A typical yield response curve based on field trials shows a “diminishing return” to N inputs. For experimental sites that have been under fertilizer N treatment over a period of time, the yield observed at the control plot tends to decrease over time (the red point on the vertical dashed line of NFN moves downward), mainly due to the gradually reducing legacy effect of N input before the setting of the control plot. See the legend of Fig. 1 for definitions of abbreviations. The slopes of red lines are NUEdiff and NUEbala.