Determining nitrogen status and quantifying nitrogen fertilizer requirement using a critical nitrogen dilution curve for hybrid indica rice under mechanical pot-seedling transplanting pattern

Field experiments of nitrogen (N) treatment at five different application rates (0, 75, 150, 225, and 300 kg ha^?1) were conducted under pot-seedling mechanical transplanting (PMT) in 2018 and 2019.  Two high-quality and high-yielding hybrids of indica rice, Huiliangyou 898 and Y Liangyou 900, were used in this study.  The N nutrition index (NNI) and accumulated N deficit (N_and), used to assess the N nutrition status in real-time, were calculated for the indica cultivars under PMT with a critical nitrogen concentration (Nc) dilution model based on shoot dry matter (DM) during the whole rice growth stage.  The relationships between NNI and N_and with relative yield (RY) were determined, and accurate N application schemes were developed for hybrids indica rice under PMT.  The results indicated that high application rate of N-fertilizer significantly increased the concentrations of shoot DM and N in aboveground organs during the observed stages in the two cultivars for two years (P<0.05).  The Nc dilution model of hybrid indica cultivars was N_c=4.02DM^?0.42 (R²=0.97) combining the two cultivars under PMT.  Root-mean-square error and normalized root-mean-square error of the curve verification were 0.23 and 10.61%, respectively.  The NNI and N_and ranged from 0.58 to 1.31 and 109 to –55 kg ha^?1, respectively, in the two cultivars for all N treatments.  NNI showed a linear relationship with N_and during the entire growth stage (0.53<R²<0.99, P<0.01).  In addition, NNI showed a linear-plateau relationship with RY (0.73<R²<0.92, P<0.01) throughout the observed stages.  These results suggest that the models can accurately diagnose the N-nutrition status and support effective N-fertilizer management in real-time for hybrid indica rice under PMT.