See the legend of Fig. 1 for definitions of abbreviations. According to Fig. 2a, NUEdiff, NUE15N, and NUEbala correspond to the slope of line BD, CD, and OD respectively. Numbers in brackets are from an example developed for the Chinese cereal cropping system. Point A was derived as the cross-point of line OD and the vertical line BC. Strictly speaking, NUEbala measures the efficiency of total N inputs instead of fertilizer inputs only, because the denominator for NUEbala is FN + NFN instead of FN. To derive the NUE for fertilizer based on NUEbala (NUEbala_F, the slope of pD, where p represents a point on the line MN and is not noted in the figure), the non-fertilizer nitrogen input (NFN) and its bioavailability (BANF, or the slope of Op) need to be quantified. When BANF is the same as NUEbala, point p overlaps with point A, and the NUEbala_F is the same as NUEbala (0.52, the red dotted line). If all NFN is harvested as crop products (BANF=1; the maximum value of BANF), then point p moves to point M, and NUEbala_F is the slope of line MD (NUEbala_F=0.38). In contrast, if no NFN is harvested as crop products (BANF=0; the minimum value of BANF), then point p moves to point N, and NUEbala_F is the slope of line ND (NUEbala_F=0.66). Therefore, based on NUEbala for the Chinese cereal cropping system example, the lowest possible value for NUEbala_F is 0.38, and it is still higher than NUE15N (0.30) and NUEdiff (0.32), indicating other important drivers for the differences between NUEbala and the other two approaches.