**5. Conclusions**

The future direction of world agriculture is towards high yield, profitability and resource use efficiency. Due to the variation in soil properties, weather conditions, and planting densities, the optimal N rate should be adjusted according to specific soil, weather, and planting density combinations. The results of this study indicated that the average EONR in a fertile black soil field (265 kg ha−1) was higher than in an aeolian sandy soil field (186 kg ha−1). The variation in weather conditions and planting density had significant effect on EONR, resulting in CV of 10% and 30% in black and aeolian sandy soil fields, respectively. The optimal planting density was defined at 70,000 and 55,000 plants ha−<sup>1</sup> for the black soil and aeolian sandy soil fields, respectively. The soil-specific EONR managemen<sup>t</sup> strategy performed better than RONR in reducing N application rate and improving NUE. The best improvement was achieved using the SYDS-EONR strategy which considered the soil, weather, and planting density combinations. More studies are needed to develop practical in-season soil (site)-specific N managemen<sup>t</sup> strategies to better account for soil, weather and planting density variation under diverse on-farm conditions.

**Author Contributions:** Y.M. and D.J.M. designed the experiment. X.W. conducted the experiment, performed the analysis, and wrote the original paper, R.D. and Z.C. assisted in the experiment, plant and soil sampling, and sample processing. Y.M., K.K., and G.M. reviewed and revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was financially supported by National Key Research and Development Program of China (2016 YFD0200600, 2016YFD0200602), Norwegian Ministry of Foreign Affairs (SINOGRAIN II, CHN-17/0019), and the UK Biotechnology and Biological Sciences Research Council (BB/P004555/1).

**Acknowledgments:** We would like to thank Yanjie Guan, Xuezhi Yue, Zheng Fang, Weidong Lou, and Guiyin Jiang for their help with the field experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.
