE in cereals, and (ii) future nitrogen fertilizer demand, and (iii) major technology and policy options for increasing NUE. I focus on cereals because they account for nearly 60%% of global N fertilizer use (IFA 2002) or roughly 20% of the global annual creation of Nr.

Global status of N use efficiency

World consumption of N fertilizers has averaged 83 million metric tons (Mt) in recent years, of which about 47 Mt is applied to cereal crops (Table 2). At a global scale, cereal production (slope = 31 x 106 Mg yr-1), cereal yields (slope = 45 kg yr-1), and fertilizer N consumption (slope = 2 Mt yr-1) have increased in a near-linear fashion during the past 40 years. However, significant differences exist among world regions, particularly with regard to N use efficiency. On a global or regional scale, partial factor productivity of N (PFPN) or the ratio of grain yield to N amount applied is the only index of NUE that can be estimated reasonably well, although not very precisely because of uncertainties about the actual N use by different crops. The PFPN is an aggregate efficiency index that includes contributions to crop yield derived from uptake of indigenous soil N, N fertilizer uptake efficiency, and the efficiency with which N acquired by the plant is converted to grain yield. Because PFPN is a ratio, it always declines from large values at small N application rates to smaller values at high N application rates. Thus, differences in the average cereal PFPN among world regions depend on which cereal crops are grown, their attainable yield potential, soil quality, amount and form of N application, and the overall timeliness and quality of other crop management operations.

At global level, average PFPN in cereal production has decreased from about 245 kg grain kg-1 N in 1961/65, to 52 kg kg-1 in 1981/85, and is currently about 44 kg kg-1. A decrease in PFPN always occurs as farmers move yields higher along a fixed N response function, unless other factors shift the response function up. In other words, an initial decline in PFPN is an expected consequence of the adoption of N fertilizers by farmers and not necessarily bad within a systems context. In developing regions, N fertilizer use was small in the early 1960s and increased exponentially during the course of the Green Revolution. Although the growth rate in N consumption has slowed substantially in recent years, it still averaged 1.45 Mt N yr-1 (3.2% yr-1) during the past 20 years. The large increase in N use since the 1960s resulted in a steep decrease in PFPN in all developing regions (Fig. 1). However, average regional N rates on cereals range from less than 10 kg N ha-1 in Africa to more than 150 kg N ha-1 in East Asia (Table 2) and, with the exception of Africa, PFPN continues to decline in all developing regions at rates of –1 to –2% yr-1 (Fig. 1). The low PFPN in East Asia, which is dominated by China, is of particular concern for the global Nr budget because this region uses the greatest amount of N fertilizer (Table 1). Without greater investment in research and extension, declines in PFPN in developing countries will likely continue.

Table 2. Current levels of cereal production, nitrogen fertilizer use on cereals, and cereal nitrogen use efficiency by world regions. Values shown represent annual means for the 1999 to 2002/03 period.

 

Developed

Transitional/Developing

World

 

North

NE

W

E Europe

Ocean.

Africa

W Asia

South

SE

East

Latin

  
 

America

Asia

Europe

C Asia

    

NE Africa

Asia

Asia

Asia

America

  

Cereal prod. (Mt)

377

19

208

216

34

98

81

307

141

447

144

2072

Cereal yield (t ha-1)

5.1

6.1

5.5

2.1

1.9

1.1

2.3

2.4

3.2

4.8

2.9

3.1

Total N use (Mt)1

12.5

0.9

9.5

4.9

1.3

1.4

4.2

14.6

4.0

24.9

5.1

83.2

Cereal share N (%)2

66

32

45

51

67

56

56

50

71

58

53

57

N use cereals (Mt)

8.3

0.3

4.3

2.5

0.9

0.8

2.4

7.3

2.8

14.5

2.7

46.7

N rate (kg N ha-1)3

112

89

113

25

48

9

68

58

65

155

55

70

PFPN (kg kg-1)4

45

71

59

90

46

123

34

44

53

32

55

44

Relative PFP5

1.0

1.6

1.4

2.1

1.1

2.8

0.8

1.0

1.2

0.7

1.3

1.0

1 Total fertilizer N consumption by all crops (FAO 2004).
2 Estimated share of cereal N use of total N consumption, calculated as weighted average of country-specific estimates of fertilizer use by crops (IFA 2002). Weights were proportional to N use by countries.
3 Estimated average N application rate on all cereal crops.
4 Average partial factor productivity of applied N = kg grain yield per kg N applied.
5 PFPN relative to world average (World = 1).

Figure 1. Regional trends in nitrogen use efficiency in cereals. A logarithmic scale was used for the NUE axis.

