Relative importance of soil and climate variability for 4 nitrogen management in irrigated wheat

10 11 Abstract 12 13 Increased efficiency of nitrogen (N) fertilizer use may be achieved with management practices that account for spatial 14 variability in soil properties and temporal variability in climate. In this study, we develop a N management decision model for an 15 irrigated wheat system that incorporates hypothetical diagnostics of soil N and growing season climate. The model is then used 16 to quantify the potential value of these forecasts with respect to wheat yields, farmer profits, and excess N application. Under the 17 current scenario (i.e. no diagnostics), uncertainty in soil and climate conditions is shown to account for an average over18 application of N by roughly 35%. Both soil diagnostics and climate forecasts are shown to increase profits significantly and 19 decrease over-application of N, with minimal changes in yield. Soil variability is roughly three times as important as climate 20 variations in terms of potential impact on profits in this region. The model was also used to simulate the effect of increases in 21 fertilizer price, which have similar positive effects on excess N application but negative impacts on profits. Finally, the role of 22 forecast uncertainty was evaluated, indicating that even limited information on soil or climate can be a useful input to 23 management decisions. Future work is needed to improve operational diagnostics of soil N and growing season climate, whose 24 cost can then be compared to benefits calculated in this study to determine their net value to N management decisions. 25 # 2003 Published by Elsevier B.V. 26 27