Objectives To identify the variation in N status indicators with different yield levels for winter wheat after rice in the middle and lower reaches of the Yangtze River, and provide a theoretical basis for the real-time diagnosis of N status in field.

Methods In this study, the big data of wheat with different yield levels were collected based on the field experiments of different N rates, sowing dates, densities and varieties in Jiangsu Province during past years. The dynamic changes of dry matter accumulation (DMA), plant N accumulation (PNA), and plant N concentration (PNC) of wheat with different yield levels at different growth stages were analyzed. The plant critical N concentration, the accumulated N deficit (N_and) and N nutrition index (NNI) were calculated, and then the characteristics of N status indicators of high-yield wheat were clarified.

Results With the development of wheat growth, the trends of DMA and PNA were consistent, which showed gradually increasing trends. The PNC decreased gradually while the N_and and NNI were fluctuated. During the whole growth stages, there was no significant difference in DMA and PNA between high and middle yield wheat. The high yield and middle yield wheat had significantly higher PNA than the low-middle yield wheat, and the low-middle yield wheat had significantly higher PNA than the low yield wheat. The high yield and middle yield wheat had significantly higher DMA than the low-middle yield wheat, while the low-middle yield wheat had significantly higher DMA than the low yield wheat. Judging the N nutrition status based on the N_and, the N_and trend of high and middle yield wheat was consistent. During the rising stage, the N_and of high yield wheat decreased from 0.3 kg/hm² to −23.0 kg/hm², and the middle yield wheat decreased from 7.0 kg/hm² to −14.6 kg/hm², and the booting-heading stage showed a increase trend again, the high yield wheat increased from −23.0 kg/hm² to −11.4 kg/hm², and the middle yield wheat increased from −14.6 kg/hm² to 2.4 kg/hm², and the fluctuation was small during the flowering-filling stage. It was showed that the N nutrition of high yield wheat was surplus except the rising stage, and the N nutrition of middle yield wheat at the jointing-booting stage was less than 0, which was greater than 0 at the rest stages, but the N_and of this yield level wheat had always been within the appropriate range fluctuation. During the rising-jointing stage, the N_and of low-middle yield level wheat decreased from 14.2 kg/hm² to 9.5 kg/hm², and then gradually increased, and the highest value was 43.9 kg/hm² in filling stage. The N_and of low yield level wheat increased from 17.3 kg/hm² to 71.1 kg/hm² in the rising-filling stage. It showed that the nitrogen nutrition of low-middle and low yield level wheat gradually decreased and it was in a deficit state throughout the whole growth stage (N_and > 0). In the jointing, booting, heading and filling stages, the actual PNC of high yield level wheat was higher than the critical PNC, and the actual PNC of middle yield at the booting stage was higher than the critical PNC, while the actual PNC of low-middle and low yield levels wheat was lower than the critical PNC throughout the whole growth stage. The NNI of high yield and middle yield wheat fluctuated around 1. Among them, the NNI of high yield increased from 0.9 to 1.1 during the rising-booting stage. In heading-filling stage, the NNI decreased first and then increased, and the values were 1.0, 0.9 and 1.0, respectively. The change trend of the middle yield and high yield wheat was consistent. The NNI at the rising-booting stage increased from 0.8 to 1.0, and then gradually decreased. The values were both less than 1, and in the heading-filling stage were 0.9, 0.9 and 0.9, respectively. The NNI of low-middle and low yield wheat was always lower than 1, and among them, the NNI of low-middle yield wheat was increased from 0.7 to 0.8 during the rising-jointing stage, and then gradually decreased. The NNI of low yield wheat was gradually decreased from the rising-flowering stage, and the NNI of the two yield level wheat showed a slight upward trend during the filling stage.

Conclusions With the increase of yield level, DMA, PNA, PNC, NNI increase correspondingly, while the N_and decreases. Higher DMA and PNA are the main reasons for the increase of wheat yield. In the process of wheat growth, both NNI and N_and can be used for diagnosing N status accurately, which can provide theoretical support for precision N management in fields.