With the exception of Oceania and Eastern Europe/Central Asia, cereal yields in many industrialized regions have continued to increase in the past 20 years without significant increases in N fertilizer use. As a consequence, average PFPN has remained virtually unchanged at 49 kg kg-1 since the early 1980s. Trends of increasing PFPN have occurred in some regions (Fig. 1), e.g., Western Europe (mostly rainfed wheat with high yields) and Northeast Asia (irrigated rice in Japan and Korea). In North America, average cereal PFPN has changed little because of low PFPN in dryland wheat areas with low and variable yields, while PFPN of maize has increased substantially (Dobermann and Cassman 2002). At present, average cereal yields in North America, Western Europe, and East Asia are 60 to 100% above the world average, even though the N rates applied are only 30 to 60% above world average rates (Table 2). High yields and high PFPN in these regions result from a combination of fertile soils, favorable climate, and improved crop and soil management practices, including N fertilizer management. Trends of increasing PFPN are likely to continue in developed countries because they primarily result from investments in research and extension on crop improvement, new fertilizer products, and better management technologies by both public and private sectors, at levels that greatly exceed those currently available in the developing world.

The very high PFPN in Africa (123 kg kg-1) and Eastern Europe/Central Asia (90 kg kg-1) are indicative of soil N mining. Fertilizer use in Africa has lagged behind other world regions and is a major reason for the low cereal yields in this region (Table 2). In Eastern Europe and countries of the former Soviet Union (FSU), N fertilizer use on cereals dropped drastically in the late 1980s as a result of political and economic turmoil. Consequently, PFPN doubled from 1988 to 2000 without improvements in yield potential or major changes in N management. Because these trends of increasing PFPN in both Africa and Eastern-Central Europe are likely associated with a mining of soil N resources, they are not sustainable over the long-term and we would expect yields to stagnate or even decline unless greater amounts of N fertilizer are used in cereal production.

The trends shown in Figure 1 depend on the reliability of the aggregate data on crop yields and fertilizer use. Both are difficult to validate. Data on fertilizer use by individual crops within countries and regions are notoriously difficult to obtain and we do not have reliable time series. For many countries, the values used were derived from estimated total N fertilizer use and expert estimates of the average N fertilizer use by crop (IFA 2002). Very few countries collect more detailed information. Despite these caveats, there are several pieces of supporting evidence. One assumption made in calculating trends in NUE (Fig. 1) is that the share of total N fertilizer consumption by cereals within a region has not changed substantially since the early 1960s. In the USA, for example, surveys of cropping practices are annually conducted with sample sizes of several thousand farmers (http://www.ers.usda.gov). Those data indicate that the cereal share of total N consumption has remained virtually unchanged since the mid 1960s. In our approach, average PFPN for rice grown worldwide was estimated at 44 kg kg-1 (data not shown). This value is in reasonable agreement with an average PFPN of 46 kg kg-1 as directly measured in on-farm studies conducted on 400 farmers’ fields in South Asia, East Asia, Southeast Asia and West Africa (Dobermann et al. 2002; Adhikari et al. 1999; Wopereis et al. 1999; Haefele et al. 2001).

The relationship between the mean national cereal yield and the mean rate of N fertilizer applied to cereal crops on a country-by-country basis is linear and it provides an estimate of the ‘global’ average agronomic efficiency of fertilizer N (AEN) in cereals (Fig. 2). On a global basis, the slope of the regression suggests that global cereal production will increase by 30 kg ha-1 for each kg of additional N fertilizer. The slopes and intercepts (yield at zero N applied), however, differ significantly among crops (Cassman et al. 2003). Rice, for example, often yields more with no N fertilizer applied than wheat or maize because of greater N supply from indigenous soil resources. Thus the slope of the regression is lower for rice (26 kg kg-1) than for wheat and maize (36-41 kg kg-1, not shown). Actual N response within countries or at farm level varies widely due to differences in climate, soil fertility and the technological sophistication of crop management.

Ladha et al. (2005) provide a summary of published research data on fertilizer-N efficiency in cereal crops. In their analysis, the average recovery efficiency of N (REN) in aboveground biomass (grain+straw) in research plots was 44% in rice, 54% in wheat and 63% in maize (Table 3). Recovery in grain alone averaged 35 to 44% for the three major cereals, which is significantly higher than the crude global estimate (33%) suggested by Raun and Johnson (1999). Not included in this is fertilizer-N recovered in roots, N recovered in subsequently grown crops, and N that remains in the soil N.

