Fig. 7 Contour map based on the grain yield, N harvest index (NHI), residual NO3-N in the 0–100 cm soil layer (Nit0−100), and accumulated NO3-N in the 100–300 cm soil layer (Nit100−300) at the crop harvest under different treatments. The red line is the safe threshold for Nit0−100 at the crop harvest. The black dashed line indicates the N and P fertilizer application amounts based on the maximum yield and safe threshold for Nit0−100.