Nitrogen fertilizer efficiency and high-yield limiting factors of summer maize in the southern Hebei Province

The output of summer maize from the Huanghuaihai region accounts for approximately 35% of the national maize output, and so increasing the summer maize output of this region is of great significance to China's food security. The formation of summer maize yield is affected by many factors, such as climate, cultivation, fertilization, and so on. To find the yield variation characteristics, improve the nitrogen fertilizer efficiency, understand the different yield responses to soil nutrients and management measures, and propose a scientific optimization plan, this study used the example of Quzhou County of Hebei Province to investigate nitrogen fertilizer efficiency and high-yield limiting factors of summer maize. The management and soil nutrients of farmer's fields were tracked in real time for 4 consecutive years, and the yield variation characteristics and nitrogen fertilizer efficiency were studied at the farm scale. Meanwhile, the management measures and soil nutrients were analyzed using the boundary line analysis method, to identify the main factors that limit high yields in the region. The results showed that the average summer maize yield for farmers during 2015-2018 was 10.26 t·hm^-2, the coefficient of variation was 15.64%, and the yield and nitrogen fertilizer efficiency fluctuated between years. From 2015 to 2018, the total yield gap was 4.02 t·hm^-2, which varied from 1.96 to 3.68 t·hm^-2 between years. Eliminating the yield gap could achieve a yield increase of 16.46%-34.72%. Over 4 years, the occurrences of farmers obtaining a high yield or nitrogen fertilizer efficiency was normally distributed. The proportions of obtaining one high yield and one high nitrogen fertilizer efficiency were the largest. The yield and nitrogen fertilizer efficiency of a farmer were in an unstable state over the years. The stable-high-yield farmers had significant differences concerning the number of productive ears, 1000-grain weight, sowing date, and density compared with the stable-low-yield farmers (P < 0.05). This study showed that, in this region, the optimal sowing date was June 9-14, the optimal harvest density was 56 000-59 000 plants·hm^-2, the optimal N application rate was 210-230 kg·hm^-2, the optimal P₂O₅ application rate was 45-65 kg·hm^-2, the optimal K₂O application rate was 50-60 kg·hm^-2, and the optimal N-bass application ratio was 0.7-0.8. The formation of the yield gap was caused by various factors, and the contribution of different factors to the yield gap was different in different years. Generally speaking, the density, soil organic matter, and sowing date were the main factors that influenced the yield gap. Increasing regional yields, shrinking yield gaps between farmers, and increasing nitrogen fertilizer efficiency, as well as the shift from low-yield farmers to stable-high-yield farmers, all require improvements in soil nutrients and the optimization of comprehensive management measures.