Figure 2. Global relationships between average cereal yields and average fertilizer-N use for 81 countries during the late 1990s. The solid line indicates the present average N response of all cereals to fertilizer N application. The dashed line indicates a possible increase in NUE due to a 20% increase in the slope of the average N response (ΔY/ΔNrate), but no change in the intercept. Drop lines and values in the table show the effect of different N response on present and required N rates and NUE at global yield levels for two future scenarios in which cereal harvest area continues to decline slowly until 2025, but NUE either increases or decreases. Scenario 1: No change in the global N response function. Yield increases are mainly associated with increasing N rates (move along the current N response function); Scenario 2: A 20% increase in the slope of the global N response function. Yield increases are associated with increasing N rate and increasing NUE (Dobermann and Cassman 2005).

In field studies with rice and dryland systems, average 15N fertilizer recovery was 3.3% in the 1st subsequent crop, 1.3% in the 2nd subsequent crop, 1.0% in the 3rd subsequent crop, 0.4% in the 4th subsequent crop, and 0.5% in the 5th subsequent crop, or 6.5% in total (IAEA 2003; Krupnik et al. 2004). Thus, together with an average first-crop REN of 51% (difference method) or 44% (15N method), total crop N recovery from a one-time application of N averages about 50 to 57% in research trials with cereals. The remainder is either stored in soil organic matter pools or lost from the cropping system. In the IAEA trials, the average amount of 15N fertilizer recovered in soil after five growing season was 15 %, suggesting that, under research conditions, about 30 to 35% of the fertilizer-N applied is typically lost from the system.

Detailed research studies provide valuable insides into N pathways and the processes that lead to N losses in agricultural systems. However, results from research plots cannot be extrapolated to obtain estimates of NUE at regional or global scales because N losses in farmers’ fields are often much larger. Unfortunately, little is known about the current level of NUE in key cropping systems of the world at the scale of typical production fields. This shortage of information reflects the logistical difficulty and high cost of obtaining direct on-farm measurements and the lack of funding for what appear to be routine on-farm evaluations (Cassman et al. 2002).

The few available regional or national scale on-farm studies generally suggest a greater disconnection between the amount of fertilizer N applied by farmers and the crop yield that is achieved, resulting in often low and highly variable NUE among and within farmers’ fields. Irrigated rice is the only cropping system for which systematic on-farm measurements of NUE have been conducted for numerous regions in Asia and West Africa (Dobermann et al. 2002; Haefele et al. 2003). Average REN in irrigated rice fields in Asia was 31% as compared to 44% in research trials (Table 3). Similarly, whereas Ladha et al. (2005) cited an average AEN in rice of 21.6 kg kg-1 and average PFPN of 63.2 kg kg-1, measured on-farm averages in south and southeast Asia were 11.5 kg kg-1 and 49.2 kg kg-1, respectively (Dobermann et al. 2002).

Table 3. Average apparent first-crop recovery efficiency of applied fertilizer-N in cereals (REN = fertilizer-N recovery in above-ground biomass).

Crop

Region

Average N rate

REN

 

(no. of observations)

(kg N ha-1)

(%)

Maize, research trials1

World (36)

102

63

Maize, on-farm2

USA (55)

103

37

Rice, research trials1

World (307)

113

44

Rice, on-farm3

Asia (179)

117

31

Wheat, research trials1

World (507)

117

54

Average research trials1

World (850)

-

51

1 Ladha et al. (2005)
2 Cassman et al. (2002)
3 Dobermann et al. (2002)

Extensive on-farm studies of similar kind and of regional or global scope have not been conducted for other major cereal crops. This makes it difficult to judge whether the findings made for rice systems are applicable to other crops and cropping systems. However, there is some evidence that this may be the case for wheat grown in rice-wheat systems of south Asia and maize grown in rainfed and irrigated systems of the USA Corn Belt (Cassman et al. 2002; Adhikari et al. 1999). On-farm studies with maize in the U.S. Corn Belt also showed much lower average REN of 37% (Table 3) than the ‘global’ average of 63% cited for maize in Ladha et al. (2005). A similar discrepancy occurs for PFPN in maize, with a computed research trial average of PFPN of 69.9 kg kg-1 (Ladha et al. 2005) as opposed to an average value of 58 kg kg-1 estimated for maize in the USA (Dobermann and Cassman 2002). The latter was estimated at national scale based on crop yield statistics and large annual surveys of farmers’ fertilizer use.

Lower NUE in farmers’ fields is usually explained by a lower level of management under practical farming conditions and greater spatial variability of factors controlling REN and other indices of NUE (Cassman et al. 2002). Considering this, NUE achieved in research trials is a good indicator of what can be targeted with good management. It is reasonable to assume that, on a global scale, at least 50% of the fertilizer-N applied is lost from agricultural systems and most of these losses occur during the year of fertilizer application.

