© 2004 "New directions for a diverse planet". Proceedings of the 4th International Crop Science Congress, 26 Sep – 1 Oct 2004, Brisbane, Australia. Published on CDROM. Web site www.cropscience.org.au 1 Nitrogen Over-use, Under-use, and Efficiency J.L. Hatfield 1 and J.H. Prueger 1 1 USDA-ARS National Soil Tilth Laboratory, 2150 Pammel Drive, Ames, Iowa 50011 USA Email: hatfield@nstl.gov Abstract Nitrogen is a critical element for plant growth and plant response to added nitrogen (N) has proven to be a valuable agronomic practice. However, N is subject to losses from a number of pathways of which leaching of nitrate is one of the most important because it is extremely mobile leading to increases in nitrate concentrations in surface and ground water. These detections are often attributed to application rates to crops that exceed agronomic needs. Nitrate in water is a world-wide problem and agriculture is being asked to develop farming systems that will reduce the leaching of nitrate from fields and decreases both the concentration and load of nitrate in water. Nitrogen fertilizer use in the world continues to increase in many countries and has remained relatively constant for the past 15 years in Western Europe and the United States. Nitrogen Use Efficiency (NUE), expressed as grain production per unit of N applied, has shown a decrease for all countries. Grain production within each country shows a linear increase with N applied. Management of N in farming systems is difficult because of the interactions between soil mineralization potential, soil water availability, and the type of crop grown. An example of the interaction between N use and water on yield is shown in a detailed study on maize (Zea mays L.) in Iowa. Yield response to N showed a decrease with N rates above 116 kg ha -1 due to water deficits during grain-filling that reduced yield and water use efficiency. Increasing N rates decreased the yield variation within fields without increasing the yield. Management of N within fields can be improved through an analysis of the soil organic matter content and the soil water holding capacity. Integrating soil water with N management will increase the efficiency of N use and decrease the environmental impact of agriculture. Media Summary Nitrogen (N) is necessary for plant growth and the increasing world population increases the demand for more food production. Nitrogen, as nitrate in water is an environmental problem and agriculture has a responsibility to develop practices that increase the efficiency of nitrogen use by crops. Adoption of new management techniques that are based on water and nitrogen use efficiency (NUE) will enhance our ability to efficiently produce food and fiber around the world. Keywords Nitrogen use efficiency, water use patterns, water use efficiency, cropping systems, spatial variability, nitrogen management Introduction Nitrogen is one of the critical elements essential for life. However, it is also one of the most complex elements when one considers all of the potential forms and processes involved in the N cycle. Keeney and Hatfield (2001) summarized the historical perspective on the N cycle and world patterns for N use to show that N use continues to increase. Data from the Food Agricultural Organization (FAO) on N use shows the largest increases are in developing countries while developed countries have fairly consistent levels of N use for the past two decades (FAO, 2000). Nitrogen fertilizer consumption by agriculture is shown in Fig. 1 for six regions throughout the world. China has shown a linear increase in N fertilizer use since 1976 with a nearly level use since 1996 while Western Europe and the United states have had no significant change in annual N use since 1978 (Fig. 1). Increased human population will require increased food production and Bouwman and Booji (1998) suggest that global cereal production will increase to meet the increasing demand with the result being N transport among regions. To meet increased protein requirements will require either more N inputs to produce grains and forage or increased efficiency of N use. Smil (1999) reached a similar conclusion on global N cycles and stated N use will increase to meet the demands for food supply. van Egmand et al. (2002) evaluated the N budget for Europe and concluded that the NO 3 -N levels were increasing and that N losses were due to denitrification, emission of NH 3 and NO y , and exported products. In a similar study for Asia, Zheng et al. (2002) found that environmental N enrichment are due to the increasing demand for food supply and one method to reduce this excess was to reduce the application rate of inorganic N fertilizers. Richter and Roelcke (2000) examined central