Improving Nitrogen Use Efficiency for Cereal Production

found, had they considered N derived from the soil (Keeney, 1982; van der Ploeg et al., 1997). Using the Worldwide, nitrogen use efficiency (NUE) for cereal production same references and assumptions in Table 1, cereal (wheat, Triticum aestivum L.; corn, Zea mays L.; rice, Oryza sativa L. and O. glaberrima Steud.; barley, Hordeum vulgare L.; sorghum, NUEs are 42 and 29% in developed and developing Sorghum bicolor (L.) Moench; millet, Pennisetum glaucum (L.) R. nations, respectively. Based on present fertilizer use, a Br.; oat, Avena sativa L.; and rye, Secale cereale L.) is approximately 1% increase in the efficiency of N use for cereal produc33%. The unaccounted 67% represents a $15.9 billion annual loss of tion worldwide would lead to a $234 658 462 savings in N fertilizer (assuming fertilizer–soil equilibrium). Loss of fertilizer N N fertilizer costs (Table 1). An increase in NUE of 20% results from gaseous plant emission, soil denitrification, surface runoff, would result in a savings in excess of $4.7 billion per year. volatilization, and leaching. Increased cereal NUE is unlikely, unless a systems approach is implemented that uses varieties with high harvest index, incorporated NH4–N fertilizer, application of prescribed rates Why Are Nitrogen Use Efficiencies So Low? consistent with in-field variability using sensor-based systems within Not until recently have scientists documented that production fields, low N rates applied at flowering, and forage produccereal plants release N from plant tissue, predominantly tion systems. Furthermore, increased cereal NUE must accompany as NH3 following anthesis (Harper et al., 1987; Francis increased yields needed to feed a growing world population that et al., 1993). Plant N losses have accounted for 52 to has yet to benefit from the promise of N2–fixing cereal crops. The Consultative Group on International Agricultural Research (CGIAR) 73% of the unaccounted N using 15N in corn research linked with advanced research programs at universities and research (Francis et al., 1993), and between 21% (Harper et al., institutes is uniquely positioned to refine fertilizer N use in the world 1987) and 41% (Daigger et al., 1976) in winter wheat. via the extension of improved NUE hybrids and cultivars and manageGaseous plant N loss in excess of 45 kg N ha21 yr21 has ment practices in both the developed and developing world. also been documented in soybean [Glycine max (L.) Merr.] (Stutte et al., 1979). Reported gaseous N losses due to denitrification from I 1996, a total of 82 906 340 Mg of fertilizer N was applied fertilizer N include 9.5% in winter wheat (Auapplied in the world, of which 11 184 400 was applied lakh et al., 1982), 10% in lowland rice (De Datta et al., in the United States (FAO, 1996). Cereal production 1991), and 10% (conventional tillage) to 22% (no-till) accounted for approximately 49 743 804 Mg of N fertilin corn (Hilton et al., 1994). Incorporation of straw and/ izer world-wide (60% of the total; Table 1) (Alexanor application of straw on the surface of zero-till plots dratos, 1995, p. 190). Of that, only an estimated can double denitrification losses (Aulakh et al., 1984). 16 572 232 Mg was removed in the grain (Dale, 1997; Fertilizer N losses in surface runoff range between Tkachuk, 1977; Keeney, 1982) (Table 1). The world 1% (Blevins et al., 1996) and 13% (Chichester and Richcereal grain NUE would therefore be estimated at 33% ardson, 1992) of the total N applied, and are generally {NUE 5 [(total cereal N removed) 2 (N coming from lower under no-tillage. When urea fertilizers are applied the soil 1 N deposited in the rainfall)]/(fertilizer N apto the surface without incorporation, losses of fertilizer plied to cereals)}, far less than the 50% generally reN as NH3 can exceed 40% (Fowler and Brydon, 1989; ported (Hardy and Havelka, 1975). Similar results in Hargrove et al., 1977), and generally greater with inNUE for West German agriculture would have been creasing temperature, soil pH, and surface residue. When fertilizer N is applied at rates in excess of that Dep. of Plant and Soil Sciences, Oklahoma State University, Stillwaneeded for maximum yield in cereal crops, NO3 leaching ter, OK 74078. Contribution from the Okla. Agric. Exp. Stn. Received 30 Sept. 1998. *Corresponding author ([email protected]). Abbreviations: ATP, adenosine-59-triphosphate; NUE, nitrogen use efficiency. Published in Agron. J. 91:357–363 (1999).