Optimizing nitrogen application depth can improve crop yield and nitrogen uptake-A global meta-analysis

Deep application of nitrogen fertilizer (DANF) is a strategy based on applying nitrogen fertilizer in the plow layer where it is not exposed to the surface of the soil. Due to its significant beneficial effects on the crop yield and environment, DANF has been applied in many regions throughout the world. However, the effects of DANF on the crop yield, nitrogen apparent recovery (NAR), nitrogen uptake (NU), and farmland management strategies have not been comprehensively evaluated, and thus it is not possible to accurately determine and assess the optimal depth for nitrogen application in widespread use. Therefore, we conducted a meta-analysis based on 716 observations from 96 studies to examine the effects of three nitrogen application depths comprising 5-10 cm, 10-15 cm, and > 15 cm on the crop yield, NAR, and NU. The results showed that compared with the surface application of nitrogen fertilizer, under the three nitrogen application depths of 5-10 cm, 10-15 cm and > 15 cm, the crop yield increased significantly by 7.57%, 11.63%, and 11.85%, respectively, NAR increased by 46.37%, 55.27%, and 58.57%, and NU increased by 19.36%, 26.24%, and 20.93%. In general, a nitrogen application depth of 10-15 cm was most effective. Further analysis showed that the positive effects of DANF on the crop yield, NAR, and NU were influenced by climate factors, soil physical and chemical factors, and management factors. Mean annual precipitation > 800 mm and mean annual temperature > 15 degrees C were most conducive to improving the crop yield under different nitrogen application depths. In addition, we found that DANF was more effective in fields with poor soil and medium and low fertility. More importantly, DANF was highly adaptable to different crops. When DANF was applied in combination with slow-or controlled-release nitrogen fertilizer, the same or higher yields could be obtained at lower nitrogen application rates, thereby improving NAR and NU. These results provide novel insights into the optimal nitrogen application scheme for global agroecosystems by considering the climatic conditions, soil fertility, nitrogen application method, and nitrogen application rate at specific sites.