Combined nitrogen and phosphorus management based on nitrate nitrogen threshold for balancing crop yield and soil nitrogen supply capacity

An appropriate combined nitrogen (N) and phosphorus (P) fertilization strategy is essential for obtaining sustained higher grain yields while maintaining soil fertility. In this study, a long-term split-plot design farmland experiment (initiated in 2009) with five N fertilizer rates combined with four P fertilizer rates was established during 2016–2019 to determine an appropriate nitrate-N (NO₃-N) threshold in an intensive managed wheat–maize double cropping system. A fertilization strategy was then proposed based on the NO₃-N threshold to balance the crop yields and soil nitrogen supply capacity. The results showed that N fertilizer increased the accumulated NO₃-N, and the combined application of phosphate fertilizer with each N rate reduced the accumulated NO₃-N to different degrees. The residual soil NO₃-N reached a steady-state soil N pool balance after long-term application of N at 150–225 kg ha⁻¹ combined with P at 60–120 kg ha⁻¹. The residual NO₃-N threshold in the root zone (0–100 cm soil layer) was determined as about 100 kg ha⁻¹ at the crop harvest to maintain the N supply capacity and prevent leaching into the deep soil (>100 cm soil layer). The fertilization guidelines are 154 kg ha⁻¹ for N fertilizer and 106 kg ha⁻¹ for P fertilizer in the winter wheat season, and 162 kg ha⁻¹ for N fertilizer and 122 kg ha⁻¹ for P fertilizer in the summer maize growing season based on the NO₃-N safety threshold. The optimized fertilizer strategy reduced the fertilizer application rate by 67 kg N ha⁻¹ per year and the residual NO₃-N by 34.2 % in the deep soil, while only reducing the average yield by 3.1 % across the crops and years. These findings provide a basis for sustainably balancing grain yields and soil nitrogen supply capacity as well as preventing nitrate pollution of farmland.