Future fertilizer needs

On a global scale, future cereal production must meet the demands of a growing population and a shift in food consumption patterns to more diverse diets and greater consumption of livestock products. Cereal production is expected to increase from 2072 Mt at present to 2860 Mt in 2025 (Rosegrant et al. 2002), which is equivalent to an annual growth rate of 1.3% for the 2000 to 2025 period. Because production increases reflect the influences of changes in cropland area and yield, trends in cereal harvest area have large impact on yield levels required to meet anticipated food demand. To achieve a global cereal production of 2860 Mt in 2025, yields must increase by 23 to 50%, depending on the changes in cereal harvest area over this period. In an optimistic scenario, global cereal area may increases by more than 50 Mha so that cereal yields would only have to rise from 3.1 to 3.8 t ha-1 (Rosegrant et al. 2002). However, worldwide cereal area has declined from a peak of 727 million ha (Mha) in 1981 to the present level of 671 Mha (2001-2003 average), which represents a linear rate of -2.44 Mha yr-1 or -0.33% yr-1 (R2 = 0.79). The decline has mostly occurred in wheat and other coarse grains (barley, sorghum, millet), whereas rice and maize area has remained relatively stable in recent years. Although it has been suggested that this decrease in cropped area has been limited to a few regions and that it should be reversible if cereal prices rise to provide greater economic rewards for grain production (Rosegrant et al. 2002), the reality is that substantial decreases in cereal area have occurred worldwide. Among the developed regions annual losses in cereal area since the early 1980s range from –0.7% yr-1 in Western Europe to –1.5% yr-1 in Northeast Asia, with Oceania as the lone exception. In the developing world, decreases in cereal area have occurred in South Asia, East Asia, and Latin America, regions that together account for 44% of the world cereal production. The largest losses have occurred in China, where cereal area has declined by 20 Mha, since 1976, primarily due to a decline in rice and wheat area. Cereal area has increased only in Africa and Southeast Asia. Thus, if current trends of decreasing cereal production area continue, average cereal yields must increase from 3.1 to 4.6 t ha-1in 2025. The annual yield growth rate required in this scenario of decreasing production area (1.6% yr-1) exceeds the yield growth during the past 20 years by a substantial margin, illustrating the challenges that lie ahead if cereal land area continues to decline (Dobermann and Cassman 2005).

Figure 2 illustrates the potential global impact of increasing NUE in agricultural systems. If losses of cereal cropping area continue at present rates and fertilizer-N efficiency cannot be increased substantially, a 60% increase in global N consumption by cereals or 74% increase in average N rates per ha would be required to meet the predicted 38% increase in cereal demand by 2025 (Scenario 1). Such a large increase in N consumption would have major environmental consequences at local, regional, and global scales through continued accumulation of different forms of Nr. On the other hand, the predicted cereal demand can be met by only a 30% increase in global N fertilizer use on cereals if the incremental cereal yield response to applied N can be increased by about 20% within a period of 20 years (Dobermann and Cassman 2005). Such a level of increase in NUE is well within the scatter of the present ‘global N response curve’, i.e., there are many countries in which even higher NUE has already been achieved. It has also been demonstrated in many studies that 30 to 50% increases in NUE can be achieved through fine-tuning of N management practices.

Not considered in this analysis are the recent rapid increases in biofuel production from cereals such as maize, particularly in North America, which are likely to have major impact on future world cereal markets and global fertilizer demand.

What can be done to increase nitrogen use efficiency?

Is a 20% increase in the incremental yield response to applied N (Fig. 2) achievable at the global scale over a time period of about 20 years? To answer this question it is important to re-iterate that a 20% increase in this incremental N efficiency, as estimated by the slope of the regression line in Figure 2, is not equivalent to a 20% increase in the overall NUE (or PFPN). On a global scale, higher cereal yields are likely to be achieved through a combination of increased N applications in regions with low N fertilizer use, such as Africa and parts of Asia and Latin America, and improved N fertilizer efficiency in countries where current N fertilizer use is already high. For example, the global PFPN in cereals only needs to increase at a rate of 0.1 to 0.4% yr-1 to meet cereal demand in 2025 (Dobermann and Cassman 2005). Such rates have been achieved in some developed regions in the past 20 years (Fig. 1), and far greater rates of increase have been achieved in several countries.

