Fig. 1. Method of optimizing N fertilization for winter wheat (left) and summer maize (right) is based on the plant N demands and soil N supply at three growth stages. Soil N_min was tested in the effective root depth at different growth periods. N_min target value included N demand by wheat/maize shoot and N demand by root. In the first year (1999–2000), for winter wheat, N_min target value from regreening to shooting was 120 kg N ha^–1, and from shooting to harvest was 125 kg N ha^–1. In the years of 2000–2001, 2001–2002, and 2002–2003, the N_min target value from regreening to shooting was adjusted to 90 kg N ha^–1 and the N_min target value from shooting to harvest was adjusted to 110 kg ha^–1.