Objectives The objective of this study was to establish the critical nitrogen dilution curve of maize in central area of Henan Province, China, to compare differences of nitrogen use efficiency among different varieties of maize, and to study the reliability of nitrogen nutrition index for diagnosing and evaluating nitrogen nutrition in maize, which would provide theoretical basis for rational nitrogen fertilization of maize.

Methods In this paper, Weike702 and Zhongdan 909, as the two different nitrogen use efficiency varieties, were studied for three consecutive years in a field trial with five nitrogen levels (0, 120, 180, 240 and 360 kg/hm²). Effects of different nitrogen application rates on dry matter of both varieties at jointing, spike formation, silking and harvest stages were analyzed, critical nitrogen dilution curves of two varieties were established based on the dry matter and plant nitrogen concentration in different periods, differences of critical nitrogen dilution curve models for maize with different nitrogen use efficiencies were analyzed, and the nitrogen nutrition index and its relationship with relative shoot biomass and relative yield of maize were analyzed.

Results The nitrogen use efficiency of Zhongdan909 was significantly higher than that of Weike702, and the shoot biomass of both varieties showed N0 < N120 < N180 = N240 = N360. With the increase of N application rate, the aboveground biomass of two varieties of maize could be divided into two parts: nitrogen limitation and non-limiting nitrogen, the critical nitrogen dilution curve models of the two cultivars (Weike702, Nc = 35.638DM ^–0.341, Zhongdan909, Nc = 30.801DM^–0.370) had good stability. Compared with the model parameters of Zhongdan909, the parameter a of Weike 702 increased by 15.70%, its parameter b decreased by 7.84%, and the value of a changed more than that of the parameter b. The nitrogen nutrition indices of the two cultivars based on this model at the same period were raised with the increase of nitrogen application rate. The nitrogen nutrition index was first increased and then decreased with the increase of nitrogen application rate in maize growth period when the nitrogen application rate was less than 180 kg/hm². When nitrogen application rate exceeded 240 kg/hm², the nitrogen nutrition index was increased continuously. The correlations between nitrogen nutrition indices and relative shoot biomass, relative yield of maize also reached a significant level.

Conclusions In this paper, the nitrogen nutrition status of maize plants can be diagnosed and evaluated using the critical nitrogen dilution curve model and nitrogen nutrition index of two varieties of maize in central Henan Province. There were differences in the parameters of critical nitrogen concentration dilution curve between different nitrogen use efficiency varieties. Varieties with high nitrogen use rate had lower nitrogen concentration per unit biomass and higher curve slope, and the nitrogen concentrations were lower than those with the low nitrogen utilization rate.