Determination of critical nitrogen dilution curve based on leaf area index in rice

• LAI-based crop N status assessment is an effective method to optimizing N management.
• N uptake and LAI relationship is strictly proportional and robust across the environment.
• N uptake is the feedback controlled by LAI expansion rather than DM accumulation.
• LAI-based approach is an excellent and rapid indicator reflecting the changes in plant N.
• LAI-based approach is suitable for multi-scale and multi-temporal research.

The environmental constraint caused by intensification of rice production in China has raised the need to adjust crop nitrogen (N) fertilizer requirement. Leaf area index (LAI)-based assessment of crop N status can be an effective approach to optimizing N management in rice production. This study was designed to develop a new methodology for establishing the critical nitrogen (Nc) dilution curve based on LAI, to compare if a Nc dilution curve based on LAI would allow a better diagnosis of N nutrition status than that on plant dry matter (PDM) basis and to assess its applicability for estimating plant N status in Japonica rice (Oryza sativa L) in east China. Three field experiments with varied N rates (0–360 kg N ha−1) were conducted using three Japonica rice hybrids, Lingxiangyou-18 (LXY-18), Wuxiangjing-14 (WXJ-14) and Wuyunjing (WYJ) in lower Yangtze River reaches. LAI and plant N concentration (PNC) were determined from active tillering to heading for growth analysis in each experiment. The LAI and PNC ranged from 0.29 to 7.46 and 0.99 to 3.11%, respectively, under the varied experiment conditions. The relationship between N uptake and LAI expansion are strictly proportional and robust across the environments. The Nc dilution curve (Nc = 3.70LAI−0.35) was validated for N-limiting and non-N-limiting growth conditions. The N nutrition index (NNI) and accumulated N deficit (Nand) ranged from 0.69 to 1.06 and 88.8 to −7.54 kg ha−1, respectively, over main growth stages. The values of ΔN derived from either NNI or Nand could be used as references for in-season N dressing management in rice. Our results indicate that Nc dilution curve as a function of LAI efficiently identified the conditions of limiting and non-limiting N nutrition and can be adopted as a novel tool for evaluating plant N status for precision N management in Japonica rice of east China.