- Long-term N and P application led to different nitrate residue levels.
- Residual nitrate increased by N but decreased by combined P to different degrees.
- Suitable N and P application achieved steady-state soil N pool balance.
- Residual NO3-N threshold in 0–100 cm soil layer determined as 100 kg ha−1.
- Using threshold maintained soil N supply capacity and prevented NO3-N leaching.