Objectives Nitrogen nutrition index (NNI) and nitrogen deficit (N_and) value are two indicators in diagnosing nitrogen nutrition of rice. The NNI and N_and values of inbred and conventional rice cultivars under different nitrogen supply levels were calculated and compared in this paper, in order to achieve precise and quantitative control of crop nitrogen status.

Methods Super-high-yield hybrid rice cultivars (YY-1 and CYQ) and conventional rice cultivars (YSM and JSM) were selected as materials in a field experiment. Nitrogen application levels of 0, 40, 80, 120, 160, 200 and 240 kg/hm² (expressed as N0, N40, N80, N120, N160, N200, and N240) were set up for each cultivar, the dry matter weight and nitrogen concentration of aboveground parts were investigated 15, 30, 45, 60, 75 days after transplanting, and at mature period, the critical nitrogen concentration curves were constructed. The curves were used to calculate the critical nitrogen concentration, the nitrogen index and nitrogen deficit value in different stages of plant for different species.

Results For hybrid rice, N0, N40, N80, N120, and N160 treatments showed significant differences in dry matter weight, N200 and N240 treatments had no significant difference but were significantly higher than those of N0, N40, N80, N120 and N160 treatments; for conventional rice cultivars, there was significant difference in dry matter weight of shoots among treatments N0, N40, N80 and N120, there was no significant difference among N160, N200 and N240 treatments, but the shoot dry weights in N160, N200 and N240 were significantly higher than those in N0, N40, N80 and N120 treatments. The nitrogen concentration of rice plant increased with the increase of nitrogen application rate, but the nitrogen concentration of rice plants showed a decreasing trend with the extension of growth period and the increase of aboveground dry matter weight. The critical nitrogen concentration curves of rice were constructed according to the aboveground dry matter weight and its nitrogen concentration, N_c=3.36DM^–0.31 (R²=0.91) for hybrid rice, N_c=2.96DM^–0.25 (R²=0.86) for conventional rice. The NNI and N_and values were calculated based on the critical nitrogen concentration curves. The NNI range was from 0.73 to 1.05 for hybrid rice and from 0.78 to 1.11 for conventional rice; and the N_and value was –9.8 to 117.8 kg/hm² for hybrid rice and –25.4 to 90.3 kg/hm² for conventional rice.

Conclusions The critical nitrogen concentration of conventional rice is higher than that of hybrid rice at the same growth stage, while the dry matter weight of hybrid rice is higher than that of conventional rice. Under the experimental conditions, the nitrogen accumulation in the critical nitrogen concentration of hybrid rice is larger than that of conventional rice according to the results of N_and calculation, the suitable nitrogen application rates of hybrid rice and conventional rice are about 200 kg/hm² and 160–200 kg/hm², respectively.