Estimation of nitrogen fertilizer requirement for rice crop using critical nitrogen dilution curve

Abstract

Estimating in-season N requirement (NR) is essential for managing N fertilizer application in crop production. Critical N (Nc) dilution curve is an effective and simple-to-use technique for assessment of in-season crop N status, yet its adaptation to make field decisions about dressing N fertilization remains to be determined. This study was endeavored to establish the relations between NR, N nutrition index (NNI) and relative yield (RY) at different crop growth stages in Japonica and Indica rice (Oryza sativa L.) eco-types and to estimate time-course NR for recommending supplemental N fertilization on Nc dilution curve basis. Four field experiments of multi-N rates were carried out in east China using three Japonica and two Indica rice hybrids. Growth analysis was carried out at different growth stages from active tillering (AT) to heading (HD). The estimated NR under varied N rates has well differentiated the sub-optimal, optimal and supra optimal growth conditions at different stages in both rice eco-types. The NR-NNI and RY-NR relations for both rice eco-types at different growth stages were highly significant with R2 values greater than 0.88 and 0.95 for NR-NNI, and 0.83 and 0.91 for RY-NR relations, respectively, the highest R2 values for both eco-types were 0.98 and 0.99 for NR-NNI and 0.94 and 0.93 for RY-NR relations at panicle initiation (PI) and booting (BT) stages. Validation of the regression models with two independent datasets exhibited a solid model performance at PI and BT stages, with R2 values greater than 0.96 for NR-NNI while 0.94 for RY-NR relations. Moreover, the root mean square error (RMSE) values lower than 20% for NR prediction from NNI, while 8% for RY prediction from NR also confirmed the robustness of the relationships at PI and BT stages. The kappa (k) coefficients at PI and BT stages for observed and predicted NR and RY were close to 1. Generally, the robust relations at PI and BT stages well elucidated the variation in NR and RY both under deficient and optimum N growing conditions, and gave reliable estimation of NR for quantifying supplemental N fertilization for rice grown in east China. The results of this study will offer a suitable approach for managing N application precisely during the growth period of rice crop.