Fig. 4 Accumulated NO3-N amounts in the 0–300 cm soil layer at the crop harvest. The codes comprising 201706, 201710, 201806, 201810, 201906 and 201910 have the same meanings as those in Fig. 1. Different capital letters indicate significant differences among the average N values at P < 0.05. Different lowercase letters indicate significant difference among the four P treatments with the same N rate at P < 0.05.