Nitrogen placement at sowing affects root growth, grain yield formation, N use efficiency in maize

Grain yields of summer maize are significantly affected by different nitrogen (N) rates and depths through regulating root growth and distribution in soil. Understanding of effects of the deep placement of N on the root and shoot growth, grain yield and N use efficiency in summer maize are limited. In this study, four N rates: 225, 191.25, 157.5 and 0 kg ha−1 applied at four depths: 5, 10, 15, and 20 cm were studied. Soil N content, root dry weight, root length density, biomass, grain yield and N use efficiency of maize were measured. Compared to 225 kg N ha−1 applied at a depth of 5 cm, a 15% reduction in the N application rate at a depth of 15 cm induced a larger root length density in the subsoil, as well as a larger rooting depth. It also facilitated maintaining a higher level of biomass and N accumulation during the later growth period, which increased the N assimilation of grain and enhanced grain yield by 3.9%, N recovery efficiency by 66.7%, N agronomic efficiency by 38.5%, and partial factor productivity of N by 22.1%. Overall, this study demonstrates that reducing the recommended N application rate of 225 kg ha−1 by 15% but applying it at a depth of 15 cm might be considered an efficient fertilization method that increases agricultural productivity and N use efficiency.