Objectives The interaction of N rate and plant density often makes troubles to evaluate the nitrogen use efficiency (NUE) of maize. It is of great importance to analyze the effects of interaction of N rate and plant density on physiological processes of N absorption and utilization and yield formation of maize for revealing mechanism and pathway of enhancing NUE and yield synchronously in maize.

Methods Field were conducted using a typical commercial hybrid Zhengdan958 as tested materials in Shulinzhao and Salaqi, Inner Mongolia. The tested plant densities included 45, 75 and 105 thousand plants per hectare (D4.5, D7.5 and D10.5), and N rates included 0, 150 and 300 kg/hm² (N0, N150 and N300). At the stages of 14-open-leaves (V14), silking (R1), grain filling (R3) and ripening (R6), plant samples were collected, and divided into different parts for measuring biomass, nitrogen contents. The nitrogen use efficiencies were calculated.

Results Compared to N150, N300 neither enhanced the total carbon and nitrogen accumulation of maize, nor improved individual plant yield capacity, which induced the low nitrogen use efficiencies under the high N rate. The D10.5 combined with N150 gained the highest yield and NUE, indicated that increased plant density with reduced N rate was an important way to improve maize yield and NUE synchronously. The N accumulation advantage of applying N and increasing plant density was mainly driven by dry matter accumulation during the stage from V14 to R3. N uptake rate was positively correlated with dry matter accumulation rate at the V14−R3 stage, and applying N and increasing plant density remarkably enhanced the response intensity of N accumulation to dry matter accumulation. Under the treatments of applying N and increasing plant density, maize could acquire higher pre-anthesis N accumulation with lower N concentration, which indicated that pre-anthesis N accumulation primarily depended on shoot dry matter accumulation, while post-anthesis N accumulation was mainly ascribe to ear dry matter accumulation. The interaction of N rate and plant density did not affect the nitrogen harvest index (NHI), but the medium N and high plant density significantly improved HI. This indicated that enhancing NUE by reducing N rate and increasing plant density had nothing to do with nitrogen distribution to grain, but was primarily ascribe to grain dry matter accumulation.

Conclusions The interaction of N rate and plant density influences the NUE significantly through affecting dry matter accumulation and N accumulation. Reasonable reduction of N input while increasing plant density could concurrently enhance maize grain yield and nitrogen use efficiency, which physiologically promoted crop growth rate during the bracketing silking and post-anthesis biomass accumulation, and further drove adequate pre-anthesis N accumulation and post-anthesis grain biomass distribution.