A high plant density reduces the ability of maize to use soil nitrogen

Understanding the physiological changes associated with high grain yield and high N use efficiency (NUE) is important when increasing the plant density and N rate to develop optimal agronomic management. We tested the hypothesis that high plant densities resulting in crowding stress reduce the ability of plants to use the N supply post-silking, thus decreasing the grain yield and NUE. In 2013 and 2014, a field experiment, with five N-application rates and three plant densities (6.0, 7.5, and 9.0 plants m(–2)), was conducted in the North China Plain (NCP). The calculated maximum grain yield and agronomic use efficiency (AE(N)) at a density of 7.5 plants m(–2) were 12.4 Mg ha(–1) and 39.3 kg kg(–1), respectively, which were significantly higher than the values obtained at densities of 6.0 (11.3 Mg ha(–1) and 30.2 kg kg(–1)) and 9.0 plant m(–2) (11.7 Mg ha(–1) and 27.8 kg kg(–1)). A high plant density of 9.0 plants m(–2) decreased the post-silking N accumulation, leaf N concentration and net photosynthesis, which reduced the post-silking dry matter production, resulting in a low yield and NUE. Although a relatively low grain yield was observed at a density of 9.0 plants m(–2), the optimal N rate increased from 150 to 186 kg N ha(-1) at a density of 7.5 plants m(–2). These results indicate that high plant densities with crowding stress reduce the ability of plants to use soil N during the post-silking period, and high rate of N fertilizer was needed to increase grain yield. We conclude that selecting the appropriate plant density combined with optimal N management could increase grain yields and the NUE in the NCP.