How do different fertilization depths affect the growth, yield, and nitrogen use efficiency in rain-fed summer maize?

An appropriate fertilizer application depth is important for improving crop yields. However, the effects of different fertilization depths on the growth, yield, and nitrogen utilization in summer maize are unclear. In order to determine whether modifying the fertilization depth could improve the yield and nitrogen utilization efficiency (NUE) in summer maize, we conducted field experiments for two consecutive years (2019–2020) in a semi-humid and drought-prone area of Northwest China. We systematically analyzed the effects of fertilization depths on the soil mineral nitrogen (NO3--N and NH4+-N) concentration, crop growth, yield, and NUE in summer maize. Five fertilizer placement depths comprising 0 cm (FD0), 5 cm (FD5), 15 cm (FD15), 25 cm (FD25), and 35 cm (FD35) were tested. The results showed that compared with other fertilization depths, FD15 improved the content and distribution of nitrogen in the soil, thus promoting root growth and development and nitrogen absorption (6.5–38.0%). Meanwhile, FD15 increased nitrogen content, chlorophyll content (SPAD value), and photosynthetic rate in the summer maize leaves, thus promoting the accumulation and distribution of dry matter. Similarly, FD15 increased the grain number per ear and 100-grain weight. Therefore, compared with the other deep fertilization treatments, FD15 increased the summer maize grain yield (13.9–98.9%) and NUE (7.1–44.3%). These results clearly demonstrate that in the semi-humid and drought-prone area of Northwest China, a fertilization depth of 15 cm was conducive to rain-fed summer maize growth, and it improved the grain yield and NUE. These findings provide a practical reference for optimizing the fertilization strategy in this region.