In the UK, our estimates suggest an average cereal NUE of 36 kg kg-1 in 1981/85, which increased to 44 kg kg-1 by 2001/02 (+23%, 1.1% yr-1). In the USA, annual surveys of cropping practices indicate that NUE in maize increased from 42 kg kg-1 in 1980 to 57 kg kg-1 in 2000 (+36%, 1.6% yr-1)(Dobermann and Cassman 2002). In Japan, NUE of irrigated rice remained unchanged at about 57 kg kg-1 from 1961 to 1985, but it increased to more than 75 kg kg-1 (+32%, 1.8% yr-1) in since 1985 (Suzuki 1997; Mishima 2001). In each of these countries, key factors that contributed to this improvement included: (i) increased yields and more vigorous crop growth associated with greater stress tolerance of modern cultivars, (ii) improved management of production factors other than N, and (iii) improved N fertilizer management. The latter may include use of better fertilizers and NUE-enhancing products as well as better application strategies and methods. The combination of these measures allowed achieving higher yields with either stagnating (USA) or declining N use (UK, Japan).

Most of these improvements were achieved without general restrictions or regulations on N fertilizer use. They were driven by investments in public and private sector research and extension. Because of the large differences in NUE among countries, regions, farms, and fields within a farm, policies that focus only on increasing or decreasing N fertilizer use at a state or national level would have a widely varying impact on yields, farm profitability, and environmental quality. Instead, achieving greater NUE at state or national levels will require policies that favor increases in NUE at the field scale with emphasis on technologies that can achieve greater congruence between crop N demand and N supply from all sources—including fertilizer, organic inputs, and indigenous soil N (Cassman et al. 2002).

Most of the fertilizer-N is lost during the year of application. Consequently, N and crop management must be fine-tuned in the cropping season in which N is applied in order to maximize system-level NUE. Numerous concepts and tools needed to increase NUE have been developed. These technologies can be divided into (1) those that enhance crop N demand and uptake (genetic improvements, management factors that remove restrictions on crop growth and N demand) and (2) management options that influence the availability of soil and fertilizer-N for plant uptake. The latter primarily include more efficient fertilizers (new N forms, modified fertilizers & inhibitors that lead to slow/controlled release), more efficient N application methods, and various forms of site-specific N management. It is important to understand, however, that many of the technology options have different effects on crop yield response to N and that it is often the combination of measures that leads to the greatest benefit.

It is beyond the scope of this paper to discuss specific technologies in more detail and the reader is referred to the recent literature on this (Ladha et al. 2005; Giller et al. 2004; Dobermann and Cassman 2004; Cassman et al. 2002). Modern N management concepts usually involve a combination of anticipatory (before planting) and responsive (during the growing season) decisions. Improved synchrony, for example, can be achieved by more accurate N prescriptions based on the projected crop N demand and the levels of mineral and organic soil N, but also through improved rules for splitting of N applications according to phenological stages, by using decision aids to diagnose soil and plant N status during the growing season (models, sensors), or by using controlled-release fertilizers or inhibitors. The latter have a theoretical advantage over other, more knowledge-intensive forms of fined-tuned N management in a sense that the knowledge is ‘embedded’ in the product to be applied. As experience with seeds shows, embedded knowledge can lead to high adoption rates by farmers, provided that the benefit : cost ratio is high. Improved fertilizer products can thus play an important role in the global quest for increasing NUE, but their relative importance varies by regions and cropping systems.

Important prerequisites for the adoption of advanced N management technologies are that they must be simple, provide consistent and large enough gains in NUE, involve little extra time and be cost-effective (Giller et al. 2004). If a new technology leads to at least a small and consistent increase in crop yield with the same amount or less N applied, the resulting increase in profit is usually attractive enough for a farmer. This is particularly relevant for developing countries or large-scale grain farms in North and South America or in Australia, where there is still potential and need to produce more food and feed. Where yield increases are more difficult to achieve, where increasing crop yield is of less priority, or where reducing the creation of Nr in agriculture is the top societal priority, adoption of new technologies that increase NUE but have little effect on farm profit may need to be supported by appropriate technology incentives.

Summary

Quantifying the status of NUE in agriculture is a difficult task because (i) definitions used in research papers and interpretation of different NUE indices vary and (ii) reliable data needed to compute NUE indices are often not available, particularly at national, regional and global scales. Worldwide, crops do not directly utilize about half of the applied N and the overall NUE has declined with increasing N fertilizer use. This trend seems to continue in many developing countries. In many industrialized countries NUE has increased in recent years, even at high levels of cropping intensity and fertilizer use. Interventions to increase NUE and reduce N losses to the environment must be accomplished at the farm level through a combination of improved technologies and carefully crafted local policies that promote the adoption of improved N management practices while sustaining yield increases.

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