Relative importance of soil and climate variability for nitrogen management in irrigated wheat David B. Lobell a,b,* , J. Ivan Ortiz-Monasterio c , Gregory P. Asner a,b a Department of Global Ecology, Carnegie Institution of Washington, Stanford, CA 94305, USA b Department of Geological and Environmental Science, Stanford University, Stanford, CA 94305, USA c Wheat Program, International Maize and Wheat Improvement Center (CIMMYT), Apdo. Postal 6-641, 06600 Mexico D.F., Mexico Received 31 July 2003; received in revised form 18 October 2003; accepted 18 October 2003 Abstract Increased efficiency of nitrogen (N) fertilizer use may be achieved with management practices that account for spatial variability in soil properties and temporal variability in climate. In this study, we develop a N management decision model for an irrigated wheat system that incorporates hypothetical diagnostics of soil N and growing season climate. The model is then used to quantify the potential value of these forecasts with respect to wheat yields, farmer profits, and excess N application. Under the current scenario (i.e. no diagnostics), uncertainty in soil and climate conditions is shown to account for an average over- application of N by roughly 35%. Both soil diagnostics and climate forecasts are shown to increase profits significantly and decrease over-application of N, with minimal changes in yield. Soil variability is roughly three times as important as climate variations in terms of potential impact on profits in this region. The model was also used to simulate the effect of increases in fertilizer price, which have similar positive effects on excess N application but negative impacts on profits. Finally, the role of forecast uncertainty was evaluated, indicating that even limited information on soil or climate can be a useful input to management decisions. Future work is needed to improve operational diagnostics of soil N and growing season climate, whose cost can then be compared to benefits calculated in this study to determine their net value to N management decisions. # 2003 Elsevier B.V. All rights reserved. Keywords: Climate variability; Management decisions; Nitrogen; Soil variability; Uncertainty; Wheat 1. Introduction The worldwide application of fertilizer nitrogen (N) in agriculture has increased more than six-fold since 1960 (FAO, 2003), and at roughly 80 Mt N per year is currently second only to biological N fixation as a source of N to terrestrial ecosystems (Galloway et al., 1995). While increased fertilizer use has played an important role in keeping food production in pace with population growth, off-site consequences such as eutrophication and N-oxide gas production have raised concerns about environmental impacts of cur- rent practices (Matson et al., 1997; Gregory et al., 2002). Improvements in the efficiency of nitrogen use in major cropping systems will be a central component of efforts to produce more food while reducing environmental impacts (Conway, 1997; Cassman, 1999). Several opportunities have been identified Field Crops Research 87 (2004) 155–165 * Corresponding author. Present address: Department of Global Ecology, Carnegie Institution of Washington, Stanford, CA 94305, USA. Tel.: þ1-650-325-1521x329; fax: þ1-650-325-6857. E-mail address: dlobell@stanford.edu (D.B. Lobell). 0378-4290/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2003.10.004